Unit 6: Genetics

Advantages and Disadvantages of Sexual VS Asexual Reproduction

Asexual = Binary fission, mitosis, budding, fragmentation, vegetation reproduction, parthenogenesis

Produce genetically identical offspring by individuals that are adapted to an existing environment

• Rapid population growth (Under stable conditions)

• Energy efficient (Lack need for mate)

BUT…

• Limited ability to adapt

• Mass death once environment changes

Sexual

Produce offspring with gene combinations and thus variation needed for adaptation to a changed environment.

• Eliminate harmful mutations over time

• Provide defence against parasites and diseases

BUT…

• Needs time & energy

• Produce small populations

Role of Meiosis and Fusion of Gametes in Sexual Life Cycle

Meiosis = Breaks up parental combinations of alleles

• Production of haploid gametes (‘n’)

Fusion of gametes (Fertilisation) = Produce new combinations

• ‘2n’ number of chromosomes

Male VS Female in Sexual Reproduction

Male Gamete = Travels to female gamete

• Smaller (~5 μm), flagellum for movement

• Less food reserves than egg (Since so many are being produced)

• Increase chance of fertilisation through quantity

Female Gamete

• Invest in quality and survival of offspring

• More energy (Larger food resources)

• More parental care after fertilization

Anatomy of the Human Male Reproductive System + Function (HINT: 11 features)

• Testes

  Produce sperm + testosterone

• Vas Deferens

  Carry sperm from epididymis → junction with urethra

• Rectum

• Cowper’s Gland

  Secrete mucus-rich fluids into semen → Protects sperm from acids in urethra

• Prostate

  Secrete alkaline buffer into semen → Protect sperm from acidic environment in vagina

• Seminal Vesicle

  Secretes fructose into semen → Provide energy for sperm

• Bladder

• Epididymis

  Produce + Store sperm

• Penis

  Deposit sperm into vagina during ejaculation; Contains urethra

• Urethra

  Carries semen during ejaculation / urine from bladder to exterior

• Scrotum

  Protects testes + Thermoregulation

Anatomy of the Human Female Reproductive System + Function (HINT: 5 features)

• Fallopian Tubes

  Ducts that carry egg to uterus

• Ovaries

  Produce / release eggs (secondary oocytes) + Secrete oestradiol

• Uterus

  Muscular structure; Where embryo is implanted and develops

  Endometrium = Inner lining of uterus

• Cervix

  Lower portion of uterus; Opening allows sperm to enter for fertilisation

• Vagina

  Muscular tube; Leads from external to cervix / Where semen is ejaculated

Changes During the Ovarian and Uterine Cycles (Hormonal Regulation)

Roles of estradiol, progesterone, luteinising hormone (LH), follicle-stimulating hormone (FSH) + Feedback Mechanism

• Pituitary gland → FSH + LH / Ovaries → Oestradiol + progesterone

  
  

Ovarian Cycle = Production & release of egg / Release of oestradiol & progesterone

1. Follicular phase (Day 1-13)

   • FSH increase production and secretion of oestradiol → Increased density of blood vessels (vascularization) in endometrium

   • Rising oestradiol levels → Increase secretion of FSH and LH (Positive feedback loop)

   • FSH & LH result in Graafian follicles

2. Ovulation (Day 14) – Release of egg

   • Mostly LH & FSH surge → Triggers release of egg; Most mature egg cell burst from follicle through ovary wall

   • Empty follicle releases progesterone

3. Luteal phase (Day 15-28)

   • Empty follicle forms corpus luteum (Endocrine structure) → Produces MOSTLY progesterone and some oestrogen

   • Progesterone → Promotes endometrium thickening

   • High levels of progesterone & oestrogen → Inhibit GnRH → Prevent production of FSH & LH → Low LSH & FH → No further Graafian follicle → Corpus luteum break down → Progesterone & oestrogen decrease → Endometrium shed

Uterine Cycle = Preparation / Maintenance of endometrium to receive egg

1. Menstruation (Day 1-5)

   • Shedding of endometrium

2. Proliferative phase (Day 6-14)

   • Estradiol → Regeneration of endometrium

3. Secretory phase (Day 15-28)

   • Progesterone → Thickening and vascularisation of endometrium

The ovarian and uterine cycles together constitute the menstrual cycle (~28 days)

Feedback Mechanisms Involved with Ovarian / Uterine Cycles

Positive Feedback

• Rising estradiol levels → Triggers LH surge for ovulation

Negative Feedback

• High levels of estradiol and progesterone → Suppress FSH and LH

  (So that eggs are not all released)

Fertilisation in Humans

• Many sperms penetrate follicle cell layer and zone pellucid using hydrolytic enzymes stored in acrosomes

Fusion of sperm cell membrane with egg cell membrane

• First sperm to reach perivitelline layer attaches to its cell membrane receptor

Entry of sperm nucleus into egg + Destruction of the tail and mitochondria; Dissolution of nuclear membrane of sperm and egg nucleus

• Sperm nucleus (Haploid) in pronucleus structure, surrounded by membrane

• Nuclear membrane of sperm and egg dissolves

• Two pronuclei come together and fuses; Only sperm nucleus enters cell

• Cortical granules fuse with the egg membrane such that the vitelline layer becomes impenetrable

All condensed chromosomes participate in a joint mitosis to produce two diploid nuclei (46 chromosomes)

Use of Hormones in IVF Treatment

• Eggs are fertilised by sperm in lab (outside of body)

• Normally, once egg matures per month and release from ovary during ovulation, controlled by natural hormones

In IVF:

Normal secretion of hormones is suspended

• Prevent secretion of GnRH by hypothalamus → No FSH or LH by pituitary → No menstrual cycle

Artificial doses of hormones induce superovulation

• Artificial FSH → Stimulates development of multiple eggs in follicle cells until appropriate size

• Inject hCG → Triggers final cell maturation

  • hCG = Structurally similar to LH + Longer half life in body than LH → Release more eggs for collection

Control of Puberty by GnRH and Steroid Sex Hormones

Increased release of GnRH by hypothalamus in childhood → Trigger increased LH and FH release by pituitary gland

• Hypothalamus releases GnRH in pulses, increasing frequency during puberty

• LH and FSH trigger gonads to produce sex hormones (male: testes = testosterone // female: ovaries = estrogen + progesterone)

Increased sex hormone production → Changes in puberty

• Growth spurt; Increase in height and body mass

• Development of secondary sex characteristics

• Growth of underarm / pubic / facial / chest hair

• Acne

• Ability to reproduce

(For females)

• Development of breasts

• Menstrual cycle begins

• Bone structure of hips widens to prepare for possible childbirth

(For males)

• Deeper voice

• Testes and penis enlarges

• Erections begin

Spermatogenesis in Humans

• Occurs in seminiferous tubules

• Sperm = Made in testes; stored in epididymis → Carried in vas deferens / Seminal vesicle adds fluid → Leaves by urethra

• Starts at germinal epithelium (outside tubules) → Move to lumen (inside tubules) during maturation

Mitosis → Cell growth → Two devisions of meiosis → Differentiation

• Spermatogonia (2n) x1 —Mitosis

• Spermatocytes (2n) —Meiosis I & II

  • Primary Spermatocyte (2n) x1 → (Meiosis I) → Secondary spermatocyte (n) x2

  • Secondary spermatocyte (n) x2 → (Meiosis II) →

• Spermatids (n) x4 —Differentiation

• Spermatozoa (n)

  • Receive nourishment from Sertoli cells

  • When mature, detach → Epididymis for storage

Oogenesis in Humans

Mitosis → Cell growth → Two devisions of meiosis → Differentiation

• Oogonia + Follicle cells → (Mitosis) → Many oogonia and follicle cells in ovaries

• Oogonium (2n) → (Growth) → Primary oocytes (2n) surrounded by layer of primary follicle

• Primary oocytes → (Begins meiosis; Arrested during Prophase I)

• (Each menstrual cycle) Few primary follicle cells → (Meiosis I) → Secondary oocyte (n) → (Begins meiosis II; Arrested at Prophase) + First polar body (n) → Degeneration

• Primary follicle layer surrounding the secondary oocyte divides forming two rings of follicle cells, separated by fluid filled space, forming a Graafian follicle

• Fluid increases between layers → Ovum (Secondary oocyte + Inner follicle layer) is released from ovary to fallopian tubes at ovulation

• At fertilisation, secondary oocyte completes Meiosis II → Fuses with spermatozoon to form zygote nucleus

Difference Between Gametogenesis in Humans

Gametogenesis results in different numbers of sperm and eggs + different amounts of cytoplasm

• Numbers

  Sperm = Millions daily → Increase chances of fertilisation

  Eggs = One / month → Only one is fertilised in pregnancy

• Cytoplasm

  Sperm = Little → Small + mobile to reach egg

  Egg = Lots → Provide nutrients for early embryo development

Mechanisms to Prevent Polyspermy

Polysemy = ≥1 sperm fertilizes an egg → Abnormal embryo development

Acrosome reaction allows a sperm to penetrate the zone pellucida

• Enzymes in acrosome are released upon contact and breaks down egg’s outer layer

Cortical reaction prevents other sperm from passing through

• First sperm fuses with egg membrane → Cortical granules inside egg fuse with egg plasma membrane → Releases cell contents

• Zona pellucida hardens + Sperm receptors are altered/destroyed (No attachment sites) → Prevents additional sperm entry

• Maintain correct chromosome number → Prevent genetic abnormalities + Ensure normal embryo developments → Increases chances of successful pregnancy

Compare & Contrast Spermatogenesis and Oogenesis (HINT: 8 VS 3)

Differences

• Location

  Testes = Testis

  Ovary = Ovary

• Production

  Testes = Lifelong production

  Ovary = Fixed amount

• Gametes per parent cell

  Testes = 4

  Ovary = 1

• Beginning of process

  Testes = Puberty

  Ovary = Fetal development

• Timing of Gamete Formation

  Testes = Continuous

  Ovary = Once a month (during menstrual cycle)

• End of process

  Testes = Lifelong; Reduces

  Ovary = Stops at menopause

• Timing of gamete release

  Testes = Anytime

  Ovary = Monthly cycle

• Meiotic divisions

  Testes = Uninterrupted

  Ovary = Arrested

Similarities

• Processes in sexual reproduction

• Produce haploid gametes

• Both undergo meiosis and mitosis

Development of Blastocyte + Implementation in the Endometrium

Blastocyst = Ball of cells, gravitating in a certain direction, allowing a fluid-filled cavity to form in middle

• Zygote undergoes rapid cell division as it travels toward uterus → Forms a blastocyst after 4-6 days

• At Day 4: Enters uterus; Specialise into inner cell mass and outer layer (precursor to placenta when implanted)

• At Day 6-10: Blastocyst attaches to endometrium; Outer layer invades endometrium and connects with mother’s blood supply (Placenta)

Pregnancy Testing by Detection of hCG Secretion

Production of hCG in embryo / developing placenta

• Implantation → Production of hCG in embryo and cells of placenta

  • Levels rise rapidly in early pregnancy

  • hCG = Prevents more eggs developing in ovary

  • Appears in blood and urine of pregnant women

• At Week 3: Embryo development reaches organ formation

Use of monoclonal antibodies that bind to hCG

• Antibodies are attached to test strip

• When urine containing hCG flows over strip, hCG bind to antibodies → Color change → Visible result

• Made by injecting antigen into mice → Retrieving plasma cells + tumour cells → Hybridoma cells → Monoclonal antibody

Role of Placenta in Foetal Development Inside Uterus

Placental structure = Large SA of placental vili for exchange

• Placenta = Temporary organ developed during pregnancy; Connects developing foetus to uterine wall

• Allows for nutrient, gas, and waste exchange

• Maternal and fetal blood do not mix directly

Exchange processes in placenta → Allow foetus to be retained in uterus til later stage of development than in mammals that do not develop a placenta

• Types of exchanges

  • Nutrient transfer (Glucose, amino acid, fatty acids)

  • Gas exchange (Oxygen → foetus / CO₂ → mother)

  • Waste removal (Urea, etc.)

  • Some hormone transfer

• Allows for longer gestation period / Fetus can develop to more advanced stage before birth (More complex brain development)

  • Mammals without placenta (e.g. Marsupials) → Shorter gestation period → Foetus is born in less developed stage

Hormonal Control of Pregnancy and Childbirth

Continuity of pregnancy is maintained by progesterone secretion; From corpus luteum → placenta

• Initial secreted by corpus luteum in ovary after ovulation

• Later produced by placenta at Week 8-12

• Functions include:

  1. Prevent uterine contractions

  2. Support foetal development

  3. Prepare breast form milk production

Changes on childbirth are triggered by decrease in progesterone levels → Allows increase in oxytocin secretion due to positive feedback

• At childbirth, progesterone levels decrease / oestrogen levels increase → Increased sensitivity of uterus to oxytocin / Increased production of prostaglandins (in uterine wall) → Widens cervix

  • Oxytocin = Primary hormone for uterine contractions; Progesterone levels drop → Oxytocin more effective

• Positive feedback loop for increasing oxytocin → More frequent contractions

  • Baby head pushes against cervix → Trigger nerve impulse so hypothalamus → Oxytocin release from pituitary gland → Stimulates uterine contractions → Pushes baby’s head harder against cervix

• Contractions continue after birth to eject placenta

• Stretch receptor no longer stimulated → Oxytocin levels fall → Contractions stop

Hormone Replacement Therapy & Risk of Coronary Heart Disease

• HRT replaces declining hormones during menopause e.g. oestrogen and sometimes progesterone → Relieve menopausal symptoms (anxiety, mood change, skin condition, feelings of loss of self, sleeping difficulties, etc.)

• Early epidemiological studies: Women undergoing HRT → Reduced CHD incidence (Cause-effect relationship?)

  • HRT patients = Higher socioeconomic status → Lower HCD risk

• Later randomised controlled trials: HRT → Small increases in CHD risk

Sexual Reproduction in Flowering Plants (Production of Gametes, Pollination, & Fertilisation)

• Sexual even if plant species is hermaphroditic

• Male = Stamens → Pollen

• Female = Carpels → Ovules

Sexual Reproduction:

Production of gamete inside ovules and pollen grains

• Male gametes produced inside pollen grains by meiosis; Pollen forms in anthers of stamens

• Female gametes produced inside ovules by meiosis; Ovules forms within ovary of carpels

  
  

Pollination

• Transfer of pollen from anther to stigma

• Can self-pollinate or cross-pollinate but must be same species

Fertilisation

• Once pollen lands on stigma, pollen tube grows down the style towards the ovary → Carry male gametes to ovule

• 2 haploids gametes fuses to produce a diploid zygote at ovule

Features of an Insect-pollinated Flower

Function of flower parts:

1. Sepal

   Protecting the developing flower while it is inside the bud

2. Petal

   Colourful → Attract pollinators

3. Anther

   Part of stamen (male parts) that produces pollen (male gametes)

4. Pollen

   Contain male nuclei for fertilisation

5. Filament

   Stalk of stamen that hold up the anther

6. Stigma

   Sticky top of carpel (female parts); Where pollen lands

7. Style

   Part of carpel that supports the stigma

8. Ovary

   Base of carpel; Contains 1+ ovules

9. Ovule

   Chamber in ovary where female nuclei develops

Methods of Promoting Cross-Pollination

• Promotes genetic diversity and stronger offspring

Different maturation times for pollen & stigma

• Prevents self pollination

  E.g. Sunflower anthers mature and shed pollen inwards before stigma lobes elongate and become receptive

Separate male and female flowers / plants

• At different heights

  E.g. Corn has male flowers in tassels at top of plant + female flowers on the ears of corn at lower parts of stalk

Role of animals of wind in transferring pollen between plants

• Through wind, insects / birds / animals, water, mechanical means

• Adaptations to attract specific pollinators

  Scent and color to attract pollinator + Reward of nectar and pollen (Diet)

Self Incompatibility Mechanisms to Increase Genetic Variation

Self pollination leads to inbreeding → Decreased genetic diversity and vigour

• Decreased ability to adapt to environmental changes

  Plants being smaller in size, making fewer seeds, increased susceptibility to disease

Genetic mechanisms in many plant species ensure male and female gametes fuse during fertilisation are from different plants

• Self incompatibility = Plant recognises genetics of pollen from same plant and reject as “self” pollen

• Specific genes create proteins on pollen → Proteins are detected by stigma when pollen lands → If proteins match, pollen tube growth is inhibited → Pollen does not reach ovule

  E.g. Roses

Dispersal and Germination of Seeds

Distinguish seed dispersal from pollination

• Seed dispersal = Spreading of seeds away parent plant; Occurs after fertilisation and seeds are produced

  • Seed dispersal agents include…

    Wind (light seeds with wings)

    Animal (consumption and excretion or stick to fur / feathers)

    Water (buoyant seeds for aquatic plants)

    Explosive (forceful ejection of seeds from fruit)

• Pollination = Transfer of pollen from an anther to stigma

Growth and development of embryo + Mobilisation of food reserves

• Seed structure:

  Seed coat → Protective outer layer

  Embryo → Undeveloped plant

  Endosperm → Food storage tissue (Starch for energy, proteins fro enzyme production and growth, lipids as concentrated energy storage)

• Steps (IAESC):

  1. Imbibtion = Seed absorbs water and swells

  2. Activation = Enzyme activates → Breaks down food reserves (Cell respiration + protein synthesis increases)

  3. Emergence = Radicle (embryonic root) emerges first; Grows down into soil due to gravity

  4. Shoot Development = Hypocotyl (below cotyledons) lifts seed above ground → Early shoot straightens; Orients growing plant perpendicular to ground + Epicotyl pushes the plumule (embryonic shoot; top of epicotyl) above earth and grows upwards, forms first true leaves

  5. Cotyledon Function = (As endosperm becomes depleted) Becomes leaves to photosynthesise and transfer nutrients to growing parts

  6. Root structure development

DNA Replication

Production of exact copies of DNA with identical base sequences

• Required for reproduction, growth, and tissue replacement in multicellular organisms

Role of Complementary Base Pairing

Allows genetic information to be replicated and expressed accurately and quickly

• When two DNA strands split open, they act as templates for synthesis of new strands at high accuracy

Complementarity is based on hydrogen bonding

• A-T (2 H-bonds)

• G-C (3 H-bonds)

Semi-conservative Model of Replication

Semi-conservative replication = Each strain in DNA double helix acts as template for synthesis of new complementary strand

• Each daughter helix = Old strand from parental DNA + New strand

Complementary base pairing and retention of one original strand of DNA allows high degree of accuracy in copying base sequences

• Using the unzipped single DNA strand, complementary nucleotides could be used to synthesise the new strand → Ensures two identical copies of DNA are produced from the original strand

Role of Helicase and DNA Polymerase in DNA Replication

Helicase = Hexomer; Only binds to one DNA strand on either side to split hydrogen bonds as they go in opposite directions (E.g. Pulling open__ )

• Unwinds and breaks hydrogen bonds between DNA strands

DNA Polymerase

• Adds free nucleotides to the leading and lagging strands in a 5’ to 3’ direction

Directionality of DNA Polymerases

Difference between 5’ and 3’ terminals of strands of nucleotides

• 5’ end = Phosphate group attached to 5’ carbon of deoxyribose

• 3’ end = Hydroxyl group on 3’ carbon of deoxyribose

  
  

DNA Polymerases can only add the 5’ end of a DNA nucleotide to the 3’ end of existing nucleotide

• DNA chain must be built in 5’ to 3’ direction → Creates alternating sugar-phosphate backbone of DNA

Replication on the Leading Strand VS Lagging Strand

Due to the specific direction of Polymerase III from 5’ to 3’…

• Synthesis of leading strand = Relatively fast; Continuous

• Lagging strand = Discontinuous; Synthesised in Okazaki fragments

  • Requires DNA ligase to bind the fragments together

Replication has to be initiated with RNA primer only once on leading strand (because it is continuously formed) but repeatedly on lagging strand

Functions of Molecules in DNA Replication (In Prokaryotic System)

• DNA Primase

  Synthesises and attaches RNA primers to start of sequence; Signals DNA polymerase I where to begin synthesis

• DNA Polymerase III

  Removes RNA primers and replaces the sequence with DNA nucleotides

• DNA Polymerase I

  Synthesise new strand of DNA based on complementary bases in the direction of 5’ to 3’

• DNA Ligase

  Joins Okazaki fragments together to form a single chain of DNA

DNA Proofreading

DNA Polymerase III removes any nucleotide from the 3’ terminal with a a mismatched base, and replaces it with a correctly matched nucleotide

1. An incorrect base may be present in growing DNA chain

2. DNA Polymerase III excises the incorrect base and replace with correct base before proceeding with replication (Stalling)

DNA Replication Process (FULL)

1. Topoisomerase uncoils DNA

2. Helicase unwinds and splits H bonds between the DNA strands, creating a replication fork

3. Primase forms RNA primer and it attaches itself to site of synthesis

4. DNA polymerase III adds nucleotides to new Okazaki fragment only at 3’ end and continues until it meets the primer of the pervious fragment

   OR

   DNA polymerase III adds nucleotides to synthesise leading strand continuously

5. DNA polymerase I hydrolyses the primer and replaces it with DNA

6. DNA ligase catalyses the formation of phosphodiester linkage, joining the Okazaki fragments together

Polymerase Chain Reaction

PCR = Amplifies amount of DNA sample; Each cycle doubles the # of copies of target gene → Production of many copies quickly; In thermocycler…

• Source DNA

• DNA primers = Short DNA strands complementary to regions with sequence of interest on either strand; Used to isolate a gene

• Free nucleotides (dNTPs)

• Taq polymerase = From T. aquaticus; Bacteria found in hot springs → Enzymes have high optimum temperatures

  • Good for PCR since utilises temp. > normal body temp. + Much more efficient. than human DNA polymerase

• Reaction buffer

Temperature cycles are…

1. Denaturing (94-95ºC) = High temp. → DNA strands to separate

2. Annealing (50-56ºC) = Lower temp. → Primers bind to DNA

3. Extending (72ºC) = Optimal temp. for Taq → Effective replication

Gel Electrophoresis

Since DNA is inherently -ve charged, will move through the gel from negative electrode to positive electrode.

Gel electrophoresis = Separation of DNA fragments based on size

• Small DNA fragments → Travel further

• Large DNA fragments → Travel less

Applications of PCR and Gel Electrophoresis

• DNA profiling for paternity

  Compare common sequences

• Forensic investigations

  Small amount of DNA from crime scenes can be amplified and searched against murder suspects’ DNA

• Human genome studies for evolutionary biology

• Historical

  Clone DNA from extinct species

• Medical

  Detect mutations by comparing DNA; Predict risk of disease and cancers

NOTE: Increasing number of markers used reduces probability of false match in DNA profiling

Generation of New Cells by Cell Division

In all living organisms, a parent cell (AKA mother cell) divides to produce two daughter cells

Cytokinesis

Splitting of cytoplasm in parent cell between daughter cells

• Occurs once cell reaches certain size

  
  

In an animal cell, a ring of contractile actin and myosin proteins pinches a cell membrane together to split the cytoplasm

• Actin and myosin filaments contract to form cleavage furrow

• Furrow deepens (Inward pinching) until pinched into two separate cells

  • Ensures equal share of cytoplasm in daughter cell

In a plant cell, vesicles assemble sections of membrane and cell wall to achieve splitting

• Vesicles which make up new cell membrane and walls assemble to form a cell plate

• Vesicles expand outwards and fuse with cell membrane → Divides cell into two

Equal and Unequal Cytokinesis

Division of cytoplasm is usually, but not always, equal; Both daughter cells must receive at least one mitochondrion and any other organelle that can only be made by dividing a pre-existing structure

• Identical daughter cells → Ensures new cell contains necessary organelles to survive and function (i.e. 1 mitochondrion / chloroplast in each cell)

Examples of unequal cytokinesis:

• Oogenesis in humans

  • 4 haploid cells are produced (Each containing 23 chromosomes)

  • One cell retains most cytoplasm → Ovum (Can be fertilised to make zygote)

  • Other cells form smaller polar bodies which degenerate

• Budding in yeast

  • Small bud receives part of cytoplasm and a nucleus → Grows and eventually detaches to become daughter cell

Role of Mitosis and Meiosis in Eukaryotes

Nuclear division is needed before cell division to avoid production of anucleate cells

• Nuclear division must occur before cells division to ensure each new cell contains a nucleus

  • Anucleate cells cannot survive or function properly

Mitosis maintains the chromosome number and genome of cells

• Mitosis results in two identical daughter cells

  • Essential for growth, repair in somatic cells + Asexual reproduction

Meiosis halve the chromosome number and generate genetic diversity

• Meiosis results in FOUR daughter cells with only HALF the DNA

  • Used to make gametes; Each daughter cell receives different combination of genes

  • For sexual reproduction; Combination of genetic material from two parents at fertilisation → Genetic diversity (Beneficial to species survival and evolution)

DNA Replication as a Prerequisite for Both Mitosis and Meiosis

After replication, each chromosome consists of two chromatids held together until anaphase

• DNA replication must occur before cell division → Ensure each new cell contains complete set of DNA

• Chromatids = Attached a centromere; Two sisters → Chromosome

Condensation and Movement of Chromosomes

Shared features of mitosis and meiosis

Role of histones in condensation of DNA by supercoiling

• DNA is usually spread out in long chains inside nucleus; Must be condensed by supercoiling before replication to prevent breaking

  • Condensed bt wrapping around histones → Nucleosomes

  • Nucleosomes further coil into → Chromatin

  • Chromatin further condense → Fully supercoiled Chromosome

Use of microtubules and microtubule motors to move chromosomes

• Centrosome = Organelle allowing microtubule spindle fibres to move chromosome

• Microtubules = Constructed / Disabled as needed; Act as highways guiding chromosomes to poles of cell

• Motor proteins = Push / pull objects around cell

  • Dynein and kinesis can transport chromosomes by walking along microtubules

Phases of Mitosis

INTERPHASE (NOT A PHASE)

• Cells spend most time here; Perform general functions (Protein synthesis, respiration, growing larger, etc.)

• Enters mitosis when ready to divide

PROPHASE

• Chromatin condense by supercoiling

• Nuclear membrane breaks down; Nucleolus disappears

• Mitotic spindle forms (New microtubules built from centrosome)

• Kinetochores attach to spindle (Region in centromere)

• Microtubules lengthen → Centrosomes move towards opposite poles

METAPHASE

• Metaphase plate is formed; Chromosomes move to equator of cell

• Centromeres align on plate

• Spindle acts to move chromosomes

• Centrosomes at opposite poles

ANAPHASE (Shortest phase)

• Chromatids move towards opposite poles of cell due to motor proteins pushing microtubule in opposing directions → Each pole has complete, identical set of chromosomes

• Centromeres move towards pole first (Point of contact with spindles)

TELOPHASE

• Nuclear membrane begins to reform

• Nucleoli reappear; Chromosomes elongate

• Spindles disappears

• Cell is elongates → Prepares for cytokinesis

Meiosis as a Reduction Division

A diploid cell undergoes two rounds of cell division to produce four haploid gametes

Diploid = 2n; 46 chromosomes

Haploid = n; 23 chromosomes

Two divisions of meiosis produces four haploid nuclei from one diploid nucleus; Need for meiosis in sexual life cycle

• At fertilisation, a full number of chromosome is achieved

Two rounds of segregation in meiosis

• MEIOSIS I (PMAT) → Separates chromosome pairs

  • Each chromosome makes an identical copy of itself which stays together

  • Chromosomes line up in pairs; As cell divides in two, one chromosome from each pair goes into new cell

• MEIOSIS II (PMAT) → Separates chromatids

  • New cells divide again; Each chromosome is split into two → Haploid cells

Compare and Contrast Meiosis and Mitosis

Down syndrome and Non-disjunction

Error in mitosis = Extra or fewer chromosomes in gamete

• Gamete with extra chromosome fertilising a normal gamete = Zygote with three copies of chromosomes (trisomy)

E.g. Down syndrome

• Extra chromosome 21 (Total 47 chromosomes)

• Causes developmental delays, physical growth limitations, distinctive facial features

• Due to non-disjunction during formation of mother’s egg cells

  • Occurs when homologous chromosomes or sister chromatids fail to separate properly

  • Can occur during Meiosis I or II

Meiosis as a Source of Variation

Meiosis generates genetic diversity by random orientation of bivalents and by crossing over

• Produce 4 cells with different chromosomes

  • Due to crossing over when genetic material swaps between non-sister chromatids → Creates recombinant chromatids; Increase variety among offspring

  • Random orientation during crossing over occurs because chromosomes can line up in different ways in Metaphase I

  • Chiasma = Point / Locus of crossing over

Cell Proliferation for Growth, Cell Replacement, and Tissue Repair

Cell proliferation = Cell division and multiplication; For growth, maintenance, and tissue repair

For example…

• Growth within plant meristems

  • Meristems = Area where plant cells divide rapidly; Undifferentiated cells will differentiate into specialised cells

  • Apical meristem tissue = Root tips and branch tips; Undergoes mitosis to lengthen plant

  • Lateral meristematic tissue = Stems; Allow plant to grow wider (Responsible for continuous growth)

• Early stage animal embryos

  • After fertilisation, embryonic stem cells in zygote divide by mitosis to produce cells which self-organise into layers → Embryo

• Skin

  • Occurs during routine cell replacement and wound healing

  • Skin cells die off and are replaced every 28 days by mitosis (New cells are made in bottom layer, dead cells flake off at top layer)

  • When broken or wounded, cells in surrounding area divides, fills gap, and repair tissue → Closes wound

Phases of Cell Cycle

Cell cycle = Allows for cell proliferation; Include G1, S, and G2 as stages of interphase, followed by mitosis, and then cytokinesis

1. Interphase (Growing)

   • G0 (Stalled growth)

   • G1 (Growth)

     • Cell grows in size; Proteins and organelles are synthesised

     • Intense cellular activity as cell prepares for DNA replication

   • S (Synthesis)

     • DNA replication

   • G2 (Growth)

     • Cell completes growth, prepares for mitosis

     • Checks DNA for errors and repairs

     • Produce organelles and proteins for cell division

2. Dividing (Mitosis)

   Nucleus and cell divides

Cell Growth During Interphase

Interphase = Metabolically active period; Longest phase

• Growth involves biosynthesis of cell components i.e. DNA and proteins

  • Ribosomes are active as they translate mRNA into proteins

# of mitochondria and chloroplast are increased by growth and division of organelles

• Growth and division via binary fission

Control of Cell Cycle Using Cyclins

Cyclins = Proteins that regulate progression of cells throughout cell cycle

• Cyclin and cyclin-dependent-kinase (CDK) forms a complex which binds to a target protein and modifies it by phosphorylation

• Phosphorylated protein triggers event in cell cycle

• Cyclin is then destroyed

  
  

Concentration of different cyclins increasing and decreasing during cell cycle

• Cyclin D – G1 Phase

• Cyclin E → Starts S phase

• Cyclin A → Starts G2 phase

• Cyclin B → Starts Mitosis

  
  

Threshold level of a specific cyclin is necessary to pass each checkpoint

• Overproduction or underproduction of cyclins lead to uncontrolled cell division → Cancer

Consequences of Mutations in Genes that Control Cell Cycle

Mutations in proto-oncogenes that convert them to oncogenes and mutations in tumour suppressor genes result in uncontrolled cell division

• Proto-oncogenes = Help cells grow

  • When mutated, becomes oncogenes; Allow cells to grow uncontrollably into tumours

• Tumour suppressor genes = Prevent uncontrolled cell division; Can slow down cell division, repair DNA mistakes, or trigger apoptosis

  • When mutated or silenced, cells grow unregulated; Damaged cells are allowed to continue dividing

Can be caused by environmental exposure or genetic inheritance

Can be inherited (germ-line) or acquired during lifetime (somatic)

Difference Between Tumours for Metastasis

Tumour = Abnormal growth of tissue that forms a mass

• Benign = Non-cancerous

  Low rate of cell division; Grows slowly

  No capacity for metastasis; Do not spread to other parts of body

• Malignant = Cancerous

  Grows rapidly; Higher mitotic index; Greater rate of cell division

  Capable for metastasis and invade neighbouring tissue

• Whether benign or malignant can be determined by biopsy, medical imaging, or blood tests

Primary tumour = Occurs at original site

Secondary tumour = A tumour that has spread elsewhere via metastasis

Mitotic Index

(Number of cells undergoing mitosis) / (Total number of cells)

High mitotic index = Rapid growth

• If mitotic index >0.75, cancerous

Polyploidy (Diploids & Haploids)

Production of haploid gametes in parents and their fusion to form a diploid zygote as a means of inheritance

• Gametogenesis = Production of haploid gametes via meiosis

  Diploid cell undergoes two rounds of cell division to produce four haploid gametes. Number of chromosome is halved and creates four genetically unique haploid cells.

• Fertilization = Fusion of gametes

  Fusion of haploid egg cell and haploid sperm forms a diploid zygote which undergoes mitotic cell division and develop into multicellular organism.

• Diploid cell = Contains two copies of each autosomal gene

  
  

Pattern of inheritance is common to all eukaryotes with a sexual life cycle

Pollination and Fertilisation in Plants

Methods for conducting genetic crosses (fertilisation) in flowering plants

• Pollination = Transfer of pollen to the stigma; Required for fertilisation to occur and produce zygote / Carry out genetic cross

  • Pollen contains male gametes

  • Female gametes are located in the ovary

• Plants such as peas produce both male and female gametes on the same plant, allowing for self-pollination and therefore self-fertilisation

  • Results in lack of genetic diversity

• Cross-pollination is when pollen is transferred between different individual plants

Genetic crosses are widely used to breed new varieties of crop or ornamental plants

• Plant breeders help fertilisation by using brushes to transfer pollen grains to stigma of other plants → Increase chance of desirable traits for general produce or ornamental plants

Genetic Cross Terminology

P generation = Parental generation

F1 generation = First filial generation

F2 generation = Second filial generation

Genes and Alleles

Genotype = Combination of alleles inherited by an organism

• Homozygous = Same alleles

• Heterozygous = Different alleles

• Hemizygous = Only one allele

  • Occurs only in males (Different sex chromosomes)

Genes VS Alleles

• Genes = Inherited features encoded by specific DNA sequences; Located at specific positions on chromosome (gene locus)

  • Every gene has a specific locus on a chromosome

  • Gene is found in same position on both homologous chromosomes (one paternal and one maternal)

• Alleles = Variations of a specific feature; Alternate forms of genes

Dominant and Recessive Alleles

Effect of dominant and recessive alleles on phenotype

• Recessive allele is only expressed when in homozygous state

• Both a homozygous-dominant genotype and a heterozygous genotype for a particular trait will produce the same phenotype

  • Dominant allele can fully express trait regardless of the state of the other allele

Phenotype + Example of Traits

Phenotype = Observable traits of an organism resulting from genotype and environmental factors

Examples of traits in humans due to…

• Genotype ONLY

  • Eye colour

  • Blood type

• Environment ONLY

  • Accent or speech patterns

• Interaction between genotype AND environment

  • Height (Nutrition and health)

  • Susceptibility to certain diseases (Lifestyle choices e.g. diet and exercise + Environment factors e.g. stress and pollution)

Phenylketonuria (PKU)

Phenylketonuria = Recessive genetic condition caused by mutation in autosomal gene that codes for enzyme needed to convert phenylalanine to tyrosine

• Deficiency / lack of enzyme → Accumulation of phenylalanine in body → Developmental issues such as intellectual disability

Example of a human disease due to a recessive allele

Single-Nucleotide Polymorphisms (SNPs) + Example

Single-nucleotide polymorphisms and multiple alleles in gene pools

• SNP = Single-base differences in DNA sequences that occur at specific loci

Any number of alleles of a gene can exist in the gene pool but an individual only inherits two

• More than two variants of the gene can exist within a gene pool → Contributes to genotypic diversity → Leads to broad range of phenotypic expression for the trait associated with that gene

  • Provide genetic variation that enables organisms to adapts to changing environments

For example…PTC Gene and Bitter Taste Perception

• tt = Non-taster / Tt = Moderate Taster / TT = Supertaster

Co-dominance + ABO Blood group as an Example

Co-dominance = Multiple alleles presented at once; E.g. ABO blood groups

• I^A and I^B are both co-dominant alleles

  • Can both be expressed individuals; E.g. AB blood type

• i is a recessive allele

  • To be expressed, must be homozygous recessive; E.g. O blood type (No antigens are expressed)

• ABO blood group is inherited in an autosomal co-dominant manner

Co-Dominance VS Incomplete Dominance

Difference between the patterns of inheritance of incomplete dominance and co-dominance at phenotypic level

• In co-dominance…

  Heterozygotes have dual phenotype

  Heterozygous phenotype is a combination of the two homozygous phenotype, with both fully expressed simultaneously

  • E.g. AB blood type (I^AI^B)

• In incomplete dominance…

  Phenotype of heterozygous individual is intermediate between (in the middle) phenotypes of two homozygous individuals; Dominant allele does not completely mask the effects of a recessive allele, resulting in a new phenotype

  • E.g. Four o’clock flower or marvel of Peru (Mirabilis Jalapa)

    R¹R¹ = Red / R²R² = White / R¹R² = Pink (Intermediate between red and white)

Phenotypic Plasticity

Phenotypic plasticity = Capacity to develop traits suited to environment experience by an organism by varying patterns of gene expression

• NOT due to genotype (No change in genotype); Occurs by regulation of gene expression

  • Different environmental cues can trigger the activation or suppression of specific genes

• Changes in traits may be reversible during a lifetime

  • Organisms can revert to their original phenotype when environmental conditions change back to previous state

Sex Determination in Humans

Sex determination in humans and inheritance of genes on sex chromosomes; Far more genes are carried by the X chromosomes than the Y chromosome

Sex-linkage = Genes found on sex chromosome

• Females = XX // Male = XY

• Sex-linked condition are usually X-linked (Few genes exist on Y chromosomes; ~2000 VS 78 genes)

  • Exhibits different patterns of inheritance; Females cannot express Y-linked traits / Males cannot mask X-linked recessive traits → More commonly expressed in males (E.g. haemophilia)

Sex chromosome in sperm determines whether a zygote develops certain male-typical or female-typical physical characteristics

• Y chromosome contains SRY gene which produces TDF (testis-determining factor) → Induces medulla of embryonic gonads to develop into testes → Produced testosterone; Initiate development of male sexual characteristics

• Absence of SRY gene and TDF → Cortex of embryonic gonads develop into ovaries → Embryo develops female characteristics

Example of a Sex-Linked Genetic Disorder

Haemophilia = Caused by recessive allele on X chromosome; Leads to uncontrolled bleeding

• Normal clotting factor allele = X^H // Faulty clotting factor allele = X^h

• Since females can be…

  X^HX^H or X^HX^h or X^hX^h

• And males can be…

  X^HY or X^hY

• Carrier females (X^HX^h) have 50% chance of passing X^h to offspring

  • X^HX^h + X^hY → 2*(X^hY) + 2*(X^HX^h)

• Affected males (X^hY) can inly pass X^h to daughter

  • X^hY + X^HX^H → 2*(X^HY) + 2*(X^HX^h)

Pedigree Charts

Pedigree Charts = Used to deduce patterns of inheritance of genetic disorders; Maps genetic history of family over several generations

• Autosomal recessive: Traits can skip generations; Affect males and females equally

  • Neither parent is affected → Offspring is affected

• Autosomal dominant: Traits do not normally skip generations; Affect males and females equally; Unaffected members are homozygous recessive

  • Both parents are affected → Offspring are unaffected

• X-linked dominant

  • 100% incidence of affected daughters + 0% incidence of affected sons from an affected father

• X-linked recessive

  • 0% incidence of affected daughters + 100% incidence of affected sons from an affected mother

Genetic basis for the prohibition of marriage between close relatives in many societies

• Consanguine marriage = Marriage between individuals who are closely related → Increased risk of recessive genetic disorders in offspring of close relatives

Polygenic Inheritance

Continuous variation due to polygenic inheritance and/or environmental factors

• Polygenic trait = Influenced by combined effects of multiple genes

For example…skin colour in humans

• Each gene contributes small effects to the overall skin colour → Continuous range of skin tones

• Environ. Factor: Exposure to UV radiation → Skin pigmentation (Amount of melanin produced in response to sun exposure)

Discussing Skin Tone and ABO Blood Group (Continuous VS Discrete Variables, Measures of Central Tendency)

Continuous variables = Infinite number of possible values within a range

• For polygenic traits, bell curve is produced (y = frequency)

  • Ends of curve = Rare cases = Extreme phenotypes

  • Middle of curve = Common cases = Average phenotype

Discrete variables = Limited set of possible values

Measures of Central Tendency

Mean, Median, Mode

• Relationship between these measures → Reveal distribution of traits within gene pool

  • Negatively skewed (Right)

  • Normal / No skew (Symmetrical)

  • Positively skewed (Left)

For example…skin tone

• Continuous variable

• To find measures of central tendency, must convert into discrete, numerical data first

  • Use Fitzpatrick scale

For example…ABO blood group

• Discrete variable

• Mode → Most common blood group // Median or Mean → Distribution of blood group frequency (?)

Mendelian Ratio and Test Crosses

Punnet grids = For predicting genotypic and phenotypic ratios in dihybrid crosses involving pairs of unlinked autosomal genes

MENDEL’S SECOND LAW (Law of Independent Assortment) = Alleles of one gene sort into the gametes independently of the alleles of another gene

• Two dual heterozygous parents → 9:3:3:1 phenotypic ratio

• Heterozygous + Homozygous recessive parents → 1:1:1:1 ratio

  (Law only applies if genes are on different chromosomes or are far apart enough on one chromosome so that recombination rate reaches 50%)

Exploring Gene Loci and Polypeptide Products

Segregation of Homologous Chromosomes in Meiosis

Segregation and independent assortment of unlinked genes in meiosis

• Homologous chromosomes align and randomly separate into different daughter cells during meiosis

  • Two genes on separate, unlinked chromosomes will assort and distribute independently into gametes during meiosis → All possible combinations of two traits in offspring

Link between the movements of chromosomes in meiosis and the outcome of dihybrid crosses involving pairs of unlinked genes

• Due to its ability to segregate and assort independently, the outcomes can be calculated using dihybrid crosses

Autosomal Gene Linkage + Notation

Linked = Genes located on same chromosome

Alleles of linked genes can fail to assort independently

• Linked genes do not assort independently (Must travel together during meiosis) → More likely to be inherited together

  • Monohybrid pattern of gene inheritance

For example…

AB on one chr, ab on another chr → Only 2 possible gamete combinations

• Gamete combinations are essentially the same across generations

Homologous Recombination (Linked Genes w/ Differing Distances; Gametes, Genotype of Offspring, Phenotype of Offspring)

Recombination changes linked gene inheritance patterns

• During recombination of linked genes, crossing over may occur, in which generating recombinant chromosomes → New genotypes

  
  

  Where linked genes (AB || AB, ab || ab) are close to each other,

  (Parental →) 48% AB, 48% ab, 2% Ab, 2% aB (← Recombinant)

  • Linked genes close together on the same chromosome has a lower probability of being separated by crossing over → Tend to be inherited together during meiosis

  • Gametes = AB, ab, Ab, aB

  • Offspring = AB || AB, ab || ab, AB || ab

  • Phenotype =

    1. Dom. trait 1 + Dom. trait 2

    2. Rec. trait 1 + Rec. trait 2

• Where linked genes (AB || AB, ab || ab) are far apart on same chr,

  25% AB, 25% ab, 25% Ab, 25% aB (← Recombinant)

  • Linked genes further apart on the same chromosome has a higher probability of being separated by recombination → All genotypes are equally likely to be created from meiosis

  • Same inheritance patterned as unlinked gene

  • Gametes = AB, ab, Ab, aB (1:1:1:1)

  • Offspring = AB || AB, ab || ab, AB || ab, Ab|| Ab, Ab || ab, aB || aB, aB || ab (New recombinant types: State of Trait₁ ≠ State of Trait₂)

  • Phenotype =

    1. Dom. trait 1 + Dom. trait 2

    2. Rec. trait 1 + Rec. trait 2

    3. Dom. trait 1 + Rec. trait 2

    4. Rec. trait 1 + Dom. trait 2

Different Types of Point Mutations

Gene Mutations = Structural changes to genes at the molecular level

• Substitution

• Insertion

• Deletion

• Inversion

Single-Nucleotide Polymorphisms (SNPs)

SNPs = Result of base substitution mutations; May (OR MAY NOT) cause different amino acid to be placed in polypeptide chain due to degeneracy of genetic code

Frameshift Mutations

Major insertions / deletions → Frameshift change; Affects every codon beyond the point of mutation → Likely cause polypeptides to cease to function

Cause of Gene Mutation

Mutagens ~ Errors in DNA replication or repair

• Mutagenic forms of radiation, e.g. UV Radiation

  High energy radiation knocks out base pair / break bonds → Modify DNA

• Chemical mutagens, e.g. Cigarette smoke, Benzoyl peroxide, BBQ

  May react / attack / replace DNA nucleotides

• Infectious agents, e.g. H. pylori, HPV

  Some bacteria produce reactive oxidative species → Induce inflammation + Damage DNA + Reduce efficiency of DNA repair in cell → Increase vulnerability to mutations

Randomness in Mutation

Mutation can occur anywhere in the base sequences of a genome, although some bases have a higher probability of mutating than others

• Uncoiled DNA = More exposure → Increased susceptibility to mutations

• Satellite DNA = Found at centromere and telomeres; Non-coding, structural function; High mutation rate (1 mutation per 1000 bp)

• CpG sites = When methylation occurs, C can mutate into T

  • CpG islands = Repeated CpG sites

• Mitochondrial DNA

• RNA sequences

  
  

No natural mechanism is known for making a deliberate change to a particular base with the purpose of changing a trait

Consequences of Mutation in Germ Cells and Somatic Cells

Inheritance of mutated genes in germ cells and cancer in somatic cells

• Somatic mutations = Occur in a single body cell; Cannot be inherited (Only tissues with mutation will be affected)

• Germline mutations = Occurs in gametes; Can be passed onto offspring (ALL cells will be affected)

Mutation as a Source of Genetic Variation

Gene Mutation = Original source of all genetic variation

• For an individual organism, most mutations are either harmful or neutral

• In a species, they are essential for evolution by natural selection in long-term

For example…

Apolipoprotein A1 = Help transport cholesterol through bloodstream + Remove cholesterol from artery wall

• Community in Italy has apolipoprotein A1-Milano mutant

  • More effective than normal type

  • Additional function as antioxidant → Prevent inflammation which occur in arteriosclerosis

Gene Knockout

Technique for investigating the function of a gene by changing it to make it inoperative

• In a KO mouse model, the gene of interest is removed → Due to evolutionary similarity, allows investigation of function of gene in humans

• Models often reproduce quickly and have few genes

CRISPR-Cas9

Use of CRISPR sequences and the enzyme Cas9 in gene editing

• Cas9 enzyme is a protein that cleaves DNA at specific loci, guided by gRNA which functions by matching desired target sequence. Together, this allows the removal and replacement of genetic sequences.

An example of the successful use:

• OB-mouse → Obesity (Leptin ~ Feelings of satiation)

• Sickle cell anemia

Conservation of Genes

Hypotheses to account for conserved or highly conserved sequences in genes

• Conserved sequences = Identical or similar across a species or a group of species

  • Highly conserved sequences = Identical or similar over long periods of evolution

• Hypotheses include…

  • Functional requirements for the protein a gene produces
    Essential gene where the lack of them or its mutation results in sever consequences which could prevent its inheritance

  • Slower rates of mutation

Transcription

Transcription = Synthesis of RNA using a DNA template

• Only the antisense strand is transcribed and the mRNA strand synthesised is the sense strand

• RNA is always shorter than DNA strand copied from as it is the complementary form of only one gene

Roles of RNA polymerase:

• Binds to promoter region on template strand

• Unzips DNA by breaking H bonds between two DNA strands

• Transcribes mRNA molecule by complementary base pairing

• Links RNA nucleotides

Hydrogen Bonding and Complementary Base Pairing in Transcription

Role of hydrogen bonding and complementary base pairing in transcription

• The formation of the new RNA strand is allowed by complementary base pairing by arranging RNA nucleotides in the complementary sequence to the DNA template strand

  • Adenine → Uracil

• H-bonds stabilise interaction between RNA polymerase and DNA, forming bonds between newly synthesised RNA strand

Stability of DNA Templates

Single DNA strands can be used as a template for transcribing a base sequence without the DNA base sequence changing. In somatic cells that do not divide, such sequences must be conserved throughout the life of a cell

• In some somatic cells such as nerve cell which do not undergo mitosis but require proteins, sequences are conserved throughout its lifetime and do not mutate

Transcription ~ Gene Expression

Not all genes in cell are expressed at a given time.

As the first stage of gene expression, transcription is a key stage at which expression of a gene can be controlled (switched on and off). 

Translation (tRNA and mRNA)

Translation = Synthesis of polypeptides from mRNA

• Base sequence of mRNA is translated into amino acid sequence of polypeptide in units of 3 base pairs known as codons.

1. mRNA binds to the small unit of the ribosome

2. Two tRNAs bind simultaneously to the large subunit

   1. The first tRNA carrying a specific amino acid attaches to the “A” site at the large subunit of the ribosome; tRNA forms hydrogen bonds with mRNA via complementary base pairing

   2. A second tRNA enters the large subunit of the ribosome and repeats the process. These tRNA simultaneously bind to the large subunit of the ribosome

3. The enzyme catalyses the condensation reaction, resulting in a peptide bond between the two amino acids

4. The first tRNA detaches from its amino acid and the mRNA, leaving the subunit

5. The ribosome moves along the mRNA molecule in a 5’ to 3’ direction so that the second tRNA will now have been in the position of the first tRNA

6. A new tRNA molecule enters and pairs with the next codon sequence, repeating the process

RNA and Ribosomes

Role of mRNA, ribosomes, and tRNA in translation

• mRNA: Carries the sequence of the sense strand from nucleus to ribosomes in cytoplasm

• tRNA: Bring amino acids in the cytoplasm to ribosomes

• rRNA: Combines with ribosomal proteins to construct cytoplasmic ribosomes

Complementary Base Pairing Between tRNA and mRNA

The antiocodon on a tRNA molecule is complementary t the codon of the mRNA. This complementary base pairing allows them to match up and produce proteins encoded by the mRNA. 

Codon = A triplet in mRNA; Three bases in DNA that determines an amino acid

Anticodon = Three bases in middle loop of tRNA determining which amino acid is attached to it

Features of the Genetic Code

Degeneracy = Each amino acid can be identified by multiple codons

Universality = All organisms share the same genetic code with few minor exceptions

Reasons for a triplet code:

• Provides many possible combinations (64) to encode all amino acids and the capability to cope with any expansion in number of amino acids during evolution

• Degenerate → Allow for possibility of mutation (Even in the presence of a few substitution mutations, the same amino acid may be obtained)

• Universal → Allows us to transfer genes between species via genetic engineering techniques → Treatment of diseases

Production of Growing Polypeptide Chain (Ribosome)

Stepwise movement of the ribosome along mRNA and linkage of amino acids by peptide bonding to the growing polypeptide chain

1. mRNA binds to the small subunit of the ribosome, and the two tRNAs to the large subunit. 

2. The ribosome holds the tRNA and mRNA close together, allowing amino acids to be connected by peptide bonds

3. tRNA move sequentially through the binding sites of the ribosome during base paring between tRNA anticodons and mRNA codons along with the mRNA, one codon at a time (Remember, they are temporarily bonded together via hydrogen bonds)

Point Mutations

Point mutation = Mutations of one base in DNA code that change protein structure

Example of point mutation affecting protein structure is: Sickle-cell anaemia

• Point mutation in the HBB gene sequence causes valine to be produced instead of glutamine → Abnormal haemoglobin forms strands that leads to sickle-shaped RBC → Decreased oxygen-carrying capacity

  • HBB gene = hemoglobin subunit beta 

  • Normal HBB gene → (CCTGAGGAG) → Pro-Glu-Glu

  • Mutated HBB^S gene → (CCTGTGGAG) → Pro-Val-Glu

Directionality of Transcription and Translation

5’ to 3’ Transcription (RNA makes sense strand; RNA synthesises a complementary and antiparallel strand to DNA template strand)

• Read in 3’ to 5’, synthesise in 5’ to 3’ direction

5’ to 3’ Translation

Initiation of Transcription at Promoter

Promoters = Regions of DNA that RNA polymerase combines with to initiate transcription; Not transcribed

• Transcription factors bind to the promoter and attracts RNA polymerase to attach to the promoter region and initiate transcription

  • Other transcription factors may also bind to other nearby areas

• RNA polymerase functions like a helicase; The front unwinds and opens the DNA double helix / The back rewinds the DNA strands

  • RNA polymerase adds RNA nucleotides to enlarging mRNA

Non-Coding Sequences

Non-coding Sequences = DNA regions that do not code for polypeptides;

• Regulators of gene expressions
  Promoters, enhancers, silencers, insulators, etc. → Control transcription

• Introns = Present in gene but removed before during RNA processing before it leaves nucleus, forming mature mRNA

  • Spliceosome excises introns into lariat loop shape, leaving a mature mRNA strand which goes to be translated into proteins

• Telomeres = Ends of chromosomes made of repetitive non-coding sequences; Protects DNA

• Genes for rRNAs and tRNAs in eukaryotes → Important for producing proteins

  • rRNA → Ribosomes // tRNA carries amino acids for translation

Post-Transcriptional Modifications in Eukaryotic Cells (Name 2 examples)

• Removal of introns and splicing together of exons by spliceosome to form mature mRNA, leaving excised introns in lariat shape

• Addition of 5’ caps and 3’ polyA tail to stabilise mRNA transcripts