Biology - Unit 2 (Genetics)

Exam Review

DNA

• hereditary molecules found in the nucleus of the cell

• contains info needed to create proteins of the cell

• shaped as a double stranded helix

DNA composition

• made up of molecules called nucleotides which pair up (base pairs) to make the double helix

• The strand shape holding the base pairs are called sugar phosphate backbone

• DNA is wrapped around proteins called histones

• Each histone with DNA wrapped around it is called a nucleosome

• A chromatin is nucleosomes tightly coiled together

• A chromosome is a chromatin that has been coiled once again (Only visible during cell replication)

Nucleotides

• A, T, C, and G

Nucleotides

• Molecules made up of a nitrogen containing base (Nucleobase), a pentose (contains 5 carbons) sugar ring, and a phosphate group

• There are 4 types of nucleobases:

• Adenine & Guanine (purines)

• Thymine & Cytosine (pyrimidines)

Nucleobases

• Guanine pairs with Cytosine

• Adenine pairs with Thymine

DNA Structure

• Base pairs are bonde

• d with hydrogen

• The side strands of DNA are deoxyribose sugar bonded covalently to phosphate

• All ofthese components makes up the DNA double helix

• The strands of DNA double helix are anti parallel and run in opposite directions (goes from 5 prime to 3 prime)

DNA replication

• DNA is first unzipped by breaking the hydrogen bonds between base pairs

• nucleotides that are freely floating in the cell bond with their complementary bases on the parent strands as daughter strands

• New nucleotides on the daughter strand form covalent bonds

Protein synthesis

• transcription - DNA is unzipped and a temporary copy of DNA called mRNA is made which can leave the nucleus

• Translation - mRNA goes into a ribosome which reads and uses the nucleic acid sequence to make protein

Purpose of mitosis

• growth and development

• reparations of damaged cells

• replacement of aging cells

mitosis

• form of asexual reproduction

• the process by which the replicated DNA of a cell is divided into two nuclei, each with the same number of chromosomes as the parent cell

• forms two genetically identical daughter cells

Chromosomes

• each chromosome contains several genes

• genes determine the expression of a trait

Genes

• Genes are sections of DNA that code for a particular trait

• Genes can be non coding and have to do with regulating the expression of a coding gene

Chromosomes

• Humans have 46 chromosomes

• 23 pairs of chromosomes

• 50% from mom 50% from dad

Homologus chromosomes

• Chromosomes that code for the same trait

Cell cycle

1. G1 is where cellular content excluding chromosomes are duplicated

2. S is where all 46 chromosomes are duplicated

3. G2 is where the cell checks for errors and edits

4. Mitosis is where cell division occurs

5. Cytokenisis is the part where the cytoplasm splits leaving two daughter cells. This includes the division of organelles as well into two daughter cells.

Interphase

• Interphase includes G1, S, G2

Mitosis

1. Prophase - Pairs of centrioles move to opposite poles, chromosomes become visible, nuclear membrane breaks down, spindle fibers form between centroles at opposite ends and chromosomes attach to spindle fibers at the centromere

2. Metaphase - The chromosomes line up in the middle of the cell on the equatorial metaphase plate

3. Anaphase - The spindle fibers pull the sister chromatids of each chromosome apart to each side.

4. Telophase - the chromsomes uncoil into chromatin, nuclear membrane reforms around each group of chromosomes, and spindle fibers break

Cell differentiation

• Process by which stem cells change into a specialized cell.

• In response to internal/external signals, certains genes are active or not and this controls the type of cell a stem cell becomes

• The stem cell will undergo mitosis and based on its genes it may specialize into different cells

Cancer

• uncontrolled cell division due to a malfunction in replication machinery, replication checkpoints, or DNA replication

Cancerous cells

• many cells that continue to grow and divide

• variations in size and shapes

• nucleus is larger and darker than normal

• abnormal number of chromosomes arranged in a disorganized fashion

• cluster of cells without a boundary

Meiosis

• parents produce gamets by meiosis

• when two sex cells undergo fertilizations a zygote is made ( the first cell of a new organism)

Female and male sex cells

• female sex cells are called ova (eggs)

• male sex cells are sperm

Germ cells

• cells that perform meiosis to produce gametes

Somatic cells

• non reproductive cells

Haploids

• cells that contain one copy of every chromosome

Diploids

• cells that contain 2 copies of every chromosome

Product of meiosis and mitosis

• a parent diploid germ cell undergoes meiosis and produces 4 haploid cells

• a parent diploid cell produces 2 diploid cells

Meiosis I

1. Prophase I - chromatin condenses into chromosomes, nuclear membrane dissolves, centrioles move to opposite poles and spindle fibers form and connect to tetrads. Homologous chromosomes form tetrads and intertwine at synapses and after synapses are formed DNA is exchanged between the homologous chromosomes at sites called chiasma. This is known as crossing over.

2. Metaphase I - Tetrads align at their centromeres at the metaphase plate.

3. Anaphase I - One chromosome from each homologous pair moves to opposite poles of the cell. each chromosome in the daughter cells is composed of two sister chromatids.

4. Telophase I - Nuclear membrane reforms then the cell preforms cytokenisis

5. Cytokinesis - after each daughter cell goes through cytokinesis immediately enters meiosis II

Meiosis II

1. Prophase II - nuclear membrane dissolves, centrioles move to opposite poles of the cell and spindle fibers form

2. Metaphase II - chromosomes align in the middle of cell

3. Anaphase II - Sister chromatids move to opposite poles of the cell

4. Telophase II - Nuclear membrane reforms

5. Cytokenisis - Nuclear membrane reforms and the cytoplasm is divided

How does genetic variation occur

• Crossing over - occurs during prophase I where the genetic exchange of homologous chromosomes at chiasma is random meaning the resulting chromsomes have random amounts of mom and dad genes

• Random assortment - occurs during metaphase I, when the tetrads line up in the centre of the cell they can be in any order.

Gametogenesis

• gametogenesis is the production of sex cells

• Spermatogenesis is the production of sperm cells

• Oogensis is the production of egg cells

Oogensis

• One of the 4 resulting gametes inherits almost all of the cytoplasm

• The other 3 cells are called polar bodies and die

Twins

• Identical (monozygotic) twins occur when 1 fertilized egg forms a single zygote that splits into 2

• Fraternal (dizygotic) twins occur when 2 fertilized eggs form 2 individual zygotes at the same time

Mistakes in meiosis

• Can cause non disjunction disorders

First division non disjunction

• meiosis I starts normally and tetrads line up in the middle of the cell during metaphase and then the tetrad(s) of homologous chromosomes does not seperate

• Meiosis II occurs normally and this results in all gametes having an abnormal number of chromosomes either too many or too few

• This occurs in anaphase I

Second division non disjunction

• At anaphase II sister chromatids move to the same pole

• Half of the resulting 4 gametes are euploid (normal) but the other half has one that is disomic and one that is nullsomic

Karyotyping

• Method of identification of chromsomes

• Pictures of chromosomes are taken as the cell undergoes mitosis and the image is enlargerd

• Individual chromsomes are cut up and are then matched into homologous pairs based on size, centrometere position, g-banding (staining

Non disjunction disorders

• occur during Anaphase I or II

• pretty common

• produces gametes that have too many (trisomy) or too few (monosomy) chromosomes

• Often causes miscarriages

• If the baby survives it develops a set of traits called a syndrome

Down syndrome

• trisomy 21

• prone to heart defects, respiratory problems, and leukemia

Kinefelters syndrome

• XXY

• Male with female characteristics

Turners syndrome

• XO

• Sterile

• Do not mature sexually

Triple X syndrome

• XXX

• Fertile

• Normal intelligence

• no physical issues

Jacobs syndrome

• XXY

• Extra testosterone

Polyploidy

• Mistakes in meiosis can also cause polyploidy\

• where zygotes are triploid or tetraploid

• a diploid egg combines with a diploid sperm producing a tetraploid zygote

• Occurs from total non disjunction

Mendels law of segregation

• Each allele seperates into different gametes during meiosis therefore only one allele is passed down to the offspring

Mendels law of independent assortment

• During gamete formation, segregating pairs of unit factors assort independently of eachother

• two traits are inherited totally independently of eachother

Test cross

• Used to determine whether an organism is homozygous or heterozygous for a dominant characteristic

• cross the organism with a homozygous recessive organism

Codominance and Incomplete dominance

• Incomplete dominance is where one allele is not completely dominant over another, the heterozygous phenotype is a blending of the two homozygous phenotypes. The alleles modify the expression of each other and create an intermediate phenotype

• Codominance is where two alleles are both expressed as a dominant phenotype and are equally expressed.

Multiple allelism

• When more than 2 alleles control a specific trait (ex: blood types include a, b, or o)

• Any combo of alleles is possible but individuals will only have 2 at a time

• results in genetic variation

Blood

• Red blood cells which contain hemoglobin and transport O2 and CO2 to and from body tissues

• White blood cells fight infection

• Platelets help blood clot

• Plasma is fluid that contains salts and proteins

Blood type

• proteins found on the surface of red blood cells and in the plasma determine blood type

• Antigens are located on the surface of red blood cells

• Antibodies are found in the plasma

• Blood type is determined by the type of antigens it has

Transfusions

• Only work if a person who is going to receive blood has a blood type with no antibodies against the blood donors anitgens

• Blood type O is known as the universal donor

• Blood type AB is known as the universal recipient

Nature vs Nurture

• Alleles can produce different phenotypes depending on the environment

• Eg: nutritition impacts height

Blood typing

• Anitbodies fight foreign blood types with antigens you don’t have

• Agglutination occurs when matching antibodies and antigens come into contact

• Red blood cells will clump together and can blood blood vessels

Rhesus factor

• a protein (antigen) on the surface of red blood cells

• If you have the antigen you are Rh+ which is the dominant allele

• Rh+ has Rh antigens but no anti-rh antibodies

• Rh- has no Rh anitbodies but has anit Rh anitbodies after exposure to Rh + blood

Erythroblastosis Fetalis

• Rh+ child with an Rh- mom

• Before birth moms blood is exposed to babies blood and the mom will develop rhesus anitbodies

• If the mom becomes pregnant again with an Rh+ baby her antibodies will go into the babies blood causing aglutination

• This can be prevented by injecting foreign rhesus antibodies in the mom before she is exposed to the babies blood during pregnancy

• Foreign antibodies trick the moms body into not producing her own anitbodies

• Foreign anitbodies only last a few weeks so the baby can be birthed safely

Sex linked inheritence

• Males have XY

• Females have XX

• These chromosomes are known as sex chromosomes

• genes located on these chromosomes are known as sex linked genes

• The X chromosome is much larger than the Y chromosome so it carries more genes

Sex linked disorders

• Much more common in males as they only need to inherit one of the alleles for it to be expressed

• Genes are only expressed on the X chromosome not the Y

Hemophilia

• Recessive sex linked

• Uncontrolled bleeding

• Individuals are missing the normal blood clotting protein

Color blindness

• Recessive sex linked

• Inability to see certain colours

Duchenne Muscular Dystrophy

• recessive sex linked

• progressive weakening and loss of skeletal muscle

Abnormal crossing over during prophase I

• Deletion - loss of segment within the chromosome

• Addition - addition of a segment within the chromosome

• Inversion - genetic material does not change but is rearranged

• Translocation - DNA from one chromosome attaches to a non homologous chromosome

• Can cause disorders

Mitosis in animal and plant cells

• In animals spindle fibers are formed by centrioles which are located in centrosomes

• In plants, there are no centrioles and spindle fibers form themselves without centrosomes

RNA

• Uses the nucleobases adenine, guanine, cytosine, and uracil instead of thymine

• adenine pairs with uracil

mRNA

• transports messages from DNA to ribosome

rRNA

• ribosomes are made of rRNA

tRNA

• Responsible for transferring amino acids to match the correct mRNA codon

• brings amino acids to the ribosome

• the anti codons of tRNA pair with the codons of mRNA and the rRNA moves along the mRNA and links them together making a polypeptide chain