mod 5 - yr 12 bio

what are DNA strands made of?

repeating nucleotides

what are nucleotides made of?

• a phosphate,

• deoxyribose sugar (or ribose sugar in RNA)

• nitrogenous base (Adenine, Thymine, Guanine or Cytosine)

how many hydrogen bonds between A and T

2

how many hydrogen bonds between C and G

3

what is the advantage of weak hydrogen bonds in DNA

little effort is required to pull bases apart so DNA can replicate or be decoded to form proteins

what is the backbone of DNA strands made of?

alternating sugar and phosphate molecules

what does DNA’s antiparallel arrangement mean

parallel but run in opposite directions

how are DNA strands formed

by strong chemical bond between phosphate of one nucleotide to deoxyribose sugar of another

DNA replication step 1

The original or parent DNA molecule untwists. (by Helicase)

DNA replication step 2

The weak hydrogen bonds of the parent DNA molecule break causing the two strands to separate (or unzip). (by Helicase)

DNA replication step 3

• Free DNA nucleotides within the cell pair with their complementary nucleotide on the unzipped strands

• Hydrogen bonds form joining the bases

• DNA polymerase joins the bases together

why is there are leading and lagging strand in DNA replication? (and what are Okazaki fragments?)

• DNA Polymerase only works in one direction so as the replication fork moves forward, only one strand of DNA can be continuously replicated (leading strand)

• the other lagging strand can only be replicated in short sections once there are enough free bases to use as a template (Okazaki fragments)

what must replication of each fragment begin with?

• short RNA sequence (primer) requiring the enzyme Primase

• the primer is later replaced with DNA by DNA polymerase

what are the fragments joined together by?

• joined together to make a continuous strand by enzyme Ligase

DNA replication step 4

The nucleotides are joined by sugar phosphate bonds and a new DNA strand is formed

DNA replication step 5

Two identical daughter strands are formed each containing:

• one strand from the parent molecule

• one newly synthesised strand

requirements for DNA replication

• DNA acts as a template

• supply of all 4 nucleotides

• enzymes to control the process

  • helicase (to untwist + separate DNA strands)

  • DNA polymerase (to join nucleotides together by joining sugar and phosphate)

• ATP for energy for DNA polymerase to join nucleotides together

importance of exact DNA replication to the cell

• all cell divisions require DNA replication to ensure daughter cells contain their genetic information for correct functioning/survival

• this method ensures a complete and exact copy of DNA is passed onto new cells - so all cells contain the information they need to survive

• if mutations occur, cell may not function properly and die

  • because DNA controls proteins that are made by the cell and proteins control almost all cell activities

how many DNA molecules is a chromosome

1

where is DNA located in eukaryotes?

• Most of DNA is located in nucleus in eukaryotes

  • However some extra DNA is found in chloroplasts and mitochondria, which is found as smaller, circular molecules

how is DNA packaged?

• DNA is wrapped around Histone proteins to form Nucleosomes

  • bead like structures made of long sequences of DNA wrapped around histone proteins

• It is then wrapped into a spiral shape

• Which is then folded back and forth into a protein scaffold

• Which is then wrapped into a chromosome

  

what do karyotypes do

display chromosome contents of a cell, organised according to their number, size and type

somatic cell

any cell that is not a reproductive cell

how many chromosomes (and in how many pairs) do human somatic cells have

46, 23 pairs

Normal somatic human cells have a karyotype with 46 chromosomes (in 23 pairs) comprising:

• 22 pairs of autosomes

• 1 pair of s*x chromosomes (determine the s*x of an individual)

• XX (female), XY (male)

homologous chromosomes

• paired chromosomes which have the same length, centromere position and same genes at the same positions along their lengths

• 1 chromosome from each pair comes from one parent and the other one comes from the other parent

While homologous chromosomes have the same genes, they may have — versions of the genes (alleles)

different

• e.g  one chromosome may have a hair colour gene which says “make protein for red hair” and the other chromosome may say “make protein for blonde hair”

why do cells divide?

• Growth of multicellular organisms – growth relies on mitotic division followed by cell assimilation, enlargement and differentiation

  • Individual cells cannot grow large because of SA:Vol limitations

  • The only way to grow larger is to produce many small cells

• to repair damaged tissue and replacement of worn-out cells

• Asexual reproduction – e.g growth of plans from cuttings

• Genetic stability – mitosis ensures the precise and equal distribution of chromosomes to each daughter nucleus, so that all resulting cells have the same chromosome number and genetic information as each other and the original parent cell

cell cycle stages

G1 - cell growth

S - DNA duplication/synthesis

• each of 46 chromosomes is duplicated by the cell

G2 - cell growth

M - mitosis

what happens in mitosis

• During mitosis an exact copy of genetic material in the parent cell must be distributed to each daughter cell

• Each stage of mitosis is designed to achieve equal and exact distribution of genetic material

• Cell division occurs in a cycle and involves 2 main steps

• Mitosis – division of nucleus

• Cytokinesis – division of cytoplasm and organelles

what happens in interphase

• DNA present as uncondensed chromatin

• Chromatin: DNA that is spread out and diffused in the nucleus

• DNA is contained within a clearly defined nucleus

• Centrosomes and other organelles have been duplicated

• Cell is enlarged in preparation for division

how many chromosomes in the nucleus before and after interphase

• before interphase, there are 46 chromosomes in the nucleus but each one is a single unit

• After interphase, there are still 46 chromosomes but with 2 identical chromatids each

what happens in prophase

• Chromatin supercoils, condensing it into chromosomes (becoming visible under microscope) each containing 2 copies of DNA and 2 sister chromatids joined by a centromere

• Nuclear membrane begins to break down and nucleus dissolves

• Spindle begins to form from broken down material and extends across the cells

what happens in metaphase

condensed chromosomes line up along the equator of the cell, each attached to spindle fibres by a centromere

what happens in anaphase

• continued contraction of spindle fibres causes sister chromatids to separate (and each become a chromosome)

• Spindle fibres contract and pull daughter chromosomes towards opposite poles of the cell, with assistance of the centromere

what happens in telophase

• Spindle fibres dissolve once the 2 chromosome sets arrive at the opposite poles of the cell

• Chromosomes decondense

• New nuclear membranes are formed around the chromosomes

what is cytokinesis

• process of cytoplasmic division where the cell splits into 2 identical daughter cells

• Occurs concurrently with telophase

how does cytokinesis happen in animal cells

• After anaphase, microtubule filaments form a concentric ring around the centre of the cell

• These microfilaments constrict to form a cleavage furrow, which deepens the periphery towards the centre

• When the furrow meets in the centre, the cell becomes completely pinched off and 2 cells are formed

how does cytokinesis happen in plant cells

• After anaphase, carbohydrate-rich vesicles form in a row at the centre of the cell

• These vesicles fuse together and an early cell plate begins to form

• The cell plate extends outwards and fuses with the cell wall, dividing the cell into 2 daughter cells

what is meiosis

• Is the division of the nucleus to produce 4 gametes

• Results in variation in an organisms offspring – i.e independent assortment of chromosomes and unique composition of alleles in daughter cells

• During meiosis number of chromosomes is halved, occurring over 2 consecutive divisions

• Only happens in gamete mother cells

purpose of meiosis

• Create haploid gametes (with half the number of chromosomes) to ensure that when fertilisation occurs the offspring receive the correct amount of genetic information

  • Haploid cell: cell that contains a single, complete set of chromosomes (23 in humans)

• Create genetic variation amongst those gametes (specialised reproductive cell that unites with another opposite-s*x cell during reproduction to form a new organism (i.e eggs and sperm))

what happens in interphase in meiosis

• Cell growth, DNA replication, cell processes

• Each uncoiled chromosome replicates to produce 2 identical chromatids

what happens in prophase I in meiosis

• Chromosomes condense by coiling and become visible

• Nuclear membrane dissolves

• Bivalents form – pair of homologous chromosomes that link together

• Crossing over occurs at points where the chromosomes touch (called chiasmata)

what happens in metaphase I in meiosis

• Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the equator of the cell

• Independent assortment occurs during this stage

what happens in anaphase I in meiosis

• Spindle fibres contract and split the bivalent

• Homologous chromosomes (still with 2 chromatids) move to opposite poles of the cell

what happens in telophase I in meiosis

• Chromosomes decondense

• Nuclear membrane may reform

• Cell divides (cytokinesis) to form 2 haploid daughter cells

what happens in prophase II in meiosis

• Chromosomes condense

• nuclear membrane dissolves

• centrosomes move to opposite poles (perpendicular to before)

what happens in metaphase II in meiosis

• spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator

what happens in anaphase II in meiosis

• centromeres split allowing spindle fibres to contract and pull the chromatids (now chromosomes) to opposite poles of the cell

what happens in telophase II in meiosis

• chromosomes decondense

• nuclear membrane reforms

• cytoplasm divides (cytokinesis) to form 4 haploid daughter cells

• within each gamete there is a different genetic combination

how does meiosis creates genetic variation: independent assortment

• occurs when homologous chromosomes line up on the equator during Metaphase I in meiosis

• homologous chromosomes line up and separate randomly and independently of all other pairs so the number of different possible gametes is very large

• the genetic content of the gametes is determined by how the chromosomes line up on the equator

• e.g 2 pairs of chromosomes in a gamete mother cell would give:

how does meiosis creates genetic variation: crossing over

• takes place between the members of a homologous pair of chromosomes during Prophase I and involves the exchange of part of one chromatid with another

• occurs at random

• produces chromosomes with a new combination of genes

  • and variation in allele combinations for different genes

• produces new variations of gametes – increases genetic diversity of potential offspring

how does meiosis creates genetic variation: random segregation

• Maternal and paternal chromosomes get sorted into gametes randomly

• E.g a parent with Aa will have half of their gametes with A, the other half with a

where are alleles of the same gene found on?

homologous chromosomes

Meiosis ensures that homologous chromosomes are — and placed in — gametes

separated, different

Each homologous chromosome has an — chance of being segregated into a particular gamete

equal

how does this explain mendel’s law of segregation? (and what is mendels law of segregation?)

• states that each organism carries 2 copies of a gene for a trait (alleles)

• and these 2 alleles separate randomly during the formation of gametes so each gamete receives only 1 allele

what is mendels law of independent assortment?

Mendel found genes separate and move into gametes independently of each other

how can mendels law of independent assortment be explained?

• by assuming the genes Mendel studied were on different chromosomes

• Chromosomes can show independent assortment, and so would genes on different chromosomes

what is genetic variation in offspring determined by?

• Is determined by the combinations of alleles of all their different genes

what is genetic variation in offspring important for

long term evolutionary change

what is genetic variation in offspring increased by

increased by both gamete formation (during meiosis) and sexual reproduction

meiosis allows for a — combination of existing genetic material to be formed

new

• can occur via independent assortment and crossing over

how does independent assortment ensure variety in gametes?

• different combinations of chromosomes (formed due to independent assortment) ensure a variety of gametes can be formed with different combinations of alleles for genes of those different chromosomes

• The number of combinations formed is determined by 2ⁿ where n is the haploid number of chromosomes

what is fertilisation

Is the fusing of a haploid male gamete with a haploid female gamete to produce a diploid zygote with a new combination of genes (one set each from mother and father respectively)

what does fertilisation involve

• random pairing of a male and female gamete and results in variation in allele combinations for each gene

  • i.e whether it is homozygous for a particular allele (2 of the same alleles of a gene) or heterozygous (2 different alleles of a gene)

fertilisation vs gamete formation: creating variation

• Fertilisation creates variation in genotype for each gene

• gamete formation creates variation in the combinations of alleles for different genes

similarities between mitosis and meiosis

• Both preceded by interphase (including DNA replication)

• Both divide according to a common pathway (PMAT)

• Both involve their cytoplasm splitting via cytokinesis

differences between mitosis and meiosis: mitosis

• Divisions – 1

• No independent assortment

• Synapsis – no tetrad/bivalent

• Crossing over – doesn’t occur

• Outcome – results in 2 daughter cells

• Ploidy – daughter cells produced are diploid

• Use – used to replicate body cells

• Genetics – produced genetically identical cells

differences between mitosis and meiosis: meiosis

• Divisions – 2

• Independent assortment

• Synapsis – tetrad/bivalents formed (pairs of homologous chromosomes)

• Crossing over occurs (non-sister chromatids of homologous pairs may exchange genetic material)

• Outcome – results in 4 daughter cells

• Ploidy – daughter cells produced are haploid

• Use – used to make sex cells (gametes)

• Genetics – produces genetically distinct cells

continuity of a species

ongoing survival of species as a result of characteristics being passed from parents to offspring in a continuous lineage

genetic stability

passing on of consistently accurate genetic information

genetic variation

occasional introduction of variation of some genetic information (e.g mutations) so that species can adapt and survive in a changing environment

what does genetic continuity do

• way of preserving genetic information across generations

• Ensures continuation of a species so new cells or organisms have all the genes they require to survive

  • Lack of genetic continuity may result in disease and sometimes extinction

what does genetic continuity depend on

• When a cells divides by mitosis, the 2 daughter cells must have the same number and type of genes as the original cell

• When 2 sexually reproducing organisms breed, the resulting offspring must have the same number of genes as the parent organism

how does stability come about at a genetic level?

At a genetic level, stability arises when chromosomes are replicated accurately to create identical daughter chromosomes

how does stability come about at a species level?

• At a species level, successful desirable traits must be passed on along with some random errors (may be introduced by mutation)

  • Allows a species to evolve if an environmental change occurs

  • Natural selection – allows individuals best suited to their

Mechanisms that have evolved to ensure genetic continuity + survival + continuity of a species (COFEN)

• Consistent and accurate DNA replication prior to cell division (mitosis and meiosis)

  • Mitosis giving full sets of chromosomes in daughter cells

  • Meiosis ensures one of each type of chromosome goes to a gamete

  • So there are homologous pairs in the next generation

• Orderly distribution of chromosomes when cells divide and when gametes form

• Fertilisation methods that ensure that individuals of the same species breed successfully

• Methods to ensure Embryo survival, such as production of large numbers or protection and nourishment of developing embryos

• Natural selection so the fittest survive to reproductive age and pass on their genes

Mechanisms that ensure genetic variation in a species

• Mutation – changes in DNA due to mutation may be spontaneous or mutagen-induced

• Mixing of parental genes during sexual reproduction

  • brought about by crossing over and independent assortment during meiosis, and random fertilisation of gametes

what ensures that genetic information is stored in a stable form and passed on consistently?

if DNA repair enzymes continuously scan the DNA for errors in replication and replace incorrect or damaged nucleotides

example of genetic errors that threaten continuity of a species: XP

• People who have the disease Xeroderma pigmentosum (XP) are unable to repair DNA

  • they are more vulnerable to DNA damage from UV rays and thus skin cancer

Why is Genetic Variation important?

• It increases the chances of survival of a population in case of environmental change

  • e.g physical environment changes, introduction of diseases

• Provides the necessary differences upon which natural selection acts

DNA in eukaryotes

• Is contained within a nucleus

• Is bound to histone proteins

• Genomes contain large amounts of non-coding and repetitive DNA

• Do not contain plasmids (but organelles such as the mitochondria and chloroplasts contain their own DNA)

• Are linear in shape

DNA in prokaryotes

• Main chromosome is circular in shape

• Smaller than eukaryotic chromosomes

• Not packaged with proteins

• Not in a nucleus but found freely in the cytoplasm

• Very little non-coding DNA – genes are closer to each other; genomes are more compact

• May also have extra small circles of DNA called plasmids (not always present)

phenotype

the physical appearance/feature of an organism (often) caused by the genetic information that it has inherited from its parents

genotype

a description of the alleles of a gene that a particular organism

contains, normally denoted by letters

gene

A part of a chromosome which contains the information for a particular

characteristic

allele

different forms of a gene

how many sets of chromosomes in each cell and how many copies of each gene in the cell?

2 sets of chromosomes; thus has 2 copies of each gene

dominant form of a gene will — show up in the organism’s phenotype

always

Homozygous

Organism which possesses 2 identical forms of a gene

Heterozygous

Organism which possesses 2 different forms of a gene