Bio Module 5 - topic 2 (cell rpelication)

mitosis + meiosis on other ards

structure of DNA

• long double stranded helix

• sugar phosphate backbone attatches to nitrogenous bases that bond in specific pairs

function of DNA

carries all genetic information coding for protein production - enabling cell growth repair and other specialised functions

nitrogenous bases in DNA and how they bond

• Adenine and Thymine join by double H-bonds

• Cytosine and Guanine has triple H-bonds

how are elements of DNA bonded together

phosphates are attatched to deoxyribose sugars by covalent bonds, strands (bases) joined by H-bonds

what is a nucleotide

each consists of sugar bonded to phosphate joined to nitrogenous base

5’ end of DNA strand

the ‘front’, has phosphate group on the end

3’ end of DNA strand

the ‘bottom’, has hydroxyl on end (from sugar)

what is a gene

it is a sequence of DNA that contains a particular sequence of nitrogenous bases that have specific role of making info for a type of protein

chromosome

DNA wound around histone proteins that is coiled over and

composition of DNA

composed of chromatin which are fibres that contains:

• 40% DNA

• 60% protein

female 23rd chromosome

XX

male 23rd chromosome

XY

‘homologous chromosome pair

chromosome pair that are similar in shape and size and contain same genes in same order

• one is from paternal and one is from maternal

how do chromosomes change throughout cell cycle

G1 phase: 1 chromatid + one chromosome

G2 phase: 2 sister (identical) chromatids

After mitosis (M phase): 2 identical chromosomes

how many DNA molecules in a chromatid

1 long molecule of DNA

when does DNA replication occur

in S phase

steps of DNA replication

1. helicase enzymes unwinds DNA strands

2. primase enzyme makes small piece of RNA - creating a ‘primer’

3. DNA polymerase enzyme binds to primer adding free nucleotide units to it complementary base partner

   • However, polymerase can only add bases continuously in one direction (from 5’ to 3’ end) - and hence only continuously add bases on the leading strand of the DNA

   • So in lagging strand: polymerase adds in sections → ‘okazaki fragments’

4. Once DNA has been made, exonuclease enzyme removes RNA primer, so that another DNA polymerase can fill gaps with bases

5. Replication errors are identified and corrected + ligase enzyme seals up fragments of DNA in both strands

what enzyme checks over replication errors and how

DNA polymerase corrects base pair errors by splicing out incorrect base + replacing w/ correct base

what do incorrect base pairs cause?

mutations

what happens if damage to DNA is not repaired

cell undergoes apoptis - programmed cell death

diff between somatic and germline mutations

• somatic is in body cells, germline in gametes

• somatic only affect individual, germline in heritable

what is mutation, what is its effect?

it is simply a change in DNA, it can be benfifical, harmful or ineffective

what does genetic continuity rely on

• Consistent replication of genetic info passed from parent to daughter cells 

• The effect of natural selection + evolution on the gene pool as a result of - more advantageous gene combinations are more likely to survive

  • Introduction of variation during sexual reproduction (in meiosis)

  • random errors arising by mutation, being replicated and passed onto offspring (new allele formation)

• Random variation that produce an advantage may be selected over those that cause no advantage/harm

what are 2 types of genetic errors

1. spontaneous mutation

2. mutagenetic mutations —> from environmental factors e.g celll exposure to radiation + chemicals

diff between prokaryotic and eukaryotic DNA

Prokaryotic

• singular, circular chromosome in cytoplasm

• is not tightly coiled around histone proteins

• has only one copy of each gene

• very little non-coding DNA

Eukaryotic:

• discrete, linear molecule in X shape located in cell membrane nucleus

• DNA is tightly wound around histone proteins

• multiple copies of genes

• large sections of noncoding DNA

—→ also mitochondria and chloroplasts in eukaryotic organisms possess their own DNA (non-nuclear DNA)

phenotype definition

the observable physical or biochemical characteristics of an individual organism, determined by both genetic makeup and environmental influences

factors that affect phenotypic expression

Genotype - is inherited

Environment - not inherited

epigenetics

studies how chemicals can change the way genes in DNA are expressed → aka why environment can affect our phenotypes

how - according to epigenetics - can environment cause genes to be expressed

relaionship between phenotype and genotype

The genotype of an organism describes the specific alleles for a particular gene, and the phenotype describes the appearance of the organism associated with the genotype

How can environment effect phenotype

environment can decrease/stimulate the expression of the genotype therefore changing allele

example of how environment can effect phenotype

• A person’s height is determined not only by genes inherited but also nutrition

• A person’s skin colour is determined by amount of melanin produced but also genes can also be stimulated to produce more melanin in response to UV rays

• Hydrangeas will create blue or pink flowers, depending upon soil’s pH