alevel biology genetics

Genome, proteome, DNA in different organism types/ organelles

Prokaryotes, plasmids, chloroplasts, mitochondria have dna not associated with histones; plasmids/chloroplasts/mitochondria have necessary/ highly used genes

Eukaryotes have dna associated with histones

Genome - full set of genes in a cell

Proteome - full set of proteins coded by a genome

Chromosome structure

Chromosome → by two sister chromatids connected by a centromere → supercondensed/coiled chromatin → uncoiled chromatin → dna wrapped around a histone

Telomeres protect ends of the chromosome from damage.

Identical sister chromatids allow for genetically identical daughter cells

Features of genes

DNA is a sequence of bases which codes for a specific sequence of amino acids, which determines the structure of a protein.

Has:

Exons and introns,

Triplets which code for a specific amino acid

Code is degenerate, meaning multiple triplet combinations code for the same amino acid

Non-overlapping, code only read once

Genes are universal and occupy a fixed locus on every organisms dna strand

mRNA + tRNA

mRNA - long single strand, with base sequences determined by DNA, exposed unpaired bases (codons)

Chloroplast/ Mitochondria dna, not associated with proteins, circular dna, contains most important genes

tRNA - clover leaf produced by nucleotides, region of amino acid to bond to, anticodons (sequence of bases is able to base pair to mRNA)

Transcription

1. DNA helicase unwinds the dna strand

2. Free activated RNA nucleotides bond to complementary base pairs on antisense (exposed DNA template strand) via base pairing

3. RNA polymerase bonds RNA nucleotides together forming sugar-phosphate backbone, with phosphodiester bonds forming

4. order of triplets on dna determines order of amino acids, by determining order of codons

Eukaryotes have further steps:

4. introns and extrons are both transcribed forming pre-MRNA

5. Splicing to remove introns and produces mRNA with only exons

   5a. alternative splicing will order the exons differently, proudcing mature mRNA

Translation

1. mRNA binds to ribosomes, ribosome moves to start codon

2. amino acids binds to tRNA, which binds to completely codon on mRNA at the ribosomes

3. peptide bond is formed between amino acids, ATP used. 2 tRNA can bind to site at a time

4. Stops binding and adding amino acids when a stop codon is reached.

Mutation

Change in the sequence of base pairs

1. Change in base pairs causes a different triplet, which hence changes the codon on an mRNA

2. Different codon results in a different amino acid in the polypeptide chain

3. So different hydrogen bonds form between the chain so secondary structure changes

4. Different hydrogen/ionic bonds, disulfide bridges forming (different interactions between the chain)

5. Causing altered tertiary structure and a denatured active site

Code is degenerate so not all changes effect protein

Mutation types

Deletion
Random deletion of base pair, changes all triplets past mutation

Insertion

Inserting of base pair into sequence, all triplets change past mutation

---

Substitution

Swapping of base pairs,

Silent - doesn’t change sequence of amino acids

Missense - Alters 1 amino acid

Nonsense - creates premature stop codon

---

Mutagenic agents - agent which increase chance of mutation

Genetic variation sources

Independent assortment: (meiosis)

Homologous pairs line up randomly, forming different chromosome combinations

---

Crossing over: (meiosis)

Alleles of non-sister chromatids are swapped, providing recombinant cells

---

Random fusion of zygotes during fertilisation (2ⁿ)

Alternative splicing (transcription)

Non-disjunction - chromosomes fail to separate (causing polydiploidy)

Variation

Differences in base sequences between individuals of a population

Genetic variation is passed across generations

Producing genetic diversity (number of different alleles in a gene pool)

producing different phenotypes

Natural selection

Certain phenotypes will be favoured as a result of selection pressures (due to environment), (natural selection occurs)

resulting in increased survival for best phenotypes, and allowing for reproduction and alleles to be passed to offspring, which changes allele frequency

---

Directional selection:

Extreme allele is favoured, so allele frequency shifts towards extreme allele end, as greater survival chance, so passes allele to offspring

---

Stabilising selection:

Extreme alleles favoured against, so allele frequency of extremes decreases, as lower survival chance, so cannot pass allele to offspring

Other factors effecting allele frequency

Founder effect: a portion of population creating a new population, Changes allele frequency in different direction due to chance

---

Bottle neck effect:

Large population has dramatic decrease due to disease etc, creating a reduced gene pool, and dramatic decrease in allele frequency as many are lost

---

Genetic drift:

Gradual change in allele frequency in small population due to chance (mutation)

Selection pressure, types of adaption

Organisms experience selection pressure due to environment and other factors, and have to adapt to enable a higher/ lower chance of survival

Anatomical - structural

Physiological - Biological processes change

Behavioural - change to behaviour

Species + recognition/ reproduction within species

two similar organisms which can reproduce to produce fertile offspring

To produce offspring courtship (as visual, chemical, audible stimuli) is needed

Required to identify a fertile mate, species member, synchronise mating

Classification

Phylogenetic classification required to class species based on evolutionary origin and relationships by using a hierarchy of groups with no overlap,

Binomial naming system (genus species, e.g. homo sapien)

---

3 domains:

Eukarya

Has nuclei, membrane bound organelles, reproduces via asexual/ sexual

---

Bacteria

No nuclei, have peptidoglycan cell wall, divide by binary fission

---

Archaea

No nuclei or peptidoglycan cell wall, transcribes rna like eukarya

Biodiveristy

Measure of variety and number of organisms in a habitat

required for an ecosystem to resist change, as higher biodiversity creates a more stable ecosystem

Species diversity - population of each species in a community

Genetic diversity - Variety of genes within gene pool of species

Ecosystem diversity - range of habitats

investigating biodiversity (ways to improve accuracy/representative), species richness, index of diversity, sdeviations+testing

Species richness - number of different species in a habitat

Index of diversity - calculation of all species and their abundance in an ecosystem

data collection needs to be representative so:

random sampling used to reduce bias

increasing sample size to make data more representative as the probability results are due to chance are reduced, and reducing the effect on anomalies

standard deviation used to show distribution of data around the mean

standard deviation shows whether overlap in data is significant between populations

Investigating species

DNA base sequence + mRNA base sequence + amino acid sequence comparisons

more similar = more closely related = recent speciation / very recent common ancestor

Immunology - comparison of proteins by binding to an antigen, more similar proteins bind to the same antigen