UNIT 2 section 2 genetics


dna function and structure

dna contains your genetic info

double helix structure= 2 seperate strands , coiled 
strands are polynucleotides= lots of nucleotides joined together
nucleoties join to form polynucleotides between the phosphate group of one nucleotide and the sugar of another = a sugar phosphate backbone 

nucleotide structure 
a phosphate group, a pentose sugar and a nitrogenous base
the sugar = deoxyribose sugar 
same sugar and phosphate on each nucleotide 
bases can vary - 4 possible bases , Adenine (A) Thymine (T) Cytosine (C) Guanine (G) 

1 of 23

specific base pairing

2 dna polynucleotide strands join together by hydrogen bonds (H-bonds) between the bases 

each base can only join with one particular partner = specific base pairing 

Adenine + Thymine 

Cytosine + Guanine

2 of 23

structure vs function

double helix = dna is very stable - doesnt break down or get easily damaged 

dna molecules are v long and are coiled up v. tightly = lot of info in a small space in the nucleus

dna molecules have a paired structure = easier to copy itself = semi conservative replication = important for cell division+ passing on genetic info. from gen to gen 

3 of 23

dna storage

Eukaryotic cells-
linear dna molecules - exist as chromosomes
dna is wound around proteins called histones = this helps to support the dna 
dna is tightly coiled to make a chromosome 

Prokaryotic cells-
short circular dna 
does exist as chromosomes 
condenses to fit in the cell by supercoiling

4 of 23

what are genes

sections of dna 

found on chromosomes

genes code for proteins 

5 of 23

making proteins using genes

amino acids make proteins 
different proteins have a different number + order of amino acids
order of nucleotide bases in a gene determines the order of amino acids in a protein
each amino acid is coded for by a sequence of 3 bases = triplet code

different base sequences code for different amino acids
dna is degenerate = more than one base triplet can code for the same amino acid 

more possible combo's of 3 bases than there are amino acids 

6 of 23

non-coding dna

genes in eukaryotic dna contains sections that dnt code for amino acids = INTRONS , non-coding sections of dna 

exons = coding sections of dna 

introns are removed during protein synthesis = splicing 

eukaryotic dna also has sections that contain multiple repeats outside of genes, =these are also non-coding

7 of 23

genes and development

enzymes speed up our metabolic pathway ------ (chemical reactions that occur in the body) 

these pathways determine how we grow and develop 

all enzymes are proteins , which are built using genes 

8 of 23


a gene can exist in more than one form = alleles 

the order of bases in each allele is slightly different, 
so they code for slightly different versions of the same gene

9 of 23

homologous chromosomes

our dna is stored as chromosomes in the nucleus of the cell 

humans have 23 pairs of chromosomes , 46 in total

paris of matcing chromosomes = homologous chromsomes

in a h.pair both chromosomes are the same genes , although they could have different alleles

alleles coding for the same characteristics will be at the same locus (position) on each chromosome

10 of 23

gene mutations

changes in the base sequence of an organisms dna 

mutations can produce new alleles of the gene 

if the sequence of a gene changes , a non-functional protein could be produced 

if theres a mutation in a gene that codes for a enzyme = this may change the shape of the enzymes active site = non-functional enzyme

11 of 23

gametes and fertilisation

gametes are sex cells - males= sperm , females= eggs

gametes join at fertilisation to from a zygote 

normal body cells have diploid number (2N) of chromosomes = each cell contains 2 of each chromosome, one from mum one from dad 

gametes have a haploid number (n) of chromosomes = only one copy of each chromosome 

fertilisation = haploid sperm + haploid egg ----> diploid zygote 

gametes are genetically different = no 2 gametes alike 

gametes formed by MEIOSIS

12 of 23

meiosis (basics)

type of cell division

cells that divide by meiosis are diploid 

cells formed by meiosis are haploid

without meiosis youde get double chromosomes when the gametes fused

13 of 23

meiosis (stages)

1. dna unravels+replicates so there are two copies of each chromosome =called chromatids
2. the dna condenses to form double armed chromosomes , made from 2 sister chromatids
3. the chromosomes arrange themselves into homologous pairs 
4. the homologous pairs are seperated, halving the chromosome number
5. the pairs of sister chromatids that make up each pair are seperated

4 haploid cells (gametes) that are genetically different from each other produced 

14 of 23

how to create genetic variation in gametes

1. crossing over chromatids

2. independant segragation of chromosomes

15 of 23

crossing over chromatids

during m1 homologous pairs join together and pair up 

the chromatids twist around each other and bits of chromatids swap over

chromatids still contain the same genes but no have diff. combos of alleles

the crossing over chromatids in m1 means that each of the 4 daughter cells formed in m2 contain chromatids with different alleles

normally in the 4 cells produced, the 2 are the same and the other 2 are the same

increases genetic variation

16 of 23

independant segragation

4 daughter cells fromed from meiosis have completely diff. combos of chromosomes

1/2 chromosomes= maternal chromosomes

1/2 chromosomes= paternal chromosomes

when gametes are produced different combos. of maternal and paternal chromosomes go into each cells = independant segregation of chromosomes

17 of 23

what is genetic diversity

variety in dna

the more closely related the 2 species are , the more dna they will have in common

dna within a species varies very little , = as members of the same species have the same genes

not at all the same alleles = its the difference in alleles that creates genetic diversity

g.d. is important = if the enviro. changes . there are likely to be some organisms with the alleles that enables them to survive

18 of 23

factors affecting g.d.

increase g.d.
mutations in the dna= forming new alleles
gene flow = where diff. alleles move between pops. eg. through migration

decreses g.d.
genetic bottlenecks
founder effect
selective breeding

19 of 23

genetic bottlenecks

event causes a big reduction in a pop.

reduces number of diff. alleles in the gene pool = decrease g.d.

survivors reproduce + a larger pop. is created from few individuals

20 of 23

founder effect

when few organisms from a pop. start a new colony

only a small number of organisms contribute to their gene pool , so g.d. is reduced

more interbreeding the new pop. , which can lead to a high increase of genetic disease

result of geographical isolation

eg, amish pop

21 of 23

selective breeding

results in reduced g.d.

selecting which domesticated animals/strains of plants to reproduce to produce useful characteristics eg. high yield

once an organism with desired charatcteristics has been made, only this type of organism will continue to be bred

results in a genetic bottleneck

22 of 23

arguments for+against selective breeding

produces high yielding
used to produce animals+plants that have increased resistance to diesease= farmers have to use fewer drugs/pesticides
increased tolerance of bad conditions eg. cold


can causes health problems eg. cows
reduces g.d. , increased incidence of genetic disease +increased susceptibility to new diseases because of lack of alleles in the pop.

23 of 23


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all DNA, genetics and evolution resources »