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4.3 genetic diversity can arise as a result of mutations or during meiosis…
4.3 genetic diversity can arise as a result of mutations or during meiosis
lesson 1: mutations
definitions
mutation
a change in base sequence of DNA (gene mutation) or a change in quantity of DNA (chromosome mutation)
gene mutation
could be a base substitution or a base deletion/addition (frame shift mutation) they occur spontaneously during DNA replication (prior to mitosis or meiosis
base substitution
when one base is substituted for another which causes a change in one amino in a polypeptide
base deletion/addition
frame shift mutation, change all base triplets codons and amino acids downstream of the mutation
explain how a gene mutation can lead to a non-functioning enzyme
gene mutation results in the substitution/deletion/addition of DNA base
this causes a change in the amino acid sequence/primary structure of the enzyme
this therefore changes the position of the ionic hydrogen and disulphide bonds between the R-groups of the amino acids causing a change in the enzymes tertiary structure/ 3D specific shape
enzyme and substrate are no longer complementary so substrate can no longer successfully enzyme so no enzyme-substrate complex is formed and reaction cannot be catalysed
explain why not all base substitutions result in a change in the protein produced
most amino acids are coded for by more than one base triplet (genetic code is degenerate)
may be no change in primary structure of protein
therefore no change in tertiary structure of protein
what increases the rate of mutation
mutagenic agents such as mutagenic chemicals (chemicals like tar found in cigarette smoke) or ionising and high energy radiation (alpha, beta and gamma radiation, UV and X-rays)
lesson 2: meiosis
features
diploid parent cell
two nuclear divisions
4 haploid daughter cells
genetically varied daughter cells
production of gametes for reproduction
process
Genetic information duplicates and the chromosomes form sister chromatids
chromosomes line up in their homologous pairs and undergo crossing over, they are then separated and one chromosome from each pair into one of the two daughter cells (first division)
the sister chromatids are then separated in the second division forming 4 genetically different daughter cells
independent segregation
homologous chromosomes are separated independently/randomly during meiosis
this creates genetic variation in the gametes
lesson 3: crossing over and non-disjunction
calculating chromosome combinations in gametes (due to independent segregation)
the number of chromosome combinations = 2^n
n = the number of chromosome pairs
number of chromosome combinations possible in a zygote from 1 set of parents
2^23 = 8 388 608 (number of combinations in 1 gamete e.g egg)
2^23 x 2^23 = 8 388 608 x 8 388 608 = 7.04 x 10^13
chromosome combinations in a zygote = number of combinations in the sperm cell x number of combinations in the egg
number of chromosome combinations in in organisms = (2^n)^2
genetic variation is created by
mutations
independent segregation of homologous chromosomes
crossing over between homologous chromosomes
random fertilisation of gametes
random fertilisation
any sperm can fertilise any egg
crossing over
what is it?
the exchange of alleles between homologous chromosomes, resulting in a mixture of parental characteristics in offspring.
process
homologous chromosomes line up during metaphase 1/meiosis 1
homologous chromosomes cross over forming a bivalent
chiasmata forms between non-sister chromatids
non-sister chromatids exchange lengths of DNA
forms recombinent chromatids with different sequences of alleles (same sequence of genes)
chromosome mutation/non-disjunction
change in quantity of DNA (number of chromosomes) caused by chromosome non-disjunction during meiosis (spontaneously occurs when homologous chromosomes/chromatids fail to separate during meiosis)
this results in:
a change in number of individual chromosomes - aneuploidy e.g downsyndrome is caused when there is an additional chromosome 21
change in number of chromosome sets- polyploidy e.g wheat is tetraploid (4n)
