3.4.3 genetic diversity as a result of mutations or mitosis

Question 1

mutations definition

Answer 1

a change in the arrangement of bases in an individual gene or in the structure of the chromosome

which changes the arrangement of genes

Question 2

frequency and repair of mutations

Answer 2

mutations can occur in gametes and somatic body cells

chance of mutation is between 2-30 x10-7

faulty DNA can be required by specific enzymes

unrepaired mutations will affect the new proteins being deleted

Question 3

describe what point mutations are

Answer 3

changes in an individual gene due to miscopying of one or more nucleotides

deletion or insertion of a nucleotide results in a FRAME SHIFT

Question 4

chrosomal mutations

Answer 4

gene deletion

Question 5

effects of mutations

Answer 5

+ production of new/superior protein = gain of reproductive advantage
+ silent mutation = no change
- production of inferior or no protein = fatal and or disease causing

Question 6

what are the causes of mutations?

Answer 6

1. X-rays
2. Ionising radiation
3. Chemicals

Question 7

how does the substitution of bases cause gene mutation?

Answer 7

nucleotide in a DNA molecule is replaced with another nucleotide that has a different base

effect of mutation is different if the new triplet code of bases still codes for the same amino acid as before
degenerate nature of genetic code

Question 8

how does the deletion of bases cause gene mutation?

Answer 8

a nucleotide is lost from the normal DNA sequence

usually, the amino acid sequence of the polypeptide is completely different so the polypeptide is now unlikely to function properly

one deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by one base as shown

Question 9

what is meiosis?

Answer 9

another form of cell division that produces gametes (sex cell, ovum or spermatozoan)

within human ovaries and testes, gametes are produced by meiosis

process halves the chromosome number

Question 10

process of meiosis

Answer 10

two divisions of each nucleus, each consisting of prophase, metaphase, anaphase, telophase

meiosis 1 = separates homologous pairs

meiosis 2 = separates chromatids

four haploid, genetically different gametes are produced

Question 11

describe what happens in meiosis 1 (first division - prophase 1)

Answer 11

chromosomes are condensed enough to make them very visible

homologous chromosomes pair up

each pair is called bivalent

when bivalents cross over;
- chromatids from each pair become twisted around each another
- tensions are created and portions of the chromatids break off
- reattach with the chromatids of its homologous partner = recombination
- point where crossing over occurs = chiasma
- one or more can occur along one length

Question 12

summary of meisois

Answer 12

genetic variation is created by independent segregation/assortment

as well as crossing over and recombination

results from random fusion of gametes during fertilisation

Question 13

how does meiosis bring about genetic variation?

Answer 13

1. independent segregation of homologous chromosomes
2. new combinations of maternal and paternal alleles by crossing over
3. random fertilisation of gametes

Question 14

how does independent segregation of homologous chromosomes create genetic variation?

Answer 14

meiosis 1: chromosomes line up alongside its homologous partner
align at random, one of each pair passes onto the daughter cell

randomisation of the pairing of homologous chromosomes, results in the combination of chromosomes of maternal and paternal origin that go into the daughter cell at meiosis 1 is due to chance
INDEPENDANT SEGREGATION

Question 15

how does independent segregation of homologous chromosomes form new genetic combinations?

Answer 15

each member of a homologous pair of chromosomes has exactly the same genes and therefore determines the same characteristics
alleles of these may differ

independent assortment therefore produces new genetic combinations

when the cells produced in meiosis are gametes these will be genetically different as a result of different combinations of maternal and paternal alleles they contain

Question 16

how does genetic recombination by crossing over cause genetic variation?

Answer 16

chromatids of each pair twist around each other, tensions are created and portions of the chromatids break off

these broken portions might then rejoin with the chromatids of its homologous partner
equivalent portions of homologous chromosomes are exchanged

new genetic combinations of maternal and paternal alleles are produced

chromatids cross over many times and broken-off portions of chromatids recombine

Question 17

what are the possible chromosome combinations following meiosis?

Answer 17

2n where n = the number of homologous chromosomes

Question 18

gene mutations

Answer 18

DNA is very stable but mistakes do occur during DNA replication before nuclear division by mitosis/meisosis

mutations are random and spontaneous

mutagens increase rate of mutations eg UV light

Question 19

mutations associated with mitosis

Answer 19

these are somatic mutations

not passed onto to offspring but may lead to cancer

Question 20

mutations associated with meiosis

Answer 20

can be inherited as they affect gametes

Question 21

types of gene mutations

Answer 21

1. base substitution = one base replaces another
2. base insertion/deletion = one or more bases are inserted or deleted from a length of DNA
   causes a frame shift

Question 22

degenerate defintion

Answer 22

more than one codon can result in same amino acids

Question 23

mis sense mutation

Answer 23

base pair substitution in a gene, which results in a single amino acid residue being changed for another in a polypeptide chain

eg sickle cell anaemia = glutamine is replaced by valine at one point in the beta chain of haemoglobin

Question 24

non sense mutation

Answer 24

caused by a substitution that changes an amino acid - specifying codon into a chain terminating (stop) codon

the polypeptide chain will be prematurely terminated

Question 25

what are the outcome of mutations?

Answer 25

mutations which cause a change in the amino acid sequence of a protein, which are usually harmful proteins
may lose function as an enzyme

very rarely, the change may be beneficial and the new allele may increase in frequency as a result of natural selection