3.4.3 - Genetic diversity can arise as a result of mutation or during meiosis Flashcards
Topic 4
What is a gene mutation?
● A random change in the base sequence of DNA (on chromosomes)
● Can arise spontaneously during DNA replication (interphase)
What is a mutagenic agent?
A factor that increases rate of gene mutation, eg. ultraviolet (UV) light or alpha particles
Explain how a mutation can lead to the production of
a non-functional protein or enzyme
- Changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA
- So changes sequence of amino acids in the polypeptide
- So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
- So changes protein tertiary structure (shape) of protein
- Enzymes - active site changes shape so no longer complementary to substrate so substrate can’t bind, enzyme-substrate complex can’t form
Explain the possible effects of a substitution mutation
- DNA base / nucleotide (pair) replaced by a different base / nucleotide (pair)
- This changes one triplet so changes one mRNA codon
- So one amino acid in polypeptide changes
○ Tertiary structure may change if position of hydrogen / ionic /
disulphide bonds change
OR amino acid doesn’t change
○ Due to degenerate nature of genetic code (triplet could code for same
amino acid) OR if mutation is in an intron so removed during splicing
Explain the possible effects of a deletion mutation
- One nucleotide / base (pair) removed from DNA sequence
- Changes sequence of DNA triplets from point of mutation (frameshift)
- Changes sequence of mRNA codons after point of mutation
- Changes sequence of amino acids in primary structure of polypeptide
- Changes position of hydrogen / ionic / disulphide bonds in tertiary
structure of protein - Changes tertiary structure / shape of protein
Describe features of homologous chromosomes
Same length, same genes at same loci, but may have different alleles
Describe the difference between diploid and haploid cells
● Diploid - has 2 complete sets of chromosomes, represented as 2n
● Haploid - has a single set of unpaired chromosomes, represented as n
Describe how a cell divides by meiosis
In interphase, DNA replicates → 2 copies of each chromosome (sister chromatids), joined by a centromere
- Meiosis I (first nuclear division)
- Homologous chromosomes pair up and their chromatids wrap arond eachother (crossing over).
- By the end of this division, homologous pairs have separated, with one chromosome from each pair going into one of the two daughter cell. > independent segregation of homologous pairs. - Meiosis II (second nuclear division) separates chromatids
● Outcome = 4 genetically
varied daughter cells
● Daughter cells are
normally haploid (if
diploid parent cell)
Outcome of meiosis
● Outcome = 4 genetically
varied daughter cells
● Daughter cells are
normally haploid (if
diploid parent cell)
Explain why the number of chromosomes is halved during meiosis
Homologous chromosomes are separated during meiosis I (first division)
Explain how crossing over creates genetic variation
● Homologous pairs of chromosomes associate / form a bivalent
● Chiasmata form (point of contact between (non-sister) chromatids)
● Alleles / (equal) lengths of (non-sister) chromatids exchanged between chromosomes
● Creating new combinations of (maternal & paternal) alleles on chromosomes
Explain how independent segregation creates genetic variation
● Homologous pairs randomly align at equator → so random which chromosome from each pair goes into each daughter cell
● Creating different combinations of maternal & paternal chromosomes / alleles in daughter cells
Other than mutation and meiosis, explain how genetic variation within a
species is increased
● Random fertilisation / fusion of gametes
● Creating new allele combinations / new maternal and paternal chromosome combinations
Explain the different outcomes of mitosis and meiosis
- Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells
○ As 1 division in mitosis, whereas 2 divisions in meiosis - Mitosis maintains the chromosome number (eg. diploid → diploid or haploid → haploid)
whereas meiosis halves the chromosome number (eg. diploid → haploid)
○ As homologous chromosomes separate in meiosis but not mitosis - Mitosis produces genetically identical daughter cells, whereas meiosis produces
genetically varied daughter cells
○ As crossing over and independent segregation happen in meiosis but not mitosis
Explain the importance of meiosis
● Two divisions creates haploid gametes (halves number of chromosomes)
● So diploid number is restored at fertilisation → chromosome number maintained between generations
● Independent segregation and crossing over creates genetic variation
How can you recognise where meiosis and mitosis occur in a life cycle?
● Mitosis occurs between stages where chromosome number is maintained
○ Eg. diploid (2n) → diploid (2n) OR haploid (n) → haploid (n)
● Meiosis occurs between stages where chromosome number halves
○ Eg. diploid (2n) → haploid (n)
Describe how mutations in the number of chromosomes arise
● Occurs Spontaneously by chromosome non-disjunction during meiosis
● The Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis
● So some gametes have an extra copy (n+1) of a particular chromosome and others have none
(n-1)
Suggest how the number of possible combinations of chromosomes in
daughter cells following meiosis can be calculated
2n where n = number of pairs of homologous chromosomes (half the diploid number)
Suggest how the number of possible combinations of chromosomes
following random fertilisation of two gametes can be calculated
(2n)^2 where n = number of pairs of homologous chromosomes (half the diploid number)
