4.3 - Genetic Diversity (Mutations + Meiosis) Flashcards
what is a gene mutation?
-a change in the base sequence of DNA (on chromosomes)
-can arise spontaneously during DNA replication (interphase)
-examples - base deletion
or substitution
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 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
1. meiosis I (first nuclear division) separates homologous chromosomes
○ chromosomes arrange into homologous pairs
○ crossing over between homologous chromosomes
○ independent segregation of homologous chromosomes
2. meiosis II (second nuclear division) separates chromatids
- outcome = 4 genetically
varied daughter cells
- daughter cells are
normally haploid (if
diploid parent cell)
describe how to draw a diagram to show the chromosome content of cells during meiosis
eg
● parent cell has 4 chromosomes = 2 homologous pairs
● these appear as X shapes due to DNA replication
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 mitosis occurs in a life cycle
-mitosis occurs between stages where chromosome number is maintained
○ eg. diploid (2n) → diploid (2n) OR haploid (n) → haploid (n)
how can you recognise where meiosis occurs in a life cycle
● meiosis occurs between stages where chromosome number halves
○ eg. diploid (2n) → haploid (n)
describe how mutations in the number of chromosomes arise
● spontaneously by chromosome non-disjunction during meiosis
● 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)
