3.4.3 genetic diversity due to mutations or meiosis Flashcards
What is a gene mutation
Change in the base sequence of DNA (on chromosomes)
Production of a non-functional protein/enzyme
Change in base sequence of DNA
Changes sequence of codons on mRNA
Changes sequence of amino acids in primary structure of polypeptide
Changes position of HID bonds in tertiary structure of protein
Changes tertiary structure of protein (& active site if enzyme)
(If enzyme) substrate cannot bind to active site and form E-S complex
Base deletion
One nucleotide removed from DNA sequence
Changes triplet sequence from the point of mutation (frameshift)
Changes sequence of codons on mRNA after point of mutation
Changes sequence of amino acids in primary structure of polypeptide
Changes position of HID bonds in tertiary structure of protein
Changes tertiary structure
Base substitution
Nucleotide in DNA replaced with another nucleotide
Change in 1 base > changes one triplet
Changes one mRNA codon and one amino acid > sequence of amino acids in primary structure of polypeptide changes
OR
Due to the degenerate nature of the genetic code, the new triplet may still code for the same amino acid
the sequence of amino acids in the primary structure of the polypeptide remains unchanged
Mutagenic agents
Increases rate of gene mutation (above the rate of naturally occurring mutations)
E.g. ultraviolet light or alpha particles
Meiosis I
Before meiosis starts, DNA replicates so there are two copies of each chromosome (sister chromatids) joined by a centromere
Meiosis (first division) separates homologous pairs
- chromosomes arrange into homologous pairs
- crossing over (prophase I) creates genetic variation in gametes
- independent segregation (metaphase I) increases genetic variation in gametes
- 2n
Meiosis II
Separates chromatids by centromeres
Creates 4 haploid cells (from a single diploid parent cell) that are genetically varied
How meiosis creates genetic variation
Crossing over between homologous chromosomes
- alleles exchanged between chromosomes
- creates new combination of maternal and paternal alleles on chromosomes
Independent segregation of homologous chromosomes
- random alignment of homologous pairs at equator > random which chromosome from each pair goes to each daughter cell
- creates different combinations of maternal and paternal chromosomes and alleles in daughter cells
Random fertilisation when 2 gametes fuse to form a zygote
Importance of meiosis
Two divisions - creates haploid gametes (half number of chromosomes)
Diploid number restored at fertilisation
Maintains chromosome number from one generation to the next
Independent segregation and crossing over creates genetic variation
Chromosome non-disjunction
Homologous chromosomes fail to separate during meiosis I or sister-chromatids fail to separate during meiosis II
One gamete has an extra copy of the chromosome and the other has none
Upon fertilisation, zygote has one few or one extra chromosome
Arises spontaneously
Causes genetic diseases
importance of centromere
holds chromatids together
attaches chromatids to spindle
allows chromatids to be separated
Number of possible different combinations (assuming no crossing over)
2^n where n=number of pairs of homologous chromosomes/diploid number
Number of different combinations of chromosomes following the random fertilisation of two gametes
(2^n)^2
E.g. number of chromosomes = 12
12/2=6
(2^6)^2=4096
Mitosis v meiosis
Mitosis- produces diploid cells, Mei= haploid
Daughter cells genetically identical to each other and parent cell in mitosis, meiosis= daughter cells are genetically varied
Mitosis produces 2 daughter cells, meiosis 4 daughter cells
Life cycle of plants
Adults alternate
Meiosis before spore as involved in spore production
