Mutation and meiosis Flashcards
Why do mutations have such a big effect?
- change in base sequence of DNA
- change in base sequence of mRNA
- change in amino acid sequence
- change in hydrogen/ionic/disulphide bonds
- change in protein function
When do mutations occur?
usually during DNA replication (interphase)
What are mutations?
a change in the sequence of DNA bases in an individual gene, or in the structure of a chromosome which changes the arrangement of genes
If mutations occur in somatic (body) cells are they passed onto offspring?
No, somatic cells are not found in the reproductive cells (testes or ovaries)
What are the 3 main types of mutation?
- substitution - a base is replaced with another base
- addition - a base is inserted
- deletion - a base is removed
What are the 3 possible consequences of a substitution mutation?
nonsense mutation - results in a premature STOP codon - polypeptide production stops prematurely
mis-sense mutation - a different amino acid is coded for so base sequence changes
silent mutation - no change in base sequence due to the degenerate code
What is the process of meiosis?
- DNA replicates, meaning that sister chromatids are joined at the centromere
- random assortment of chromatids following crossing over
- cell splits in half, leading to independent segregation of homologous chromosomes, ‘reduction division’ - number of chromosomes per cell reduces by half
- “mitotic” division during meiosis 2, chromatids separate to form 4 haploid daughter cells, each containing half the number of chromosomes of the diploid parent cell
why is it important that the chromosome number is halved in meiosis? (gametes are haploid)
because the diploid number is restored during fertilisation. If the gametes were not haploid then then chromosome number would double with each new generation
What is crossing over?
- when homologous chromosomes line up on the equator during prophase of meiosis 1, they can sometimes get tangled
- the DNA forms a chiasma as it is broken and rejoined differently
- when they pull apart, blocks of genes may get swapped between chromosomes, leading to a new combination of alleles
How do meiosis and sexual reproduction lead to genetic variation?
- crossing over
- independent segregation of homologous chromosomes
- random assortment of chromatids
- fertilisation - random fusion of gametes
what does cell division by meiosis give rise to?
gametes (sex cells)
are daughter cells identical in meiosis?
they are not identical to either the parent cell or each other - they are genetically unique
what is independent assortment?
When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other.
What are the differences between meiosis and mitosis?
- meiosis has 2 divisions, mitosis has 1 division
- meiosis produces 4 daughter cells, mitosis produces 2 daughter cells
- meiosis is genetically variable, mitosis is genetically identical
what is non-disjunction?
the failure of homologous chromosomes to separate properly during meiosis, often eggs fuse during fertilisation to form a trisomy 21 containing 3 chromosomes
