[9.2] meiosis and genetic variation Flashcards
what are the 2 steps of interphase, which must happen before meiosis 1?
- DNA which was previously coiled around histones unravels into single chromosomes
> DNA in this form is called chromatin - DNA is copied via semi-conservative replication so there is twice as much
> still in its unravelled form, chromatin
what are the names of the steps of meiosis?
- prophase I
- metaphase I
- anaphase I
- telophase I and cytokinesis
- prophase II
- metaphase II
- anaphase II
what happens during prophase I?
- chromosomes condense and pair up with its homologous partner
- crossing over also happens
the two chromosomes swap some genetic information so you have some maternal genes on the paternal chromosome and vice versa
what happens during metaphase I?
- spindle fibres capture chromosomes and move them towards the centre
- each chromosome is attached to microtubles from just one pole of the spindle
- homologous pairs, not individual chromosomes, line up
what happens during anaphase I?
- homologous chromosomes move to opposite poles
- sister chromatids remained attached
what happens during telophase I and cytokinesis?
cell divides to produce 2 haploid daughter cells, which enter meiosis II
these happen at the same time
what happens during meiosis II?
- prophase II: chromosomes condense, centrioles move apart and spindle fibres form
- metaphase II: chromosomes line up individually
- anaphase II: sister chromatids separate and are pulled to opposite poles
what is the product of meiosis?
four genetically different haploid daughter cells
how is meiosis a driver of variation?
- allows the formation of gametes (sperm or egg) with half the DNA
- when these fuse together they create a new diploid organism with a mixture of chromosomes from the mother and father
what is independent assortment?
describes how different genes independently separate from one another during meiosis I and II
- eg. Bb and Yy chromosomes initially.
- based on which direction they go to the centre in metaphase I, different combinations will be produced
- by, By, Yb, or BY could be produced
- when these chromosomes duplicate again in meiosis II, there will be 2 copies of each assortment in the haploid cells
describe the process of crossing over (6)
- maternal and paternal members of each homologous pair are positioned parallel to each other
- non-sister chromatids exchange DNA segments
- crossing over creates variation in chromosomes at the end of metaphase I
- sister chromatids are segregated to opposite poles during anaphase II
- each former chromatid is sorted into a different daughter cell
- four genetically different gametes are generated by this one crossover event
explain how crossing over can contribute to genetic variation (3)
- during cross over, sections of chromatids are exchanged
- this section contains different alleles
- a new chromatid is created with a new combination of alleles
what are the benefits of crossing over? (4)
- to reproduce
- to pass on a gene / allele
- possible adaptations which may be beneficial with changing environments
describe what happens to chromosomes in meiosis (6)
- chromosomes condense
- chromosomes associate in homologous pairs at middle of cell
- chromosomes cross over
- chromosomes join to spindle fibres via centromere
- homologous chromosomes move to opposite poles
- pairs of chromatids separated in 2nd division
in which 3 ways does meiosis created genetic variation?
- crossing over
- independent assortment in meiosis I
- independent assortment in meiosis II
