Meiosis and genetic variation Flashcards
What are gametes?
Sperm cell = male
Egg cell = female
When do gametes join together?
During fertilisation to form a zygote which divides and develops into a new organisms
What number do normal body cells have?
Diploid number (each cell has 2 chromosomes)
What number do gametes have?
Haploid number (1 chromosome)
How is a cell with a diploid number formed?
A haploid sperm fuses with a haploid egg
What does it mean for fertilisation to be random?
Random fertilisation produces zygotes with different combinations of chromosomes. Mixing of genetic material increases the genetic diversity within a species
What is meiosis?
A type of cell division
Where does meiosis take place?
In the reproductive organs
What does meiosis produce?
4 non-identical daughter cells with half the number of chromosomes
What happens before meiosis starts?
The DNA unravels and replicates so there are 2 copies of each chromosome called chromatids. The DNA condenses to form double armed chromosomes. Sister chromatids are joined in the middle by a centromere.
Explain meiosis stage I
Prophase I - DNA supercoils and chromosomes become visible. Nuclear membrane breaks down and homologous pairs line up. Non sister chromosomes join up and trade sections. Centromeres migrate and spindles form.
Metaphase I - homologous chromosome line up in middle of cell. Random assortment takes place
Anaphase I - homologous chromosomes attach to spindles which contract to pull the chromosomes apart
Telophase I - nuclear membranes reforms and chromosomes disperse. Cytokinesis begins
Explain meiosis stage II
Prophase II - chromosomes recondense and centromeres migrate. Nuclear membranes disperse
Metaphase II - chromosomes migrate to equator and spindles form
Anaphase II - spindles contract and chromosomes migrate
Telophase II - nuclear membrane reforms. Chromosomes disperse and cytokinesis completes cycle
What does crossing over of chromatids in miosis I mean?
Each of the 4 daughter cells formed contains chromatids with different alleles
Explain the process of crossing over
- chromosomes of homologous pairs come together
- chromatids cross over by twisting around each other and swapping bits with each other
- 1 chromosome from each homologous pair ends up in each cell
- each cell has a different chromatid and therefore a different set of alleles which increases the genetic variation
What is independent segregation of chromosomes?
When homologous pairs are separated in meiosis it is completely random meaning that the 4 daughter cells have completely different combinations of those maternal and paternal chromosomes. This leads to genetic variation
