Meiosis Flashcards
What is a gamete?
haploid sex cell. Sperm = male (small) egg=female (big)
What does haploid (n) mean?
a cell with a single set of unpaired chromosomes (one of each homologous pair)
What does diploid (2n) mean?
A cell with two complete sets of chromosomes (both homologous pairs- one from each parent)
What does homologous pairs mean?
One chromosome from each parent. Same genes but different alleles
What happens in fertilisation? How does it increase genetic variation within species?
A haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes. Random fertilisation produces zygotes with different combinations of chromosomes to both parents. Mixing of genetic material increases genetic diversity within a species
What is meiosis?
Cell division that produces gametes. Involves two nuclear divisions resulting usually in the formation of four haploid daughter cells (genetically different from each other) from a single diploid parent cell. +crossing over and independent segregation
before meiosis (early and late interphase)
DNA unravels and replicates so there are two copies of each chromosome called chromatids
Early interphase (2n) - 6 chromosomes, 3 homologous pairs
Late interphase (2x2n) - 6 chromosomes, 3 homologous pairs
What happens in prophase 1
(2x2n) chromosomes condense and become visible, nuclear membrane breaks down
What happens in metaphase I?
(2x2n) homologous pairs line up at the equator, spindle fibres form attach to the centromeres
What happens in anaphase I?
(2x2n) homologous pairs separate and are pulled apart to opposite poles
What happens in telophase I?
Nuclei reform and cell divides (2xn and 2xn)
What happens in prophase II?
Nuclei break down (2xn and 2xn)
What happens in metaphase II?
Chromosomes line up at the equator. Spindle forms attaches to centromeres (2xn and 2xn)
What happens in Anaphase II?
sister chromatids separate and pulled apart to opposite poles (2xn and 2xn)
What happens in Telophase II?
(n, n , n , n) Nuclei reform. Cell divides. Four haploid cells genetically different to each other are produced.
What is crossing over of chromatids.
During metaphase of meiosis I, chromatids of homologous pairs cross over and swap alleles. Chromatids still contain same genes but now have a different combination of alleles
What is independent segregation of chromosomes?
When homologous pairs are separated it’s completely random which chromosome from each pair ends up in which daughter cell. So the four daughter cells have different combinations of chromosomes leading to genetic variation.
How to tell where meiosis happens in life cycle
Meiosis needed for sexual reproduction (produces gametes)
outcome of meiosis
four genetically different daughter cells, with half the chromosome number as the parent cell
Outcome of mitosis
two genetically identical daughter cells, with the same chromosome number as the original cell
