Meiosis Flashcards
When does meiosis occur?
in sex cells for sexual reproduction producing gametes
What is gametogenesis?
formation of haploid gametes
What is fertilisation?
fusion of gametes to form a diploid zygote
How many stages of meiosis are there?
2
How does meiosis result in genetic variation?
crossing over during late prophase 1
independent assortment of chromosomes during metaphase 1
random fertilisation of gametes produced by genetically different parents
How does crossing over occur?
homologous chromosomes pair up to form bivalents
adjacent paternal and maternal chromatids cross over
thus when chromosomes separate during anaphase 1, chromatids carry a combination of paternal and maternal genes
What are points of crossing over in prophase 1 called?
Chiasmata
What does independent assortment during metaphase 1 result in?
Gametes having different combinations of paternal and maternal chromosomes
Why does random fertilisation result in genetic variation?
gametes are produced by genetically different parents
all gametes have an equal probability of being involved in fertilisation
What are the stages of meiosis?
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Cytokinesis
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
How many daughter cells are made from one parent cell in meiosis?
4 gametes
What happens to the DNA, what does this mean?
DNA replication happens once
2 divisions therefore results in haploid daughter cells
What id the diploid number of chromosomes in humans?
46 (23 pairs)
What is the haploid number of chromosomes in humans?
23
What happens during prophase 1 in meiosis?
chromosomes condense and attach to the nuclear envelope
synapsis occurs (pair of chromosomes line up close together) a tetrad composed of 4 chromatids is formed.
genetic recombination via crossing over may occur
the centrioles migrate away from one another and both the nuclear envelope and nucleoli break down
What stage does a cell enter in meiosis after prophase 1?
metaphase 1
What happens during metaphase 1 of meiosis?
chromosomes from mother and father pair up (homologous pairs)
tetrads align at the metaphase plate (on the equator of the spindle)
independent assortment occurs due to random alignment and division
What stage happens after metaphase 1 of meiosis?
anaphase 1
What happens during anaphase 1?
chromosomes move to opposite cell poles (microtubules interact to pull chromosomes to opposite ends of poles
sister chromatids remain together after homologous chromosomes move to opposite poles (unlike mitosis)
WHat stage happens after anaphase1 of meiosis?
Telophase 1
What happens during telophase 1?
chromosomes uncoil and lengthen
spindle fibres break down
nuclear envelope reforms
nucleolus reappears
What occurs after telophase 1 in meiosis?
cytokinesis
What happens during cytokinesis?
the division of the cytoplasm
What happens in meiosis 2 in prophase 2?
the centrioles separate and organise a new spindle at right angles to the old spindle
What happens during metaphase 2 of meiosis?
chromosomes line up on the equator, each chromosome attached to a spindle fibre by its centromere
Independent assortment occurs as chromatids of chromosomes can face either pole
What happens during Anaphase 2 of meiosis?
the centromeres divide and the spindle fibres shorten, pulling the chromosomes to opposite poles
What happens during Telophase 2 of meiosis?
at the poles, the chromatids lengthen and can no longer be distinguished in the microscope. The spindle disintegrates and the nuclear envelope and nucleoli reform
What does cytokinesis after telophase 2 of meiosis form?
4 haploid daughter cells (all genetically different)
What happens before meiosis 1 that does not happen before meiosis 2?
DNA replication, the DNA is copied and doubled before meiosis 1
crossing over of chromosomes
What are the differences between metaphase 1 and 2?
metaphase 1 aligns homologous pairs either side of the equator
indecently assorts homologous chromosomes
metaphase 2 aligns chromosomes on equator
independently assorts chromatids
What are the differences between anaphase 1 and anaphase 2 of meiosis?
separation at anaphase 1 forms chromosomes
2 daughter cells
separation at anaphase 2 forms chromatids
4 daughter cells
(all daughter cells are haploid)
