Meiosis Review Flashcards
what is meiosis
- form of cell division by which gametes: with 1/2 the number of chromosomes are produced
- meiosis is sexual reproduction with 2 divisions
(SPERM-23)+(EGG-23)=(ZYGOTE-46)
Sex cells divide to
produce gametes (sperm or egg), gametes have 1/2 the number of chromosomes,
where does meiosis occur
in the gonads (testes or ovaries)
male (sperm) process
spermatogenesis- production of 4 haploid sperm cells
female (ovary) process
oogenesis- production of one egg and 3 polar bodies that die and are reabsorbed/recycled by the body
INTERPHASE 1
- chromosomes replicate in S phase
- one duplicated chromosome=2 identical sister chromatids attached at their centromeres
- centriole pairs also replicate
INTERPHASE 1: nucleus and nucleolus visible
Process of meiosis
- cell division that reduces the chromosome number by 1/2: the four phases
- prophase 1
- metaphase 1
- anaphase 1
- telophase 1
(general) PROPHASE 1
- longest and most complex phase (90%)
- chromsomes condense
- synapsis occurs (formation of tetrad with 2 homologus chromsomes)
tetrad= 2 chromosomes/ four chromatids (sister and non-sister chromatids)
Non-sister chromatid homologs
- homologs contain DNA that codes for the same genes. but different versions of those genes
- genes occur at the same loci
Sister Chromatids vs Homologs Chromosomes
Sister: exact replicas
Homlogus: not exact replicas-regions code for the same gene
Homologus Chromosomes
- pair of chromsomes: maternal and paternal that are similar in shape and size
- homologs pairs (tetrads) carry the genes controlling the exact same region on the homologus
how many pairs of homologus chromsomes do humans have
22- autosomes pairs
1- sex chromosome pair
= 23 total
Crossing Over- PROPHASE 1
- crossing over MIGHT occur between non-sister chromatids at chiasmata
- Crossing Over: segments of non-sister chromatids break and re-attach to the other chromatid
- Chiasmata are where chromosomes touch and exchange genes (CROSSING OVER)
- Causes genetic recombination
GENERAL EVENT MEIOSIS 1
-homologs separate
Prophase 1
- nucleus and nucleolus disappear
- spindle forms
- chromosomes coil and synapsis (pairing) occurs
- tetrads form and crossing over occurs
Metaphase 1
- shortest phase
- tetrads align on the equator
- independent assortment occurs: chromosomes separate randomly causing genetic recombination
In terms of independent assortment, how many different combinations of sperm could a male produce
ANS: using 2^n
2n=46 (Number of human chromsomes)
n=23, 2^23= around 8 million
Anaphase 1
- homologous chromsomes separate and move towards the poles
- sister chromatids remain attached at their centromeres
Telophase 1
- each pole now has a haploid set of chromosomes
- cytokinesis occurs and 2 haploid daughter cells are formed
Meiosis 2 main event
- sister chromatids separate
- no interphase 2 (or its very short)
- dna replication
- MEIOSIS 2 IS SIMILAR TO MITOSIS
Prophase 2
- same as prophase in mitosis
- nucleus and nucleolus disappear, chromosomes condense, spindle fibres form
Metaphase 2
- same as metaphase in mitosis (chromosomes not homologous line up at the equator now)
Anaphase 2
- same as anaphase in mitosis
- sister chromatids separate
Telophase 2
- same as telophase in mitosis
- nuclei. and nucleoli reform, spindle disappears
- cytokinesis occurs
- 4 haploid daughter cells are produced=GAMETES
Genetic recombination/variation
- raw material for natural selection
- ensure all organisms are not alike
- strongest “most fit’ traits survive and are passed along
OCCURS THROUGH:
- crossing over (prophase 1)
- independent assortment (metaphase 1)
- random fertilization
A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?
10 chromosomes (haploid=n)
20 chrom as diploid=40 haploid
40/2=20 first phase
20/2=10 second phase
Karyotypes
- organized picture of chromosomes arranged in pairs by size from largest to smallest
- pairs 1-22= autosomes
- last pair= sex chromsomes
What is fertilization?
- the fusion of a sperm and egg to form a zygote
- a zygotę is a fertilized egg
A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce how many cells containing chromosomes?
10 haploid cells
chromatid=haploid
40/2=20 first phase
20/2=10 second phase
how does meiosis involve genetic variation
independent assortment, crossing over, and random fertilization
