MEIOSISMEIOSIS
MeiosisMeiosis
Division of sex cells, called gametes
MeiosisMeiosis is sexual reproductionsexual reproduction
Humans have 46 chromosomes
Numbers of Chromosomes
Diploid= 2 sets (2n) Haploid= 1 set (n)
Meiosis vs Mitosis
Meiosis is the division of gametes (sex cells)
Mitosis the division of somatic cells
There are two cell divisions during meiosis and only for mitosis!
Gametes
Sperm and egg
Fertilization
When sperm (n) and egg (n) fuse to make a zygote (2n)
A zygote is the product of the fusion of an ovum (egg) and a sperm. It contains two copies of each chromosome, one from each parent. Egg and sperms cells, on the other hand, each contain only one copy of each chromosome. The zygote develops into an embryo.
Sister Chromatids
Exact copies of DNA
Homologous Chromosomes
Pair of chromosomes have 2 chromatids each, with exact same genes on each
Homologous pairs (tetrads) are 2 chromosomes paired together
Homologous Chromosomes Each locus, where alleles or traits
are located, are in the same position on all chromosomes
Homologous Homologous ChromosomesChromosomes
Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are similar in shape and size.
Homologous pairs (tetrads) (tetrads) carry genes controlling the same inherited traits.
Each locuslocus (position of a gene) (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous homologous chromosomes.chromosomes.
a. 22 pairs of autosomesautosomesb. 01 pair of sex chromosomessex chromosomes
Phases of Meiosis
Meiosis I (four phases)Meiosis I (four phases)
Cell division Cell division that reduces the chromosomechromosome number by one-half.one-half.
four phasesfour phases:
a.a. prophase Iprophase I
b.b. metaphase Imetaphase I
c.c. anaphase Ianaphase I
d.d. telophase Itelophase I
Interphase Interphase Chromosomes replicate during the
S phase of the cell cycle.
Each duplicated chromosome consists of 2 sister chromatids
atttached at their centromere.
Interphase Interphase
Centrioles duplicate
Chromosomes are visible
Interphase Interphase
NucleusNucleus and nucleolusnucleolus visible.
nuclear membrane
nucleolus
cell membrane
chromatin
Prophase 1Prophase 1
Chromosomes condense.
Synapsis occurs when 2 homologous chromosomes are paired (Which is a tetrad)
Prophase IProphase I - - SynapsisSynapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
Prophase IProphase I
centriolesspindle fiber
asterfibers
Prophase 1 Prophase 1
Crossing Over may occur between chromatids of different chromosomes in a tetrad.
Crossing Over - Crossing Over - variation variation
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
Metaphase IMetaphase I
TetradsTetrads align in the middle of the cell.
INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Mixes genetic info from maternal and paternal
chromosomes 2. Leads to different combinations of
chromosomes
Metaphase IMetaphase I
metaphase plate
OR
metaphase plate
Independent Assortment
Anaphase IAnaphase I
Homologous chromosomes Homologous chromosomes separate and move towards the poles.
Sister chromatids Sister chromatids remain attached at their centromerescentromeres.
Anaphase IAnaphase I
Telophase ITelophase I
Each pole now has haploidhaploid set of chromosomeschromosomes.
CytokinesisCytokinesis occurs and two haploid daughter cells are formed.
Telophase ITelophase I
Meiosis IIMeiosis II
No interphase II No interphase II (or very short - no more DNA DNA replicationreplication)
Remember:Remember: Meiosis II Meiosis II is similar to mitosismitosis
Prophase IIProphase II
Metaphase IIMetaphase II
metaphase platemetaphase plate
Anaphase IIAnaphase II
sister chromatids separate, sister chromatids separate, daughter chromosomes move daughter chromosomes move to different ends of the cellto different ends of the cell
Telophase IITelophase II
Nuclei form.
CytokinesisCytokinesis occurs.
Four haploid daughter cells produced.Four haploid daughter cells produced.
gametes = sperm or egggametes = sperm or egg
Telophase IITelophase II
VariationVariation
Important to population as the raw Important to population as the raw material for material for natural selectionnatural selection..
Question:Question:
What are the three sexual What are the three sexual sources ofsources of
genetic variation? genetic variation?
Answer:Answer:
1. crossing over (prophase I)1. crossing over (prophase I)
2. independent assortment 2. independent assortment (metaphase I)(metaphase I)
3. random fertilization3. random fertilization
Remember: variation is good!
Question:Question: A cell containing 20 chromosomes20 chromosomes
(diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?
Answer:Answer:
10 chromosomes (haploid)10 chromosomes (haploid)
KaryotypeKaryotype A method of organizing the A method of organizing the chromosomes chromosomes
of a cell in relation to number, size, and of a cell in relation to number, size, and type.type.
Karyotype
Karyotype
Diploid= 2 copies
Triploid= 3 copies
Polyploid= more than 2 copies