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Meiosis
Key Terms
- Meiosis- Gametes- Recombination / Crossing Over- Chiasma- Independent Assortment
Chromosomes contain genetic information
Human somatic cells have 46 chromosomes
Divided into 23 pairs of homologous chromosomes, or homologs; one set of 23 from mother, one set of
23 from father.Specific traits are coded for in genes
Example: eye colour, hair colour, etc
Learning the Basics…
Each of your parents contribute to genes towards a specific trait, but they are different forms of the same gene, called alleles. Mom’s gene codes for blue eyes Dad’s gene codes for green eyes Both are alleles for the “eye colour
gene”
Recall:Mitosis involves cell division that
ensures both daughter cells receive a full set of chromosomes (diploid cells)
Sexual reproduction involves the “merging” of two gametes (sex cells) together (eg. sperm and egg)
Twice the chromosomes = New species?!?
Meiosis:
Division process that prevents this “doubling” of genetic material from occurring.
Creates gametes with half the number of chromosomes (haploid cells)
Fertilization is the fusion of 2 gametes (male + female) to get back to the diploid number
Stages of Meiosis
Prior to the start of meiosis, cell undergoes S Phase (chromosome replication)
Meiosis has two rounds of cell division:Meiosis IMeiosis II
Each round is divided into 4 sub phases:Prophase, Metaphase, Anaphase,
Telophase
Meiosis I
Interphase (S-phase)
Prophase I Chromosomes condense
& shorten, visible
Spindle fibres form
Centrioles moving towards poles of the cell
Homologous chromosomes form a tetrad made of 4 chromatids Process of forming a
tetrad is called synapsis
Tetrad of a homologous pair
Prophase I
In the tetrad, chromatids often break at the end and swap places with the sister chromatid
This process is called recombination or crossing over and allows for genetic variation
The location of recombination is called the chiasma
Recombination/Crossing Over:
Homologous pair of chromosomes(tetrad)
Metaphase I
Tetrads line up along equator randomly
= Independent Assortment
Spindle fibres attach to the pair of sister chromatids
Anaphase IThe sister chromatids do NOT
separate here (different than mitosis)Instead, the pairs of chromosomes
move apart to opposite poles
Telophase IChromosomes
condense slightly, nuclear membrane may form
Cytokinesis occurs forming two genetically different daughter cells
Each daughter cell has half the number of chromosomes as the parent cell (haploid - n)
Prophase I(early)
(diploid)
Prophase I(late)
(diploid)
Metaphase I(diploid)
Anaphase I(diploid)
Telophase I(diploid)
NucleusSpindlefibres
Nuclearenvelope
Meiosis I
Meiosis II
Meiosis II
Similar to mitosis, but no duplication of chromosomes during a very short interphase (no G1 or S phase)
Each chromosome (made of two "mixed" chromatids) lines up at equator, centromeres split, and each chromosome is pulled to opposite poles
End result – four haploid cells
Prophase II
Nuclear envelope begins to break down
Spindle fibres begin to form
Centrioles begin to move to poles
Metaphase II
Chromosomes align along equator of cell.
Spindle fibres attach to centromeres of sister chromatids
Anaphase II
Spindle fibres contract and pull sister chromatids apart
Telophase IINuclear
envelope assembles
Chromosomes decondense
Spindle disappears
Cytokinesis divides each cell into two
Results of Meiosis:
4 haploid gamete cells
1 copy of each chromosome
1 allele of each gene
Different combinations of alleles for different genes along the chromosome
Prophase II(haploid)
Metaphase II(haploid)
Anaphase II(haploid)
Telophase II(haploid)
Four Non-
identical haploid daughter
cells
Meiosis II
Sexual reproduction
Growth and repair
Role
At sexual maturityThroughout lifeWhen
Germline cellsSomatic cellsWhere
Half of parentSame as parentChromosome #
NoYesGenetically identical?
42Number of
daughter cells
21Number of
divisions
MeiosisMitosisVIDEO http://www.youtube.com/watch?v=kVMb4Js99tA
Chromosome Accounting
INSTRUCTIONS:o Complete the table below to show the number of chromosomes present
in different stages of cell division in a variety of species.
# of Chromosomes Present in Meiosisat Beginning of Each Phase
# ofChromosomesin Daughter
Cells ofMitosis
Diploid # Haploid # # of Pairs ofHomologous
Chromosomes Prophase I ProphaseII
TelophaseII
Cabbage18 18 9 9 18 9 9
Trillium5
Blackbear
38
Human23
Fruit Fly8
Peanut40