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MEIOSISChapter 7 Section 1
MITOSIS VS. MEIOSISMitosis:
occurs because the cell gets too big and for healing.
produces genetically identical diploid daughter cells.
Meiosis: occurs in the production of gametes.
produces haploid daughter cells.
SOMATIC VS. GAMETES Somatic Cells –
They are all the cells in the body except for sperm and eggs
Always diploid!Produced by mitosis.
Gametes –In males – sperm/pollen
In females – eggsAlways haploid!Produced by meiosis.
GAMETESEach sperm and egg cell contains half the normal number of chromosomes – essential to sexually reproducing organisms.
When a egg is fertilized by the sperm, the offspring inherits one chromosome for each homologous pair from each parent.
It gains the normal number of chromosomes.
OVERVIEW OF MEIOSISMeiosis produces gametes through two stages of cell division.
Meiosis begins with a cell that has the normal number of chromosomes.
Due to the cell having completed the S (synthesis) phase of the cell cycle, the chromosome has replicated DNA.
G1 Phase
After S Phase
G2 Phase
OVERVIEW OF MEIOSIS Meiosis I: separation of homologous chromosomes – with each of the 2 new daughter cells get one chromosome from each pair.
Meiosis II: each chromosome is separated into two chromosomes, so that each of the four daughter cells get half the normal number of chromosomes
First reduction
Second reduction
Unreplicated chromosomes
PHASES OF MEIOSISDuring meiosis, two cell divisions – divided into 2 stages – stages I and II.Each stage, the cell undergoes the same phases as mitosis: prophase, metaphase, anaphase and telophase.
PHASES OF MEIOSISProphase I:
Nuclear envelope breaks down.
Homologous chromosomes pair up
Crossing over occurs.
CROSSING-OVERCrossing-over occurs in Prophase I after the homologous chromosomes pair up.
DNA strands from one chromosome join with the other, breaking off and piecing themselves back together.
END RESULT: each chromosome contains pieces of DNA from its homolog
RESULT OF CROSSING-OVEREach homolog has a slightly different set of alleles.
Crossing-over shuffles the genetic material so that each sister chromatid has a different combination of alleles.
Instead of the two original allele combinations, there are now four
RESULT OF CROSSING-OVERCrossing over makes it possible for offspring to inherit unique combinations of alleles.
This increases the genetic variation in the gene pool of a species.
PHASES OF MEIOSISMetaphase I:
Spindles attach to the centromeres of the homologues.
Homologous chromosomes align at the equator.
PHASES OF MEIOSISAnaphase I:
Spindles shorten
Homologous chromosomes separate
PHASES OF MEIOSISTelophase I:
Cytokinesis occurs.Two cells have formed.
At the end of telophase I, one parent cell has been divided into two new daughter cells with half the number of chromosomes – haploid cell.
PHASES OF MEIOSISProphase II:
Chromosomes condense
Spindle fibers form
Prophase I
Prophase II
PHASES OF MEIOSISMetaphase II:
Spindles attach to the centromeres of the sister chromatids.
Chromosomes align at the equator.
Metaphase I
Metaphase II
PHASES OF MEIOSISAnaphase II:
Spindle fibers shorten.
Sister chromatids separate.
Anaphase I
Anaphase II
PHASES OF MEIOSISTelophase II:
Chromosomes begin to unwind and stretch out.
Nuclear envelope reforms.
Cytokinesis occurs.
At the end of telophase II, two daughter cells have been divided into four new daughter that are haploid.
Telophase I
Telophase II
REVIEW TIME! Mitosis Meiosis I Meiosis II
Number of daughter cells
Chromosome number
Crossing-over occurs (yes/no)
Sister chromatids separate
Daughter cells genetically identical to parent cell
Daughter cells genetically identical to each other