Mitosis and Meiosis

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Mitosis and Meiosis
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Chromosomes
Sister Chromatids

• Chromosomes are made up of DNA which carries the
  cells genetic information
• Human body cells have 46 chromosomes (in 23
  pairs)
• Sex cells (gametes aka egg and sperm) only have
  23 chromosomes
• Before cell division occurs, each chromosome
  replicates (or copies itself)

Centromere
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Cell Growth and Division

• Cell Cycle is a series of events that cells go
  through as they grow, prepare for division and
  divide into two identical daughter cells
• Two major stages in a cells life are
• Interphase
• M Phase or cell division

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Interphase

• Interphase is the phase where the cell grows and
  prepares to divide.
• Interphase is divided into three phases
• G1 Phase - growth
• S Phase replication (copying) chromosomes
• G2 Phase further growth
• A lot is happening during G1, S and G2 phases!

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M Phase

• M Phase is cell division which includes two
  steps
• Mitosis- this is division of the nucleus
• Cytokinesis this is division of the cytoplasm

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Mitosis

• Mitosis is divided into four phases
• Prophase chromosomes condense, becoming
  visible centrioles move to opposite sides of
  nucleus nuclear envelope dissolves
• Metaphase chromosomes line up across middle of
  cell
• Anaphase sister chromatids split
• Telophase - sister chromatids move to opposite
  sides of cell and new nuclear envelopes form

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Cytokinesis

• Cytokinesis is occurring at the same time as
  telophase
• The cell membrane pinches the cytoplasm into two
  equal parts.
• Two new daughter cells are now formed and are
  genetically identical to the parent cell.
• This is the end of the cell cycle.

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MEIOSIS

• Meiosis is process where the number of
  chromosomes per cell is divided in half by
  separating homologous chromosomes in a diploid
  cell.
• Gametes
• are produced by meiosis.
• What does all that mean???

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• Chromosomes are made up of DNA which carries the
  cells genetic information
• Genes are found on the chromosomes.
• Genes are chemical factors that determine traits.
  (Ex. Eye color, hair color, height, etc.)
• Different forms of a gene are called alleles
• (Ex. Blue or brown eyes, brunette or auburn)
• The symbol N represents the number of
  chromosomes in a gamete (sex cell)

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• Homologous chromosome means that each chromosome
  that comes from the male has a corresponding
  chromosome from the female
• Homologous chromosome have DNA segments, or
  genes, for the same trait

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• Diploid cells have both sets of homologous
  chromosomes (one set from momma and one from the
  babys daddy)
• All cells in your body (except egg and sperm) are
  diploid
• These cells are called somatic cells
• 2N

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• Haploid cells have half the number of chromosomes
  as their parent cells
• Gametes (egg and sperm) are haploid
• Gametes are genetically different than the parent
  cell and one another
• N

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N
N
N
2N
N
N
N
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• Human has 46 chromosomes. How many chromosomes
  in human gametes?
• 23
• Pineapple has 25 chromosomes in its gametes.
  What is its haploid number?
• 25
• A giraffe has a diploid number of 62. What is
  its haploid number?
• 31

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Draw this!
46
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First Step

• Replication
• During meiosis, chromosomes are replicated once
  in S phase (just like mitosis) into sister
  chromatids.
• This occurs so that each of the 46 chromosomes
  exists as a duplicated sister chromatid.

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Meiosis I

• HOMOLOGUES (1-23) are segregated into different
  cells.
• Results in 2 cells that each have 1 copy of each
  of the 23 chromosomes (still duplicated from DNA
  replication).

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Prophase I

• Crossing over
• Homologous chromosomes in the cell pair up
  alongside each other lengthwise forming a tetrad
• Swap bits and pieces of their chromosomes,
  shuffling the genome
• Results in NEW Chromosomes that did not exist
  before in the parent

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Meiosis II

• SISTER CHROMATIDS of each of the 23 chromosomes
  are pulled apart just as they are in mitosis
• Results in 4 cells that have 1 copy of each of
  the 23 chromosomes (now unduplicated again)
• Now officially haploid

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Meiosis occurs in two stages

• Meiosis I
• Two cells form with sets of chromosomes and
  alleles that are different from each other
• Meiosis II
• Cells divide again without DNA replication
  forming four cells with half the number of
  chromosomes of the parent cell

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Summary of Mitosis/Meiosis

• Mitosis
• Results in 2 identical cells that are diploid
  (2N) and are exactly like the parent cell
• Meiosis
• Results in 4 cells that have half the number of
  chromosomes, haploid (N), as the parent cell and
  are genetically different from one another
• Genetic Recombination

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Meiosis Square Dance
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• Linked Genes - Genes located closely together are
  linked
• Crossing over is more likely to occur between
  genes that are far apart rather than linked which
  are close together.
• Polyploidy The occurrence of one or more extra
  chromosomes in an organism.
• This is not good in animals but is beneficial in
  plants (crop plants are stronger and bigger)