MEIOSIS

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MEIOSIS

• Chapter 10

Human sperm contacts an egg. Note the
difference in size.
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• Why must organisms reproduce?

Reproduction is not essential to the life of an
individual. However, it is essential for
species survival.
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• A. Types of Reproduction
• 1. Asexual Reproduction cell doubles its contents
  then divides into two identical individuals.
• involves only 1 parent
• offspring are genetically identical to the parent
  (clones)
• advantageous when organism is well-adapted to a
  stable environment
• Ex. binary fission, parthenogenesis
• Binary fission is common in single-celled
  organisms like amoeba.
• Parthenogenesis - reproduction without a mate
  eggs of female divide by mitosis to yield viable
  offspring. Common in insects (aphid, bees),
  salamanders, lizards, snakes, turkeys, roundworms
  flatworms.

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• 2. Sexual Reproduction
• involves 2 parents
• parents produce haploid (1n) gametes which fuse
  to form a diploid (2n) zygote (first cell of
  offspring).
• offspring are genetically diverse
• advantageous in a changing environment
• Haploid gametes are produced by a type of cell
  division called meiosis.

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• Sexual reproduction - the combination of genetic
  material from 2 individuals (parents) to create
  genetically diverse offspring.
• Haploid - nuclei contain only one set of
  chromosomes.
• Ex. Human gametes (eggs sperm) contain 23
  chromosomes.
• Diploid - nuclei contain two sets of chromosomes.
  Human body cells (somatic cells) contain 46
  chromosomes (two sets of 23).

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• B. Meiosis
• Type of cell division that halves the number of
  chromosomes.
• involves 2 nuclear divisions (meiosis I meiosis
  II)
• 1 diploid (2n) cell ? 4 haploid (1n) cells
• occurs in germ cells
• In humans, where would germ cells be located?
• In humans, germ cells would be located in the
  testes (male) ovaries (female)

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• Germ cell replicates its chromosomes in S phase
  of interphase.
• 1. Meiosis I (reduction division)
• Prophase I
• replicated chromosomes condense
• spindle apparatus forms fibers attach to
  chromosomes
• nucleolus disappears
• nuclear membrane breaks down
• synapsis crossing over occurs
• Meiosis I is called reduction division because it
  reduces the number of chromosomes (separates
  homologous chromosomes). Human germ cells have
  46 chromosomes, so at the completion of meiosis
  I, resulting daughter cells will have only 23
  chromosomes.

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• Synapsis - homologous chromosomes (homologs) pair
  up.
• Homologs - chromosomes that carry genes for the
  same traits in the same sequence.

A diploid human cell has 23 homologous pairs.
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• Homologs carry genes for the same traits (ie.
  hair color, eye color, finger length). However,
  they may carry different forms (alleles) of the
  same gene.
• For example
• Both chromosomes in diagram carry genes for the
  same traits (same letters of alphabet) however,
  each homolog carries a different version of the
  gene (upper case letters vs. lower case letters).
  A may code for black hair, while a codes
  for blond hair
• One chromosome of each homologous pair was
  inherited from the female parent (maternally
  derived), while the other chromosome was
  inherited from the male parent (paternally
  derived).

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• Crossing over - non sister chromatids of homologs
  exchange chromosomal material.

Chiasma - visible point of cross over between
homologs.
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• Crossing over helps to generate genetic diversity
  by mixing up parental traits. Since each homolog
  comes from a different parent, crossing over
  results in chromosomes that have some genes from
  the mother some genes from the father.

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Animal cell in prophase I
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• Metaphase I
• paired homologous chromosomes line up along
  equator of spindle

Note if this were mitosis rather than meiosis,
would see 4 chromosomes in line rather than 2
pairs of chromosomes in line.
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• Anaphase I
• homologous chromosomes separate move to
  opposite poles

Note centromeres do NOT part sister
chromatids remain attached.
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• Telophase I
• spindle apparatus breaks down
• chromosomes decondense
• nuclear membranes partially reform around two
  nuclei

Note these two cells are haploid because they
contain 1 rather than 2 sets of chromosomes
(one 1 one 2).
Cytokinesis usually occurs between meiosis I II
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• 2. Meiosis II (equational division)
• Prophase II
• chromosomes condense
• spindle apparatus forms fibers attach to
  chromosomes
• nuclear membrane breaks down
• Metaphase II
• chromosomes line up single-file along equator of
  spindle
• Meiosis II produces four cells from the two
  formed in meiosis I.

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• Anaphase II
• centromeres part sister chromatids (now called
  chromosomes) separate move toward opposite
  poles
• Telophase II
• spindle apparatus breaks down
• chromosomes de-condense
• nuclear membranes reform
• nucleoli reappear
• Cytokinesis divides two cells into four
  nonidentical cells.

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Meiosis II
Four non-identical haploid cells
Prophase II Metaphase II Anaphase II
Telophase II
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(No Transcript)
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• Differences between mitosis meiosis.
• Mitosis
• 1 nuclear division
• results in 2 diploid cells that are genetically
  identical
• no synapsis crossing over of homologs
• chromosomes align individually (metaphase)
• occurs in somatic cells
• occurs throughout life cycle
• used for growth, repair asexual reproduction
• Meiosis
• 2 nuclear divisions
• results in 4 haploid cells that are genetically
  diverse
• synapsis crossing over of homologs (during
  Prophase I)
• homologous pairs align (metaphase I)
• occurs in germ cells
• in humans, completed only after sexual maturity
• used for sexual reproduction

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• C. Gametogenesis in Humans
• Spermatogenesis (meiosis in the male)

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• Gametogenesis - formation of gametes (meiosis).
• Spermatogenesis occurs in seminiferous tubules of
  testes.
• Spermatogonia are (2n) germ cells. Have the
  ability to undergo mitosis (to replace
  themselves) meiosis (give rise to sperm). Will
  not undergo meiosis until puberty.
• 1. Spermatogonium (2n) grows, accumulates
  cytoplasm replicates its chromosomes. Is now
  called a primary (1o) spermatocyte (2n).
• Note several spermatogonia may be attached to
  each other by bridges of cytoplasm they may
  undergo meiosis simultaneously.
• 2. Primary spermatocyte divides by meiosis I to
  form two secondary (2o) spermatocytes (1n).
• 3. Secondary spermatocytes divide by meiosis II
  to form two spermatids (1n).
• 4. Spermatids undergo period of maturation (eject
  most of cytoplasm grow a flagellum) to form
  spermatozoa (sperm).

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• Structure of human sperm

• Head
• nucleus - 23 chromosomes
• acrosome - contains enzymes that help sperm
  penetrate egg
• Midpiece
• mitochondria
• Tail
• Sperm are stored in the epididymis until
  ejaculation.

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• 2. Oogenesis (meiosis in the female)

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• Oogenesis occurs in ovaries.
• 1. Oogonia are (2n) germ cells. Oogonia divide
  by mitosis up to the 3rd month post-conception.
  Before she is born, these oogonia grow,
  accumulate cytoplasm replicate their
  chromosomes, developing into primary (1o) oocytes
  (2n). At birth, she possesses about 1 million 1o
  oocytes in her ovaries. These oocytes began
  dividing (meiosis I), but they were arrested in
  Prophase I. only 400,000 remain at puberty
• 2. Beginning at puberty continuing until
  menopause, one or a few complete meiosis I,
  forming a large secondary (2o) oocyte a very
  small 1st polar body (unequal cytokinesis).
• 3. 2o oocyte begins dividing by meiosis II, but
  is arrested in Metaphase II. The 2o oocyte is
  ovulated enters oviduct.
• 4. Meiosis II will not be completed unless a
  sperm penetrates the 2o oocytes outer membrane.
  At completion of meiosis II, a large ovum small
  2nd polar body are formed. Sperm ovum nuclei
  fuse to form zygote.
• 1st polar body may or may not divide. All polar
  bodies degenerate. Thus, only 1 functional cell
  results (ovum).

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Comparison of Oogenesis Spermatogenesis
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• Oogenesis
• a woman is born with all the primary oocytes she
  is ever going to produce. when she is 40 years
  old, her oocytes are 40 years old
• oogenesis begins before birth, but is not
  completed until fertilization occurs (sperm
  stimulates completion of meiosis II).
• oogenesis is not a continuous process (halted
  twice - prophase I metaphase II).
• women ovulate over short period of lifetime
  (puberty to menopause). She will ovulate 400
  secondary oocytes in her lifetime.
• Spermatogenesis
• spermatogenesis begins at puberty is a
  continuous process (spermatogonium -gt sperm takes
  74 days).
• sperm are produced from puberty till death.