Meiosis and Sexual Reproduction Chapter 10

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Meiosis and Sexual ReproductionChapter 10
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Outline

• Reduction in Chromosome Number
• Meiosis Overview
• Homologous Pairs
• Phases of Meiosis
• Meiosis I
• Meiosis II
• Meiosis Compared to Mitosis
• Genetic Variation
• Crossing-Over
• Independent Assortment
• Fertilization
• Human Life Cycle

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Discovery of Meiosis

• Meiosis was first observed by the Belgian
  cytologist Pierre-Joseph van Beneden in 1887
• Gametes (eggs and sperm) contain half the number
  of chromosomes found in other cells (haploid)
• The fusion of gametes is called fertilization
• It creates the zygote, which contains two copies
  of each chromosome (diploid)

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• Sexual reproduction
• Involves the alternation of meiosis and
  fertilization

• Asexual reproduction
• Does not involve fertilization

Contains two sets of chromosomes
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MeiosisHalves the Chromosome Number

• Special type of nuclear division
• Used only for sexual reproduction
• Halves the chromosome number prior to
  fertilization
• Parents diploid
• Meiosis produces haploid gametes
• Gametes fuse in fertilization to form diploid
  zygote
• Becomes the next diploid generation

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Homologous Pairs of Chromosomes

• In diploid body cells chromosomes occur in pairs
• Humans have 23 different types of chromosomes
• Diploid cells have two of each type
• Chromosomes of the same type are said to be
  homologous
• They have the same length
• Their centromeres are positioned in the same
  place
• One came from the father (the paternal homolog)
  the other from the mother (the maternal homolog)
• When stained, they show similar banding patterns
• Because they have genes controlling the same
  traits at the same positions

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Homologous Chromosomes
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The Stages of Meiosis

• Meiosis consists of two successive divisions, but
  only one DNA replication
• Meiosis I
• Separates the two versions of each chromosome
  (homologous chromosomes)
• Meiosis II
• Separates the two sister chromatids of each
  chromosome
• Meiosis halves the number of chromosomes

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Overview of Meiosis
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• Meiosis I (Reductional Division)
• Prophase I
• Nuclear membrane breaks down
• Homologous chromosomes pair up and exchange
  segments (crossing over)
• Metaphase I
• Homologous chromosome pairs align at random in
  the equatorial plane such that maternal or
  paternal member may be oriented toward either
  pole (independent assortment)
• Anaphase I
• Homologous chromosomes (each still consisting of
  2 chromatids) separate and move to opposite poles
• Telophase I
• Individual chromosomes gather together at each of
  the two poles
• Cytokinesis produces 2 daughter cells which are
  haploid

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Meiosis I
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Crossing Over

• Meiosis I
• Prophase I

• The longest and most complex stage of meiosis
• Homologous chromosomes undergo synapsis
• Pair up along their lengths

• Crossing over occurs

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Independent Assortment
Meiosis I Metaphase I
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Independent Assortment
Three chromosome pairs
23 combinations

• In humans, a gamete receives one homologue of
  each of the 23 chromosomes
• Humans have 23 pairs of chromosomes
• 223 combinations in an egg or sperm
• 8,388,608 possible kinds of gametes

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• Meiosis II
• After meiosis I there is a brief interphase
  (interkinesis)
• No DNA synthesis occurs
• Meiosis II is similar to mitosis, but with two
  main differences
• 1. Haploid set of chromosomes
• 2. Sister chromatids are not identical

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• Meiosis II

• Prophase II
• Brief and simple, unlike prophase I
• Cells have 1 member of each homologous pair
• Metaphase II
• Chromosomes line up at the equator
• Anaphase II
• Spindle fibers contract, splitting the
  centromeres
• Sister chromatids move to opposite poles
• Telophase II
• Nuclear envelope reforms around four sets of
  daughter chromosomes
• Cytokinesis occurs

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Meiosis I
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Meiosis II
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• Overview of meiosis
• 2 divisions, 4 daughter cells (not identical)
• Cells are diploid at beginning of meiosis
• Pairs of chromosomes are called homologues
  (homologous chromosomes)
• Meiosis I
• Homologues line up side by side at
  equator-synapsis
• When pairs separate, each daughter cell receives
  one member of the pair
• Cells are now haploid
• Meiosis II
• No replication of DNA occurs in this division
• Centromeres divide and sister chromatids migrate
  to opposite poles to become individual
  chromosomes
• Each of the four daughter cells produced has the
  haploid chromosome number and each chromosome is
  composed of one chromatid

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Genetic Variation Crossing Over and Independent
Assortment

• Meiosis I brings about genetic variation in two
  key ways
• Crossing over-exchange of segments of DNA between
  homologues (Prophase I)
• Independent assortment of chromosome pairs
  (Metaphase I)
• When homologues align at the metaphase plate
• They separate in a random manner
• The maternal or paternal homologue may be
  oriented toward either pole of mother cell
• Promotes genetic variability
• Both assure that gametes will contain different
  combinations of chromosomes
• When fertilization occurs, the resulting
  offspring will genetically unique

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• In comparison of meiosis to mitosis note that
• DNA replication occurs only once prior to both
• Meiosis requires 2 divisions, mitosis only 1
• Meiosis produces 4 daughter cells, mitosis
  produces 2
• Daughter cells from meiosis are haploid, those
  from mitosis are diploid
• Daughter cells from meiosis are genetically
  unique, while those from mitosis are genetically
  identical

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Comparing Meiosis and Mitosis

• Meiosis and mitosis have much in common
• However, meiosis has two unique features
• 1. Crossing over
• Homologous chromosomes pair all along their
  lengths in meiosis I and exchange pieces of DNA
• 2. Reduction division
• There is no chromosome duplication between the
  two meiotic divisions
• This produces haploid gametes

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Meiosis Compared to Mitosis
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Meiosis versus Mitosis

• Mitosis
• Requires one nuclear division
• Chromosomes do not synapse nor cross over
• Preserves chromosome number
• Produces two daughter nuclei
• Produces daughter cells genetically identical to
  parent and to each other
• Used for asexual reproduction, growth,
  development, and repair

• Meiosis
• Requires two nuclear divisions
• Chromosomes synapse and cross over
• Halves chromosome number
• Produces four daughter nuclei
• Produces daughter cells genetically different
  from parent and each other
• Used only for sexual reproduction

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Comparison of Mitosis and Meiosis
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Evolutionary Consequences of Sex

• Sexual reproduction increases genetic variation
  through three key mechanisms
• 1. Crossing over
• 2. Independent assortment
• 3. Random fertilization

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Random Fertilization

• The zygote is formed by the union of two
  independently-produced gametes
• Therefore, the possible combinations in an
  offspring
• 8,388,608 X 8,388,608
• 70,368,744,177,664
• More than 70 trillion!
• And this number does not count crossing-over

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Genetic Variation Significance

• Asexual reproduction produces genetically
  identical clones
• Asexual reproduction is advantageous when
  environment is stable
• Sexual reproduction produces genetically unique
  combinations
• However, if environment changes, genetic
  variability introduced by sexual reproduction may
  be advantageous
• Genetic diversity is the raw material that fuels
  evolution

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• The Human Life Cycle
• Requires both mitosis and meiosis
• The formation of gametes (eggs and sperm) is
  called gametogenesis.
• In females meiosis is part of the process of
  oogenesis
• In males meiosis is part of spermatogenesis
• At fertilization, the resulting zygote divides by
  mitosis for the processes of growth and
  development
• Mitosis is used for repair throughout life

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Life Cycle of Humans
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• Spermatogenesis
• Begins at puberty and continues throughout life
• Occurs in seminiferous tubules of testes
• Primary spermatocytes (2n) divide in meiosis I to
  form 2 secondary spermatocytes (1n)
• Secondary spermatocytes divide in meiosis II to
  produce 4 sperm

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• Oogenesis
• Occurs in the ovaries
• Primary oocyte (2n) divides in meiosis I to
  produce 1 secondary oocyte (1n) and 1 polar body
• Division is unequal as secondary oocyte receives
  most of the cell contents (nearly all cytoplasm
  and organelles) and half the chromosomes
• Allows ovum to have all the cellular machinery
  it needs for embryonic development
• Polar body functions only to receive half of the
  chromosomes
• Secondary oocyte begins meiosis II but stops at
  metaphase II polar body may also divide
• At puberty, after ovulation secondary oocyte is
  activated if fertilized to complete division
• Meiosis II produces 1 ovum and up to 3 polar
  bodies

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Spermatogenesis and Oogenesis
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• Summary
• Spermatogenesis and oogenesis both utilize
  meiosis
• Spermatogenesis begins at puberty and continues
  throughout life
• Spermatogenesis produces 4 sperm per primary
  spermatocyte
• Results in production of many sperm
• Oogenesis results in 1 oocyte and up to 3 polar
  bodies per primary oocyte
• Divisions are unequal, ovum receives most cell
  contents
• Oogenesis begins prior to birth, stops until
  puberty, then resumes in a cyclic pattern
• Cyclic release of oocytes continues until
  menopause when the process stops

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Human Chromosomes

• Human somatic cells have 23 pairs of chromosomes
• 22 pairs of autosomes
• Autosome-any chromosome other than a sex
  chromosome
• 1 pair of sex chromosomes
• XX in females
• XY in males

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Human Chromosomes

• Failure of chromosomes to separate correctly in
  meiosis I or II is termed nondisjunction.
• This leads to an abnormal number of chromosomes,
  or aneuploidy.
• Humans with one less autosome are called
  monosomics.
• These do not survive development.
• Humans with one extra autosome are called
  trisomics.
• The vast majority do not survive
• Trisomy for only a few chromosomes is compatible
  with survival
• However, there are severe developmental defects
• The only one compatible with a reasonable chance
  of survival is trisomy 21 (Down Syndrome).

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Down Syndrome
Most common trisomy in humans. Short stature,
eyelid fold, flat face, stubby fingers,, round
head, mental retardation 3 copies of chromosome
21 75 of cases- egg has 2 copies, sperm has
1 Can be detected by a karyotype
1 in 1,500 if mother is under 30
1 in 16 if mother is over 45
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• When nondisjunction occurs during meiosis I both
  members of a homologous pair migrate into the
  same daughter cell.
• When nondisjunction occurs in meiosis II, the
  centromere fails to divide and both daughter
  chromatids enter the same gamete.
• Egg with 24 chromosomes fertilized by sperm with
  23- trisomy
• 47 chromosomes in zygote
• Egg with 22 chromosomes fertilized by sperm with
  23 chromosomes- monosomy
• 45 chromosomes in zygote
• Normal development depends on the presence of
  exactly 2 of each kind of chromosome

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Nondisjunction of Chromosomes During Oogenesis
Followed by Fertilization with Normal Sperm
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Nondisjunction Involving Sex Chromosomes

• Aneuploidies of sex chromosomes have less serious
  consequences than those of autosomes
• Chances of survival are greatest if monosomy or
  trisomy involves the sex chromosomes
• However, they can lead to sterility

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Nondisjunction of the X Chromosome
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• Syndrome-disorders characterized by groups of
  symptoms.
• Turners syndrome
• Monosomy X (XO), zygote has one X chromosome and
  no other X or Y
• Capable of survival, phenotypically female,
  infertile
• Ovarian failure
• Klinefelter syndrome
• XXY
• Underdeveloped testes and prostate gland, no
  facial hair
• Phenotypically male, infertile
• Poly-X females
• More than 2 X chromosomes
• XXX females may be unusually tall
• XXXX females are usually severely retarded

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Turner and Klinefelter Syndromes
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• Nondisjunction of the Y chromosome
• Jacobs syndrome
• Due to nondisjunction in meiosis II
• Yields YY gametes and ultimately XYY zygotes
• XYY genotype can only result from nondisjunction
  in spermatogenesis
• Frequency of XYY is 1 in 800 males (live births)
• In general, these individuals are phenotypically
  normal
• Taller than average, persistent acne

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Syndromes from Abnormal Chromosome Numbers