Sex and Division

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Sex and Division

• Sexual eukaryotic organisms combine their DNA to
  produce progeny
• To do this they must prepare their DNA for
  combination with the DNA of another individual
• This process used to prepare the DNA is called
  meiosis

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Sex and Diploidy (2N)

• Sexual organisms combine their genetic
  characteristics by separating and subsequently
  recombining their genes.
• Sex cells ( gametes) are formed they have 1 set
  of chromosomes, and are thus 1n or haploid.
• When they recombine the new cell (the zygote) has
  both sets one from the mother, one from the
  father. It is 2n or diploid.
• There are two homologous chromosomes of each
  chromosome type in a 2n organism. One comes from
  the mother, one from the father.

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Sexual Reproduction Provides Greater Variability
in the Genotype

• Improves ability to adapt to the environment
• 2n organisms do not merely separate their
  homologs during preparation of the sex cells.
• The homologs come together they synapse.
• They form a synaptonemal complex, in which there
  is breakage and crossover of genetic material
  between chromatids of the two homologs.
• The resulting homologs are no longer the same as
  the parental homologs they have a mixture of
  genes, which produces a sex cell that is somewhat
  different in genetic complement from either
  parent.
• Thus, when it is joined with another
  complementary sex cell during fertilization, the
  resulting zygote has a different genetic
  complement from either parent.

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The Synaptonemal Complex

• DNA is precisely cut and recombined
• The synaptonemal complex holds the chromatids of
  the homologs close together until crossover is
  complete

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Crossover Chiasmata

• The region of crossover forms an X-shaped
  structure.
• This X-shape is called a chiasma (from the Greek)
  (chiasmata, plural)

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The First Meiotic Division

• Premeiotic interphase DNA, centriole replication
• Prophase I Chromosome condensation synapsis and
  crossover
• Metaphase I Movement toward the equator
• Anaphase I Separation of duplex homologs
• Telophase I Reformation of nuclear envelope
  cytokinesis
• Interkinesis No DNA synthesis otherwise like
  interphase

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Second Meiotic Division

• The second meiotic division (Meiosis II) proceeds
  exactly as if it were mitosis
• Prophase II
• Chromosomes condense again
• Metaphase II
• Chromosomes move to equator, with centromeres
  lined up on equator
• Anaphase II
• Sister chromatids separate
• Telophase II
• Nuclear envelopes reform
• Haploid (1n) daughter cells gametes
• Now ready for fertilization process

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The 1st and 2nd Meiotic Divisions Compared
Homologs separate in MI Then chromatids
separate in MII
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Second Meiotic Division and Polar Body Formation

• The crossed-out pictures in the next slide refer
  to the loss of a line in the female
• In the formation of ova (nonfertilized eggs) only
  one of the daughter cells lives from a division.
  This is because the division is highly asymmetric
  (not through the center of the cell) and one of
  the resulting cells, the polar body, is very
  small
• This seems to be a mechanism to conserve
  resources by concentrating it in the cell that
  will undergo development into an embryo

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Meiosis II in Females Is Different
Becomes 1st polar body in female
So
ONLY ONE OVUM
So
Becomes 2nd polar body in female
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Mitosis Meiosis Compared

• Mitosis is most like Meiosis II
• Chromatids separated
• Singlet chromosomes produced

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Life Cycles

• Different organisms time their sexual stages
  differently
• Females produce ova via oogenesis males produce
  sperm via spermatogenesis, but both are haploid
  gametes.
• Fusion of the 1n gametes is fertilization, always
  results in a 2n zygote, which develops into
  embryo
• In animals, formation of the 1n cell forms a
  gamete, which is fused to form the zygote which
  is the main form of the life form called
  diploplontic life cycle
• Many simpler organisms, however do not form 1n
  gametes immediately sometimes the 1n form is the
  main form of life for that organism true in
  protists called haploplontic life cycle
• Plants can vary this theme alternate between 1n
  (gametophyte) and 2n (sporophyte) generations in
  subsequent divisions called alternation of
  generations

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Life Cycles Pictorial Depiction

• Timing of the formation of the zygote varies in
  the life of the organism
• 2n stage length varies in plants can be very
  complex

Diploplontic Haploplontic Alternation
of generations
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Chromosome Alterations
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Types of Chromosome

• Chromosomes are placed into broad categories
  depending on the position of the centromere.
• metacentric centromere in the middle, with arms
  of equal length.
• telocentric centromere at one end, with only 1
  arm.
• acrocentric centromere near one end, with arms
  of very different lengths
• sub-metacentric centromere near the middle, with
  arms of slightly different lengths.

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Types of Chromosome
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Variations in Chromosome Number

• The suffix -ploidy refers to the number of
  haploid chromosome sets. Thus, haploid 1 set,
  diploid 2 sets, triploid 3 sets,etc.
• The suffix -somy refers to individual
  chromosomes. Thus, trisomy having 3 copies of
  a chromosome, and monosomy having 1 copy of a
  chromosome. Down syndrome, the most common from
  of mental retardation in humans, is caused by
  trisomy-21, 3 copies of chromosome 21.

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Aneuploidy

• In general, organisms need a balanced number of
  chromosomes equal numbers of each chromosome.
  This condition is called euploid.
• If the organism is supposed to be diploid but
  instead has a different number of chromosome sets
  (such as triploid), it is abnormal euploid.
• Having an extra chromosome (trisomic) or missing
  a chromosome (monosomic) is very bad, usually
  lethal. The chromosomes in this case are
  unbalanced, not equal numbers of all types. This
  condition is called aneuploid.

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Abnormal Euploidy

• Most diploids dont survive as haploids, because
  they are usually heterozygous for recessive
  lethal alleles. Similarly, making an organism
  homozygous at most genes (through repeated
  matings between close relatives) is usually
  lethal. Heterozygosity helps diploid organisms
  cope with different environmental conditions.

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Parthenogenesis

• Parthenogenesis means producing offspring from
  unfertilized eggs. If the egg cells have not
  undergone meiosis, the offspring are diploid.
  Some fish and shrimp reproduce by
  parthenogenesis. It is generally not very
  successful over the long term, because there is
  no way to remove randomly occurring mutations.
• Many plants can reproduce vegetatively, by taking
  a cutting from the plant body and causing it to
  develop roots. This ability makes it very easy
  to develop unusual genetic lines of plants they
  never have to undergo meiosis and fertilization.
  For instance, commercial potatoes are propagated
  vegetatively, through eyes on the tubers.

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Triploids

• Triploid organisms are usually sterile.
  Triploidy is a common way of making seedless
  fruit, such as in watermelons. Recall that the
  seed is a multicellular organism, many cell
  divisions after fertilization.
• The reason triploids are sterile can be found in
  metaphase and anaphase of meiosis 1. Homologues
  pair up in metaphase of M1, then they are pulled
  to opposite poles in anaphase.
• In triploids, there are 3 members to each set of
  homologues. They line up as triples at
  metaphase. In anaphase, 1 homologue goes to the
  upper pole, and one homologue goes to the lower
  pole. The third homologue goes randomly to
  either pole.
• The result is that each cell after M1 has 1 copy
  of some chromosomes and two copies of other
  chromosomes. This is an aneuploid condition,
  which nearly always results in dead embryos.
• In humans, triploid fetuses are the result of
  dispermy, fertilization of an egg by two sperm
  simultaneously. Triploid humans usually die
  before or just after birth. About 15 of
  spontaneous abortions are due to triploidy.

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Polyploids

• Any abnormal euploid condition above triploid can
  be called polyploid.
• Polyploidy can arise in two ways
• 1. autopolyploidy all of the chromosome sets
  come from the same species.
• 2. allopolyploidy the chromosome sets come
  from two or more different species.

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Autopolyploidy

• Autopolyploidy generally results from a failure
  in meiosis, which gives diploid sperm and egg
  cells.
• To produce autopolyploids, the meiotic spindle
  can be inhibited with the drug colchicine.
• Autopolyploids are often large and healthier that
  the original diploids. Thus, autopolyploids are
  commonly found in fruits and vegetables. For
  instance, commercial chrysanthemums and daylilies
  are usually tetraploid.

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Allopolyploidy

• Allopolyploidy means having chromosome sets from
  two or more species. The species must be closely
  related, and there needs to be some mechanism for
  keeping the chromosomes from different species
  from pairing with each other.
• Many commercial grains are allopolyploids. For
  instance, wheat is a hexaploid, with genomes from
  3 different grass species. Hybridization between
  these species occurred naturally in two stages,
  several thousand years apart, in what is now
  Turkey. Humans recognized the tetraploid and
  hexaploid types as valuable, and propagated them.
• Raphanobrassica is an allotetraploid created from
  a cross between a radish and a cabbage. The
  Russian geneticist Karpechenko did this in 1928,
  hoping to produce a plant with the root of a
  radish and the top of a cabbage. Unfortunately,
  he got approximately the opposite.

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Aneuploid Organisms

• Aneuploidy is the result of non-disjunction in
  meiosis. Non-disjunction is the failure of
  chromosomes to go to opposite poles in meiosis.
• Non-disjunction results in aneuploid gametes and
  embryos. It can occur in either M1 or M2.
• In humans, the rate of non-disjunction rises
  rapidly with the age of the mother. This is a
  leading cause of Down syndrome, trisomy-21.

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Maternal Age Effect on Non-Disjunction
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Some Human Aneuploidies

• A normal human has 46 chromosomes, which can be
  designated 46, XX or 46, XY
• The sex chromosomes are the most tolerant of
  aneuploidy, due to X chromosome inactivation.
• Klinefelter syndrome 47, XXY. The Y makes these
  people male, but they have a female pattern of
  body hair and they usually develop breasts.
  Treatment with testosterone alleviates most
  symptoms.

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Turner Syndrome

• Turner syndrome 45, X. Often written as XO.
  They have only one sex chromosome, an X. No Y
  means they are female, but they lack ovaries and
  are thus sterile. Also, they dont produce the
  surge in estrogen that causes body changes at
  puberty, although this can be treated with
  hormones. Interesting changes in spatial
  perception have also been noted.

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Other Sex Chromosome Abnormalities

• 47, XYY. Male, usually tall, acne-ridden, and
  slightly sub-normal in intelligence. Once
  thought to confer criminality, but this has
  been disproven.
• 47, XXX. Female, with normal intelligence and
  only occasional fertility problems. Usually not
  detected except by accident.

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Autosomal Aneuploidies

• Approximately 2 of sperm cells are aneuploid,
  with all possible extra and missing chromosomes
  occurring in equal numbers.
• However, only 3 trisomies (and no monosomies)
  occur frequently enough to have a named syndrome.
• 47, trisomy-21, Down syndrome, is the most
  common. People with Down syndrome are mentally
  retarded, with characteristic thick bodies and
  tongues, along with heart defects that used to
  kill most of them at an early age. They often
  get Alzheimers Disease at an early age.

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Other Autosomal Aneuploidies

• Trisomy-13, Patau syndrome, results in severe
  cleft palate the facial bones fail to close
  during fetal life. Average life span 6 months.
  Can result in a cyclops, a person with only 1
  eye in the middle of the forehead.
• Trisomy-18, Edwards syndrome. Multiple defects
  in many organs, unusual clenched fist, average
  life 3 months.

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Mosaics and Chimeras

• A mosaic is an organism which is derived from a
  single fertilization but which contains cells
  with two or more different chromosome
  compositions. For instance, it is possible to be
  46,XY / 45,X. Some cells are normal male (XY)
  cells, while others are Turner syndrome female
  cells. This is caused by chromosome loss or
  non-disjunction in one of the first few mitoses
  of a newly formed embryo.
• A chimera is an organism which is composed of two
  genetically different organisms, which have fused
  together. Usually seen as a person with blood
  cells from a fraternal twin whose body was
  absorbed a person with two different blood
  types.