Genetics PCB 3063

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Genetics - PCB 3063

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• Problem set
• Please turn in today.
• Todays focus
• Meiosis
• We will focus on one major question today
• 1. How does meiosis and the chromosome theory of
  heredity explain Mendelian genetics?

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Chromosomes and Ploidy

• Two terms are used to describe sets of eukaryotic
  chromosomes haploid and monoploid.
• The haploid chromosome set is the set of
  chromosomes present in normal gametes.
• This set is usually represented by the symbol
  N.
• For humans, the haploid set is 23.
• The monoploid chromosome set is a non-redundant
  set of chromosomes.
• Thus, one of each type of chromosome will be
  present in the monoploid set.

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Chromosomes and Ploidy

• The monoploid chromosome set is a non-redundant
  set of chromosomes.
• Thus, one of each type of chromosome will be
  present in the monoploid set.
• The monoploid chromosome set is typically
  represented as X.
• The number of monoploid sets present in somatic
  cells is the ploidy of an organism.
• For diploid (2X) organisms, the haploid set is
  the same as the monoploid set.
• This is not true for polyploids.

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Meiosis Reductive Cell Division

• Mendelian genetics requires the segregation of
  alleles to gametes.
• Remember that peas are diploid, so the rules we
  have been discussing apply to diploids. The
  patterns of inheritance in polyploids will
  differ.
• In March we will return to the issue of
  polyploidy and discuss its role in evolution.
• The segregation of different genes into gametes
  occurs independently.
• The chromosome theory of heredity postulates that
  genes are located on chromosomes.
• Meiosis, the specialized form of cell division
  that produces gametes, explains Mendelian
  genetics in the context of the chromosome theory
  of heredity.

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Meiosis and Mitosis

• Like mitosis, meiosis involves DNA replication
  followed by cell division.
• Unlike mitosis, meiosis
• Involves two rounds of cell division, producing
  four cells (or nuclei) rather than two cells.
• Does not result in the transmission of all
  chromosomes present in the parent cell to all
  progeny cells.
• The two cell divisions of meiosis are called
• Meiosis I - the reductive division which results
  in the segregation of chromosomes.
• Meiosis II - the equal division of duplicated
  chromosomes.

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The Process of Meiosis

• During the interphase that proceeds meiosis, the
  DNA is replicated.
• Thus, for a diploid organism like peas or humans,
  the DNA content increases from 2X to the
  equivalent of 4X.
• Then two cell divisions occur
• The stages of the cell division have names
  identical to the similar processes in mitosis.
• Although some of the specific processes are
  slightly different.
• During meiosis I, the chromosomes have been
  replicated, but only one (duplicated) copy of
  each chromosome is passed on to each progeny
  cell.
• However, the chromosomes are modified by
  recombination during meiosis I.
• At this point, the DNA content is equivalent to
  2X - however it is not the same 2X as the
  diploid genome in the original cell.

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The Process of Meiosis

• During the interphase that proceeds meiosis, the
  DNA is replicated.
• Thus, for a diploid organism like peas or humans,
  the DNA content increases from 2X to the
  equivalent of 4X.
• Then two cell divisions occur
• During meiosis I, the chromosomes have been
  replicated, but only one (duplicated) copy of
  each chromosome is passed on to each progeny
  cell.
• However, the chromosomes are modified by
  recombination during meiosis I.
• At this point, the DNA content is equivalent to
  2X - however it is not the same 2X as the
  diploid genome in the original cell.
• During meiosis II, duplicated chromatids (single
  copies of each chromosome) are passed on to the
  progeny cells.
• This reduces the DNA content back to 1X for
  diploid organisms.

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Meiosis Segregates Chromosomes

• Imagine a diploid organism with two chromosomes
• For this organism, NX2 and 2N2X4.
• If we use a notation for chromosomes similar to
  that we have been using for genes, we can
  designate one of the chromosomes A and the other
  B.
• Since this organism is a diploid, we will
  designate one of the chromosomes with A and a
  and the other with B and b.
• This assumes that the copies of the same
  chromosome can be distinguished, e.g. by the
  presence of a gene with different alleles.
• Thus, during meiosis, we see

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Meiosis I and Meiosis II Resemble Mitosis

• Meiosis I is responsible for segregation of
  chromosomes into gametes in way that resembles
  Mendels first law.
• However, it is not immediately apparent why genes
  show independent segregation unless they are on
  different chromosomes.
• Meiosis I occurs after the duplication of
  chromosomes and can be divided into four stages,
  just like mitosis
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

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Meiosis I and Meiosis II Resemble Mitosis

• Meiosis I occurs after the duplication of
  chromosomes and can be divided into four stages,
  just like mitosis
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Meiosis II occurs after an interphase that does
  not involve DNA replication.
• The stages are given the same names as meiosis I
  and mitosis, although they are designated
  Prophase II and so forth to distinguish them.

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Meiosis IComparison to Mitosis

• However, specific aspects of each phase differ
  between meiosis I and mitosis
• Prophase I - the chromosomes condense and become
  visible. Unlike mitosis, the homologous
  chromosomes pair.
• The paired chromosomes are called bivalents.
• Since the paired chromosomes have been
  duplicated, they are made up of four chromatids,
  collectively called a tetrad.
• The physical pairing of homologous chromosomes is
  called synapsis. This pairing involves the
  alignment of genes on each chromosome.
• Regions of homologous chromosomes are exchanged
  at points of contact called chiasmata.

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Meiosis IComparison to Mitosis

• Metaphase I - the bivalents align along the
  metaphase plate.
• Each chromosome is attached to the spindle
  fibers.
• Anaphase I - Chiasmata dissociate. Unlike
  mitosis, the centromeres do not divide, instead
  the bivalents are pulled to the poles of the
  cell.
• This is fundamentally different from mitosis,
  since it reduces the set of chromosomes present
  in the cells.
• Since there has been exchange between
  chromosomes, even genes on the same chromosome
  will segregate independently.
• Telophase I - Nuclear envelopes reform and the
  daughter cells will enter meiosis II.
• The number of chromosomes has been reduced but
  they have been duplicated.

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Meiosis IIComparison to Mitosis

• After the segregation of chromosomes (as
  bivalents) during meiosis I, there is an
  interphase without DNA replication that is
  followed by meiosis II
• Prophase II - the chromosomes condense and become
  visible again.
• At this point the nuclei are haploid but each
  chromosome is duplicated (since replication
  occurred prior to meiosis I).
• The duplicated chromosomes consist of two sister
  chromatids.
• Metaphase II - chromosomes align along the
  metaphase plate.

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Meiosis IIComparison to Mitosis

• After the segregation of chromosomes (as
  bivalents) during meiosis I, there is an
  interphase without DNA replication which is
  followed by meiosis II
• Prophase II - the chromosomes condense and become
  visible again.
• Metaphase II - chromosomes align along the
  metaphase plate.
• Anaphase II - the sister chromatids separate and
  migrate to opposite poles of the cell.
• Thus, there exactly one copy of each chromosome
  will be in each nucleus.

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Meiosis andMendelian Genetics

• Telophase II - nuclear envelopes reform in each
  meiotic product.
• The products of meiosis are collectively called a
  tetrad.
• Note The four chromatids present in the
  bivalents that are aligned on the metaphase plate
  in metaphase of meiosis I are also called a
  tetrad. Dont confuse the chromatids in this case
  with the products of meiosis. In general, I will
  use tetrad for the products of meiosis.

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Meiosis andMendelian Genetics

• Lets look at some images
• http//www2.sandi.net/uchs/AP.BIOLOGY/1997-98/Meio
  sis/Meiosis2.html
• Now lets look at movies of this process
• Mendels laws flow naturally from the process of
  meiosis
• The segregation of alleles into gametes reflects
  the segregation of chromosomes during meiosis I.
• The existence of multiple chromosomes and the
  mixing of chromosomes during meiosis I generates
  the phenomenon of independent assortment.

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Meiosis is different fordifferent sexes and
species.

• In animals, the production of sperm involves
  generating four functional spermatozoa while the
  production of eggs involves generating one egg
  cell and three non-functional polar bodies.
• These differences reflect equal or unequal
  cytokinesis.
• In plants, the fate of cells produced by meiosis
  can be quite variable
• In angiosperms, there are few haploid cells.
• Pollen is generated from microspore mother cells
  which undergo meiosis to generate tetrads of
  microspores. Each microspore divides once to form
  a tube cell and a generative cell. Either before
  or during germination, the generative cell
  divides forming two sperm. The tube nucleus
  supports the pollen tube while the two sperm
  nuclei fertilize the female gametophyte.

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Meiosis in Plants.

• In angiosperms, there are few haploid cells.
• Within the ovule, the megaspore mother cell
  divides meiotically generating four megaspores,
  three of which disintegrate. The fourth megaspore
  develops into the female gametophyte, which at
  maturity is a structure with seven cells and
  eight nuclei. This structure is also called the
  embryo sac.
• Angiosperms are characterized by the process of
  double fertilization. Double fertilization
  involves the formation of a zygote by syngamy of
  one sperm and the egg, coupled with the formation
  of triploid endosperm due to the fusion of the
  other sperm nucleus with two polar nuclei.
• In some cases, the polar nuclei fuse prior to the
  fertilization (as shown in your textbook on page
  70) but in many cases they do not.
• This process is part of the alternation of
  generations typical of plants.

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Alternation of Generations

• In plants, there is a multicellular haploid
  generation that produces gametes.
• This generation is called the gametophyte.
• There is also a diploid generation that is called
  the sporophyte.
• Similar terminology is used in the algae.
• If the haploid and diploid generations have a
  similar appearance - true for most red algae,
  many green algae, and a few brown algae - the
  life cycle is exhibits an isomorphic alternation
  of generations.
• If the sporophyte and gametophyte exhibit
  substantial differences, the life cycle exhibits
  a heteromorphic alternation of generations.
• So angiosperms exhibit a heteromorphic
  alternation of generations in which the
  sporophyte is much larger than the gametophyte.

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Alternation of Generations

• In plants, there is a multicellular haploid
  generation that produces gametes.
• This generation is called the gametophyte.
• There is also a diploid generation that is called
  the sporophyte.
• Similar terminology is used in the algae.
• If the haploid and diploid generations have a
  similar appearance - true for most red algae,
  many green algae, and a few brown algae - the
  life cycle is exhibits an isomorphic alternation
  of generations.
• If the sporophyte and gametophyte exhibit
  substantial differences, the life cycle exhibits
  a heteromorphic alternation of generations.
• Other groups of plants are different - e.g., the
  bryophytes also show heteromorphic alternation of
  generations but the gametophytes are large and
  sporophytes are small.

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Other Patterns of Meiosisin Different Organisms

• Fungi exhibit other patterns in their life
  cycles.
• The budding yeast Saccharomyces cerevisiae can
  proliferate as either a diploid or haploid.
• This organism have two mating types (designated
  a and a)
• When (haploid) cells of different mating types
  are close to each other, they recognize the other
  mating type using pheromones.
• The haploid cells undergo syngamy and form a
  zygote, which can then proliferate as a diploid
  cell.

• Diploid yeast undergo sporulation under
  nutritional stress. The tetrad of meiotic progeny
  form in a sac called the ascus.

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Other Patterns of Meiosisin Different Organisms

• Fungi exhibit other patterns in their life
  cycles.
• Fungi that form spores in an ascus are called
  ascomycetes.
• The spores are called ascospores.
• The nearly equal division of time between haploid
  and diploid phases is not typical of all
  ascomycetes.

e.g., Neurospora
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Other Patterns of Meiosisin Different Organisms

• Neurospora crassa (an ascomycete) grows almost
  exclusively as a haploid.
• This organism have two mating types (designated
  A and a)
• N. crassa is much more complex than yeast, and it
  forms female sexual structures called
  protoperithecia.
• Both mating types can form these structures.
• Upon fertilization with a nucleus of the opposite
  mating type, the zygotes will immediately undergo
  meiosis and form tetrads of ascospores.
• The asci form inside a fruiting bodies called
  perithecia.
• There is substantial diversity in life cycles for
  other organisms.