Biol 121- 2010.01.29- Genetics- Asexual, Sexual Reproduction, Meiosis (Ch. 12).docx

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Biol 121 225
Freeman 243-55 (Ch. 12) Jan. 29, 10
Define... fertilization.
-process of uniting sperm and egg (in sexual reproduction [SR])
-cells in a newly growing offspring
-all the cells are products of mitotic divisions from the first nucleus that formed at
-reproductive cells: sperm or egg
-haploid (1n) 1 set of chromosomes
Define meiosis
-nuclear division that leads to a halving of chromosome number
-precedes formation of eggs and sperm in animals
Sex chromosome, autosome
-sex chromosome is the chromosome associated w/ sex of the individual
-autosomes are non-sex chromosomes
Homologous chromosomes
-“same proportion (same size + shape)
-are pairs of sex chromosomes or autosomes
-also known as homologs
-homologous chromosomes carry the same genes (but may have diff. alleles)
Define gene
-section of DNA that influences one or more hereditary traits in an individual
-different versions of the same gene
-number and types of chromosomes present
-have two versions of each type of chromosome
-diploid organisms have two alleles of each gene one on each of the homologous pairs
of chromosomes
Chromosome number in
-chromosome number in each cell is reduced during meiosis
-results in haploid cells
Brief summary of meiosis I
-the homologs in each chromosome pair separate from each other
-one homolog goes to one daughter cell; the other homolog goes to the other daughter
-therefore, diploid (2n) becomes (n) in daughter cells of M1
Brief summary of M II.
-sister chromatids from each chromosome separate, each going to one daughter cell
-cells produced by M II also have one of each type of chromosome, but now the
chromosomes are unreplicated
-when two gametes fuse during fertilization, a full complement of chromosomes is
-the cell that results from fertilization is diploid and is called a zygote
-homologous chromosomes are referred to as either maternal or paternal in origin
Synapsis and tetrads
-during early prophase I, homologous chromosome pairs come together
-this pairing process is called synapsis
-happens because regions of homologous chromosomes that are similar at the molecular
level attract one another
-structure that results from synapsis is called a tetrad
-a tetrad consists of 2 homologous chromosomes, with each homolog consisting of two
sister chromatids
-the chromatids from the homologs are non-sister chromatids
-plural of chiasma
-during late prophase I, non-sister chromatids begin to separate at many points along
their length but stay joined in certain locations and look like they cross over one another,
forming an X shape
-normally, >1 chiasmata in every pair of homologous chromosomes
-chromatids involved in chiasma formation are homologous but not sisters
-paternal and maternal chromatids break and rejoin as each chiasma, producing
Biol 121 225
Freeman 243-55 (Ch. 12) Jan. 29, 10
chromatids that have both paternal and maternal segments crossing over
-resulting chromosomes have mixture of maternal and paternal alleles
Meiosis I
1) Early prophase I
-replicated chromosomes condense
-spindle apparatus forms
-nuclear envelope disappears
-synapsis of homologs form pairs of homologous chromosomes (tetrads)
-spindle fibres attach to kinetochores at centromeres of chromosomes
2) Late Prophase I
-crossing over results in a mixing of chromosome segments from maternal and paternal
3) Metaphase I
-pairs of homologous chromosomes (tetrads) migrate to metaphase plate and line up
4) Anaphase I
-homologs separate and begin moving to opposite ends of cell
5) Telophase I
-homologs finish moving to opposite sides of cell
-in some species, a nuclear envelope re-forms around each set of chromosomes
-when meiosis I is complete, the cell divides
Meiosis II
1) Prophase II
-spindle apparatus forms
-nuclear envelope breaks apart (only if it was re-formed at end of M I)
2) Metaphase II
-replicated chromosomes, consisting of 2 sister chromatids, are lined up at metaphase
3) Anaphase II
-sister chromatids separate
-unreplicated chromosomes that result begin moving to opposite sides of the cell
4) Telophase II
-chromosomes finish moving to opposite sides of cell
-nuclear envelope forms around each haploid set of chromosomes
-in plants, products of meiosis do not form gametes instead, the haploid cells that
result form reproductive cells called spores
Key difference between
mitosis and meiosis
-homologous chromosomes pair early in meiosis, but not during mitosis
-therefore, they can migrate to the metaphase plate together and then separate during
anaphase of meiosis I, resulting in a reduction division
Define asexual reproduction
and sexual reproduction
-A.R. refers to any mechanism of producing offspring that does not involve fusion of
-S.R. refers to the production of offspring through the fusion of gametes
-offspring of asexual reproduction
-exact copies of their parent
Genetic recombination
-any change in the combination of alleles on a given chromosome
Genetic variation due to
steps in meiosis
-when pairs of homologs line up during M I and homologs separate, and variety of
combinations of maternal and paternal chromosomes can result
-each daughter cell gets a random assortment of maternal and paternal chromosomes
-in crossing over, new combinations of alleles on the same chromosome are produced
combinations that did not exist in either parent
-this phenomenon is known as recombination