Chapter 13 Meiosis and Sexual Life Cycles

1
Chapter 13Meiosis and Sexual Life Cycles
2
I. Terms

• Heredity Continuity of biological traits from
  one generation to the next. Transmission of
  genes from parents to offspring results in
  similarities among closely related individuals
• Variation Inherited differences among
  individuals of the same species
• Genetics The scientific study of heredity and
  variation

3

• Genes and Chromosomes
• Genes Units of hereditary information that are
  made of DNA and are located on chromosomes
• Specific nucleotide sequences
• Most program cells to synthesize specific
  proteins which code for individual traits
• Locus Specific region of DNA on a chromosome
  that corresponds to a gene
• Chromosome A single DNA molecules that is
  highly folded and coiled along with a protein.
  Each chromosome contains many genes

4
II. Types of Reproduction

• Asexual
• Single individual is the sole parent
• Single parent passes all of its genes to
  offspring
• Parent and offspring are genetically identical
• Sexual
• Two parents give rise to offspring
• Each parent passes on half of its genes to
  offspring
• Offspring have a unique combination of genes
  inherited from both parents
• Results in greater genetic variation

5
III. Sexual Life Cycles

• Terms
• Life Cycle Sequence of stages in an organisms
  reproductive history, from conception to
  production of offspring
• Somatic Cell Any cell other than gametes (sperm
  or egg)

6

• Karyotype - A display or photomicrograph of an
  individuals somatic-cell metaphase chromosomes
  arranged in a standard sequence

7

• Homologous Chromosomes A pair of chromosomes
  that have the same size, centromere position and
  staining pattern
• Autosomes carry same genetic loci
• XY sex chromosomes carry different genetic loci

8

• Autosome a non-sex chromosome (22 pairs in
  humans)
• Sex Chromosomes X, Y chromosomes that determine
  an individuals sex

9

• Diploid Condition in which cells contain two
  sets of chromosomes abbreviated as 2n
• Haploid Condition in which cells contain one
  set of chromosomes gametes abbreviated as n

10

• Gamete A haploid reproductive cell sperm/ova
• Fertilization The union of two gametes to form
  a zygote
• Zygote A diploid cell that results from the
  union of two haploid gametes gives rise to all
  other cells by mitosis

11
B. Types

• Animal In animals, including humans, gametes
  are the only haploid cells
• Meiosis occurs during gamete production. Gametes
  undergo no further cell division prior to
  fertilization
• Fertilization produces a diploid zygote that
  divides by mitosis to produce a multicellular
  animal

12

• Plant Alternate between multicellular haploid
  and diploid generations
• Alternation of generations
• Diploid stage sporophyte Meiosis produces
  haploid spores
• Haploid spores divide mitotically to generate
  haploid gametophyte
• Haploid gametophytes produce gametes by mitosis
• Fertilization produces a diploid zygote ?
  sporophyte

13
(No Transcript)
14
IV. Meiosis

• Stages of Meiosis I
• Interphase I (precedes meiosis)
• Chromosomes replicate ? sister chromatids with
  centromere
• Centriole pairs in animal cells replicate into
  two pairs
• Prophase I (90 of meiosis)
• Chromosomes condense
• Synapsis Homologous chromosomes come together
  as pairs
• Homologous pair in synapsis appears as a tetrad
• Non sister chromatids linked by X-shaped
  chiasmata, sites where homologous strand exchange
  (crossing over) occurs
• Centriole pairs move, spindle microtubules form
• Nuclear envelope and nucleoli disperse

15
(No Transcript)
16

• Metaphase I Tetrads are aligned on metaphase
  plate homologues are aligned, kinetochores
  point towards opposite poles
• Anaphase I Homologues separate and are moved
  towards the poles by spindles
• Sister chromatids remain attached at their
  centromeres
• Homologues separate
• Telophase I/Cytokinesis
• The spindle apparatus continues to separate
  homologous chromosome pairs until the chromosomes
  reach the poles
• Each cell contains haploid set of chromosomes -
  sister chromatids still attached by centromere
• Cytokinesis usually occurs simultaneously with
  telophase I cleavage furrow or cell plate forms
• In some species nuclear membrane and nucleoli may
  reform

17
(No Transcript)
18
B. Stages of Meiosis II

• Prophase II Spindle apparatus forms and
  chromosomes move toward metaphase II plate
• Metaphase II Chromosomes align singly on
  metaphase plate, kinetochores of sister
  chromatids point towards opposite poles
• Anaphase II Sister Chromatids separate
• Telophase II and Cytokinesis
• Nuclei form at opposite poles of the cell
• Cytokinesis occurs producing four haploid cells

19
(No Transcript)
20
(No Transcript)
21
V. Genetic Variation Sexual reproduction
provides genetic variation by

• Independent Assortment Random distribution of
  maternal and paternal homologues to gametes
• 50/50 chance that a daughter cell will receive
  paternal/maternal homologues
• In humans, there are 223 or 8 million
  combinations
• Maternal and paternal homologues contain
  different genetic information on man loci

22

• Crossing Over Exchange of genetic material
  between homologues. Produces chromosomes that
  contain genes from both parents

23

• Random Fertilization In humans, an egg cell
  that is one of 8 million possibilities will be
  fertilized by a sperm cell that is also one of 8
  million possibilities. Resulting zygote can have
  one of 64 trillion possible genetic diploid
  combinations

24
VI. Evolution and Genetic Variation

• Natural Selection
• Increases frequency of inheritable variations
  that favor the reproductive success of some
  individuals
• Results in adaptation (the accumulation of
  inheritable variations that are favored by the
  environment)
• In the face of environmental change, genetic
  variation increases the likelihood that some
  individuals in a population will have inheritable
  variations that help them cope with new
  conditions.