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Biology 1010

Principles of Biology

Fall  2000



Asexual reproduction

Mitosis genetically identical offspring (clone)


  • Organism well-adapted to the environment
  • Environment never changes
  • Organisms never move to a new location

Sexual Reproduction

Provides for variation in offspring

Offspring similar but not identical

Sexual reproduction is an evolutionary imperative for multicellular organisms

I. Sexual Reproduction

A. Requires formation and fusion of gametes (syngamy) to form zygote

1. Gamete = a sex cell (sperm or egg)

2. Zygote = first diploid cell resulting from (syngamy) fertilization

3. Gamete chromosome numbers must be reduced in gametes

B. Life Cycles Vary

1. Reproductive events between one generation and the next

2. All sexual reproduction cycles have a zygote (2n) stage

3. Mitosis = nuclear division with constant chromosome number

4. Meiosis = nuclear division reducing chromosome number (2n --> n)

C. Homologous Chromosomes

1. Diploid cells, chromosomes occur in pairs

a.  Homologous chromosomes corresponding chromosome, one from each parent containing the same genes

b. Homologues

Same genes at a specific locus (location)

2. Chromosome Replication

a. Replication occurs prior to the onset of meiosis

b. Only one member of each homologous pair is passed to gamete

II. Overview of Meiosis

A. Keeps chromosome number constant across generations

Gametes receive only one member of each homologous pair

B. Two nuclear divisions

Meiosis I and II

 Produces 4 haploid cells

C. Meiosis I Overview

1. DNA replicates prior to meiosis I

2.  Crossing-over occurs during prophase I

a. Produces genetic variation

b. Exchange of genetic material between homologous chromosomes

c. Homologous chromosome pair line up in synapsis

d. Sets of chromatid pairs lay alongside each other as bivalents

e. Chiasmata regions of attachment due to crossing-over

f. Exchange like regions of DNA

3. Prophase I

  • Nucleolus and nuclear envelope disappears
  • Spindle fibers assemble
  • Chromatin condenses
  • Homologous chromosome undergo synapsis and crossing-over occurs

4. Metaphase I

  • Bivalents independently align at equatorial plane
  • Independent assortment occurs
  • Another source of variation
  • Maternal and paternal chromatids sort into gametes

5. Anaphase I

  • Homologues of each bivalent move toward opposite poles
  • Centromeres do not divide
  • Chromosomes still two chromatids

6. Telophase I

  • Only occurs in some species
  • When it does occur
    • Nuclear envelope and nucleoli reappear
    • Chromosomes decondense

7. Interkinesis

  • Period between meiosis I and II
  • Similar to interphase of mitosis
  • No DNA replication occurs

D. Meiosis II

1. Nearly identical to mitosis

2. Metaphase II

Chromosomes align with centromeres on equatorial plane

3. Anaphase II

Centromeres divide and chromosomes move to opposite poles

III. Meiosis vs. Mitosis

A. Mitosis more frequent

Growth and tissue repair

B. DNA is only replicated once

C. One nuclear division mitosis, two divisions meiosis

D. Crossing-over occurs during meiosis

E. Bivalents align on metaphase plate during metaphase I

F. Chromatids split in anaphase of mitosis

    Split in anaphase II of meiosis

G. Mitosis

  • 2 daughter cells
  • Genetically identical

H. Meiosis

  • 4 daughter cells
  • Genetically unique

IV. Significance of Meiosis

A. Maintains constant chromosome number

B. Ensures gametes get one of each kind of chromosome

C. Produces genetic variation

1. Independent assortment

2n chromosome combinations

223 = 8,388,608

2. Fertilization

(223)2 = 70,368,744,000,000

3. Crossing-over

(223)2 = 70,368,744,000,000

4. Fertilization and crossing-over

(423)2 = 5 X 1027 combinations

5. Variation sources

  • Fertilization
  • Independent assortment
  • Crossing-over

D. Advantages of Meiosis

  1. Tremendous storehouse of genetic variation
  2. Allows for adaptation to changing environments
  3. Allows for sources of variation other than mutations

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