meiosis and sexual reproduction chapter 6 part 2
TRANSCRIPT
Meiosis and Sexual Reproduction
Chapter 6 part 2
Mini-Quiz (not graded)
Name the following steps in the cell cycle:
a
d
b
e
c
Impacts, Issues:Why Sex?
Introducing Alleles
Asexual reproduction produces genetically identical copies of a parent (clones)
Sexual reproduction introduces variation in the combinations of traits among offspring
Meiosis• Process that enables organisms to make special
reproductive cells (gametes)
Halving the Diploid Number
A diploid cell has 2 nonidentical copies of every chromosome• pairs are homologous chromosomes
In germ cells meiosis changes diploid haploid, producing gametes• Eggs and sperm have 23 unpaired chromosomes
Somatic cell
Diploid #
Homologouschromosomes
23 unpaired
chromosomes Haploid #
Gamete
Two Divisions, Not One
MEIOSIS DNA is replicated once and divided twice
(meiosis I and meiosis II), making 4 haploid nuclei
In meiosis I, each duplicated homologous chromosome is separated from its partner
In meiosis II, sister chromatids are separated
Tour of Meiosis I
Tour of Meiosis II
How Meiosis Introduces Variation in Traits
Crossing Over in Prophase I
Crossing over• The process by which a chromosome and its
homologous partner exchange heritable information in corresponding segments
Fig. 10-6b, p. 160
Crossing Over in Prophase I
Fig. 10-6c, p. 160
Crossing Over in Prophase I
Fig. 10-6d, p. 160
Crossing Over in Prophase I
Random Assortment
Random assortment produces 1023 (8,388,608) possible combinations of homologous chromosomes!!!
Sperm Formation in Animals
Egg Formation in Animals
Mitosis
Fig. 10-11b, p. 165
one diploid nucleus two diploid nuclei
Prophase Metaphase Anaphase Telophase
• Chromosomes condense.
• Chromosomes align midway between spindle poles.
• Sister chromatids separate as they are pulled toward spindle poles.
• Chromosome clusters arrive at spindle poles.
• Bipolar spindle forms; it attaches chromosomes to spindle poles.
• New nuclear envelopes form.
• Nuclear envelope breaks up. • Chromosomes
decondense.
Meiosis I
Fig. 10-11c, p. 164
one diploid nucleus two haploid nuclei
Prophase I
• Chromosomes condense.
• Homologous chromosomes pair.
Metaphase I Anaphase I Telophase I
• Crossovers occur.
• Chromosomes align midway between spindle poles.
• Homologous chromosomes separate as they are pulled toward spindle poles.
• Chromosome clusters arrive at spindle poles.
• Bipolar spindle forms; it attaches chromosomes to spindle poles.
• New nuclear envelopes form.
• Nuclear envelope breaks up.
• Chromosomes decondense.
Fig. 10-11d, p. 165
two haploid nuclei four haploid nuclei
Prophase II Metaphase II Anaphase II Telophase II
• Chromosomes condense.
• Chromosomes align midway between spindle poles.
• Sister chromatids separate as they are pulled toward spindle poles.
• Chromosome clusters arrive at spindle poles.• Bipolar spindle
forms; it attaches chromosomes to spindle poles.
• New nuclear envelopes form.
• Nuclear envelope breaks up.
• Chromosomes decondense.
Meiosis II
Table of DifferencesKind of cell Kind of
ReproductionProphase Metaphase Anaphase Telophase
Mitosis
Meiosis
