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Sexual Reproduction
& Genetics
Part 1: Meiosis
Concepts for Review
• Chromatin is the relaxed form of
DNA (interphase)
• Chromosome is the coiled form of
DNA (mitosis or meiosis)
• Sister chromatids are identical copies
of DNA (after S-phase/replication)
Key Terms
• Homologous chromosomes are
chromosomes that make up a
pair, one from each parent
– Not necessarily identical but…
– Are the same length
– Have the same genes
Key Terms
• Diploid refers to complete
chromosome sets in a cell (or 2N)
– N= the number of chromosomes in a
gamete (sex cell)
– N chromosomes from the female
parent + N chromosomes from the
male parent= 2N
– For humans, 2N = 46
Key Terms
• Haploid refers to single
chromosomes in a cell (or N)
– One-half of the 2N set (the maternal
N or the paternal N)
– Gametes (i.e. sperm, egg sex cells) are
haploid cells
– For humans, N = 23
Bottom Line
• diploid means that there are the
full number of chromosomes
where as haploid means that
there are half the number of
chromosomes.
Fertilization
2n
1n + 1n
copyright cmassengale
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Meiosis Animation
copyright cmassengale
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Key Terms
• Meiosis is a type of cell division
that reduces the number of
chromosomes
– Forms gametes (i.e. sex cells)
– Occurs in the reproductive structures
of organisms that reproduce sexually
– Occurs in 2 consecutive cell divisions:
meiosis I and meiosis II
Before meiosis Interphase occurs….
• Replication of DNA occurs.
• Chromosomes Condense
Sounds familiar right???
Meiosis I: “Reduction Division”
PROPHASE I
• Homologous
chromosomes pair up
• Crossing over occurs
• Nuclear envelope
breaks down
• Spindles form
Crossing Over
• Chromosomal segments (genetic information)
are exchanged between a pair of homologous
chromosomes
METAPHASE I
• Spindle fibers attach to
the centromeres
• Homologous
Chromosomes Line up
at the equator
ANAPHASE I
• Homologous
chromosomes separate
and move to the
opposite poles of the
cell
TELOPHASE I
• The cell divides
• The result is two
daughter cells.
• Each daughter cell is
haploid.
• The daughter cells are
not genetically identical
**During Telophase I Cytokinesis occurs at the same time!!**
Meiosis II
1. DNA does not replicate in the two
cells resulting from Meiosis I.
--The genome is reduced from
diploid to haploid
2. The rest of meiosis II is similar to
mitosis
Meiosis II
• Nucleus breaks up
• Spindles form and
attach to
centromere
• Sister
chromatids
randomly
align at
middle of
cell
• Sister
chromatids
separate
• Chromosomes
move to
opposite poles
of cell
• Cells divide
• 4 haploid
daughter cells
result
• Each cell is
genetically
different
Summary of Key Concepts
• Chromosomes contain genes (humans
have 46 chromosomes or 23 homologous
pair)
• Homologous chromosomes may contain
different forms of the same genes
• Meiosis reduces chromosome # by ½ and
results in sex cells (gametes)
• Crossing-over increasing genetic variation
Click here for detailed animation of meiosis
Spermatogenesis & Oogenesis
Meiosis
• The most important thing to remember
in distinguishing Mitosis from Meiosis is:
• Mitosis deals in body cells (somatic cells)
• Meiosis deals with sex cells (Gametes)
• Mitosis results in the production of 2
genetically identical diploid cells.
• Meiosis produces 4 genetically different
haploid cells.