chapter 10 sexual reproduction & genetics part 1: meiosis
TRANSCRIPT
Chapter 10 Sexual Reproduction
& GeneticsPart 1: Meiosis
Concepts for Concepts for ReviewReview• Chromatin is the relaxed form of
DNA• Chromosome is the coiled form of
DNA• Sister chromatids are identical copies
of DNA (after S-phase/replication)
Key Terms 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 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 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.
Key Terms 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 Before meiosis Interphase occurs….Interphase occurs….• Replication of DNA occurs.• Chromosomes Condense
Sounds familiar right???
Meiosis I: Meiosis I: “Reduction “Reduction Division”Division” Reducing
2N cells
to…
N cells
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: “Mitotic Meiosis II: “Mitotic Division”Division”
Separatin
g sister
chromatid
s
• 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 Summary of Key ConceptsConcepts• 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
which set of chromatids illustrates the result of a single crossover of the homologous chromosomes?