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Chromosomes and Cell
Reproduction
Chapter 6 Section 1
Structure of Chromosomes
Gene
Segment of DNA that codes for a protein or RNA molecule
DNA
Thousands of genes
Chromosomes
DNA and the Proteins associated with it
Chromatids
2 exact copies of DNA that make up each chromosome
Chromosome Structure
Homologous Chromosomes
 Each human somatic (any cell other than
sperm or egg) cell normally has two copies of
23 different chromosomes, for a total of 46
chromosomes.
 Each of the 23 pairs of chromosomes
consists of two homologous chromosomes

Chromosomes that are similar in size, shape &
genetic content
Diploid & Haploid
 Diploid
 Somatic cells
 Two sets of
chromosomes
 2n = 46
 Haploid
 Gametes
 One set of
chromosomes
 n = 23
Sex Chromosomes
 Autosomes are chromosomes that are not
directly involved in determining the sex
(gender) of an individual.
 Sex chromosomes, one of the 23 pairs of
chromosomes in humans, contain genes that
will determine the sex of the individual.
Karyotype
 Photo of the
chromosomes in a
dividing cell that shows
the chromosomes
arranged by size.
The Cell Cycle
Chapter 6 Section 2
The Cell Cycle
 The cell cycle is a repeating sequence of
cellular growth & division during the life of an
organism.
 A cell spends 90% of its time in the first three
phases (interphase) of the cell cycle.
 A cell will enter that last two phases of the cell
cycle only if it is about to divide.
Interphase
1.
2.
3.
4.
5.
G1 Phase: Grows rapidly & carries out its
routine functions.
S Phase: DNA is copied
G2 Phase: Nucleus prepares to divide
Mitosis: Nucleus of a cell is divided into two
nuclei
Cytokinesis: Cytoplasm divides
The Cell Cycle (pg. 126 fig. 7)
Control of the Cell Cycle
 Three principal
checkpoints



G1
G2
Mitosis
 If control is lost, cancer
occurs
Mitosis & Cytokinesis
Chapter 6 Section 3
Prophase
 Chromosomes coil up &
become visible
 Nuclear envelope
dissolves
 Spindles form
Metaphase
 Chromosomes move to
the center of the cell &
line up along the
equator
 Spindle fibers link the
chromatids of each
chromosome to
opposite poles
Anaphase
 Centromeres divide
 Chromatids (pairs of
chromosomes)
separate and move
toward opposite
poles as the spindle
fibers attached to
them shorten
Telophase
 Nuclear envelope forms
around the
chromosomes at each
pole.
 Chromosomes uncoil
 Spindle dissolves
 Spindle fibers break
down & disappear
Cytokinesis
 Cytoplasm of the cell is
divided in half
 Cell membrane grows
to enclose each cell,
forming two separate
cells
End Result of Mitosis &
Cytokinesis
 Two genetically identical daughter cells each
with its own nucleus wit identical
chromosomes.
 The Cell Cycle Animated
Mitosis
Meiosis and Sexual
Reproduction
Chapter 7 Section 1
Meiosis
 Meiosis is a form of cell division that halves
the number of chromosomes when forming
specialized reproductive cells.
 Meiosis involves two divisions of the nucleus


Meiosis I
Meiosis II
Prophase I
 Chromosomes become
visible
 Nuclear envelope
breaks down
 Crossing-over occurs

When portions of a chromatid
on one homologous
chromosome are broken &
exchanged w/ the
corresponding chromatid
portions of the other
homologous chromosome.
Metaphase I
 Pairs of homologous
chromosomes move to
the equator of the cell.
Anaphase I
 Homologous
chromosomes move to
opposite poles of the cell.
 Chromatids do not
separate at their
centromeres—each
chromosome is still
composed of two
chromatids. The gentic
material, however, has
recombined.
Telophase I & Cytokinesis
 Chromosomes gather at
the poles of the cell
 Cytoplasm divides
Prophase II
 A new spindle forms
around the
chromosomes
Metaphase II
 Chromosomes line up
at the equator
Anaphase II
 Centromeres divide
 Chromatids move to
opposite poles of the
cell
Telophase II & Cytokinesis
 A nuclear envelope
forms around each set
of chromosomes.
 The cytoplasm divides
Meiosis & Genetic Variation
 The result of meiosis is four genetically
different haploid cells.
 Three mechanisms make key contributions to
this genetic variation:



Independent assortment
Crossing-over
Random fertilization
Independent Assortment
 Independent assortment is the random
distribution of homologous chromosomes
during meiosis.
 Each of the 23 pairs of chromosomes
segregates (separates) independently.
 Thus, 223 (about 8 million) gametes with
different gene combinations can be produced
from one original cell.