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Chapter 8 - Cell Reproduction
I. Chromosomes
A. Structure
1. single DNA molecule wrapped around proteins (histones)
(maintain shape and aid in tight packing of DNA)
2. long and threadlike in nondividing cell (chromatin)
3. tightly coiled during cell division (condensed - chromosome)
B. Chromosome Types and Numbers
1. each species has certain # in each cell (many species have
same #)
a. humans have 46 total chromosomes
2. sex chromosomes (X and Y)
a. determine sex, carry some genes
b. XX normal female, XY normal male
3. autosomes - all other chromosomes (44 in humans)
a. carry most of the genes
4. every somatic (body) cell has 2 of each autosome (one from
each parent)
a. homologous chromosomes - identical chromosomes
(size, shape, genes for same traits)
5. karotype - photomicrograph of chromosomes arranged in
pairs (p. 147)
6. diploid (2n) - cell has pairs (2 of each kind) of chromosomes
a. all somatic cells
b. humans: 2n = 46
7. haploid (monoploid) (n) - cell has half the total # of
chromosomes (one of each kind)
a. gametes (reproductive cells) - sperm and egg
b. humans: n = 23
II. Cell Division - prokaryotes
A. Binary fission
1. single chromosome replicates
2. cell grows to twice original size
3. cell wall forms in middle and splits cell into two identical
cells
G0
III. Cell Division - eukaryotes
The Cell Cycle
A. Interphase
1. cells spend most of their life in this phase
2. period between cell divisions - normal cell activities occurring
3. G1 phase
a. time between cell division and DNA replication
b. cells grow to mature size
4. S phase
a. replication (copying) of DNA (chromatin)
5. G2 phase
a. time between DNA replication and cell division
b. cell prepares for cell division
6. Gº phase
a. cell exits cell cycle
b. no DNA replication, no preparation for cell division
c. fully developed cells of CNS (Central Nervous System)
stop dividing at maturity and normally never divide
again
d. why a Gº phase?
1) most organisms can’t grow indefinitely
2) most cells can reenter cycle to replace dead or
damaged cells when needed
B. Mitosis (division of nuclear material preceding cell division)
1. prophase
a. DNA (chromatin) shortens, tightly coils and forms
visible chromosomes
b. two copies of each chromosome, called chromatids, are
connected by a centromere
c. nucleolus and nuclear membrane break down and
disappear
d. centrosomes appear and contain 2 centrioles (* no
centrioles in plants)
e. centrosomes migrate to opposite poles
f. spindle fibers radiate from centrosomes (all fibers form
mitotic spindle)
1) kinetochore fibers attach to kinetochore (diskshaped protein) of centromere of each chromatid
i.) extends from each chromatid to centrosome
2) polar fibers extend from centrosome to centrosome
2. metaphase
a. kinetochore fibers move chromosomes to center
(equator) of cell
b. held in place by those fibers
3. anaphase
a. centromeres divide, chromatids of each chromosome
move toward opposite poles (k. fibers move them)
b. each chromatid now considered an individual
chromosome
4. telophase
a. chromosomes reach the poles
b. spindle fibers disassemble
c. chromosomes revert to chromatin (less coiled)
d. nuclear membrane reforms
e. nucleolus reforms
f. cytokinesis - cytoplasm of cell divides
1) animal - cell membrane pinches at cleavage furrow
and two separate cells formed
2) plant - cell plate forms down middle of cell
Result of Cell Cycle:
 2 equal cells
 half the size of original
 same number chromosomes
 same kind of chromosomes
IV. Meiosis
A. Sexual Reproduction
1. fertilization - joining of 2 sex cells (gametes)
2. zygote - fertilized egg
B. Meiosis
1. reduction division - reduces chromosome number by 1/2
2. keeps chromosome number constant from generation to
generation
C. Stages of Meiosis (cells go through G1, S, and G2, so meiosis
begins with duplicate set of chromosomes)
Meiosis I
1. prophase I
a. synapsis - pairing of homologous chromosomes
1) pair called a tetrad (4 chromatids)
b. crossing-over - chromatids in homologous pair may
twist around one another (portions may break off and
attach)
1) allows exchange of genetic material between
maternal and paternal chromosomes (get genetic
recombination) (p. 154)
2. metaphase I
a. tetrads randomly line up at equator
b. spindle fibers from one pole attach to chromatids of one
homolog, fibers from other pole attach to other homolog
3. anaphase I
a. centromeres don’t divide
b. tetrads (homologous chromosomes) separate and move
to opposite poles
c. independent assortment - random separation of the
homologous chromosomes
1) maternal and paternal chromo. randomly separated
(genetic recombination)
4. telophase I
a. chromosomes reach poles
b. cytokinesis
c. 2 haploid cells but cell still contains 2 copies of each
chromosome
d. some species form nuclear membrane, some don’t
Meiosis II (no replication of DNA)
1. prophase II
a. centrosomes migrate, spindle fibers form
2. metaphase II
3. anaphase II
identical to mitosis
4. telophase II
Result of meiosis
* 4 haploid cells (1/2 the number of chromosomes as original)
Meiosis Square Dance
http://www.fofweb.com/Science/default.asp?ItemID=WE40
D. Gamete Formation (p. 155)
1. testes in males (spermatogenesis)
primary spermatocyte (2n) meiosis I secondary spermatocyte (n)
meiosis II
4 spermatids (n)
mature to 4 sperm cells (n)
a. both divisions equal
2. ovaries in females (oogenesis)
a. primary oocyte (2n) (1st meiotic division unequal)
get secondary oocyte and a polar body (#1)
b. secondary oocyte (n) (2nd meiotic division unequal)
get ovum and polar body
1st polar body divides - get 2 polar bodies
c. result is 1 ovum (n) (survives) and 3 polar bodies
(degenerate)
E. Types of Reproduction
1. Asexual (offspring from one parent, no gametes, clones)
a. types
1) fission, budding, spores, vegetative propagation
2. Sexual (usually 2 parents, gametes, offspring genetically
different from the parents)
a. fertilization restores diploid number - one chromosome
from each homologous pair comes from each parent
b. advantage - better adaptation to new conditions due to
variations