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Chapter 8
Cell Reproduction
8-1 Chromosomes
It is estimated that human DNA
contains around 3 billion
(3,ooo,ooo,ooo) nucleotides.
-During cell division the DNA
(chromatin) in a eukaryote is coiled
into chromosomes
-DNA wraps around proteins called
Histones
Histones –P. 151
-maintain the shape of the chromosome
Aid in tightly packing DNA
Nonhistones
-control activities of specific sections of DNA
Chromosomes- P. 152
Two chromatids held together by a centromere
-the two chromatids are identical to each
other.(DNA has copied itself)
Prokaryotes
-one chromosome that is a circular DNA
molecule
-attaches to the inside of the cell membrane
Chromosome Numbers
-each species has its own characteristic number of chromosomes. P. 152
Ex. Humans- 46 chromosomes (23 pairs)
Fruit flies- 8 chromosomes (4 pairs)
Animal chromosomes are either
a) Sex chromosomes
b) Autosomes
In humans, 2 sex chromosomes and 44 autosomes
Sex Chromosomes
- determine the sex of the organism
- in humans an X or Y
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Normal Female
Two X = XX
Normal Male
An X and a Y = XY
Autosomes
-all the other chromosomes
-In humans, 44 autosomes (22 pairs)
-one copy of each autosome from each parent
Homologous Chromosomes (Homologues)
-the two copies of each autosome
-the same size and shape
-contain the genes for the same trait
Karyotype P. 153
-a pictomicrograph of the chromosomes
-chromosomes are arranged by size and shape
-determines the babies sex and any chromosomal abnormalities.
Diploid and Haploid
Diploid- 2 sets of the chromosomes- 2n-all normal cells except egg and sperm
Haploid- 1 set of chromosomes -1n- egg and sperm (gametes)
Ex. Human: Diploid -46, Haploid -23
8-2 Cell Division
Binary Fission P. 154
-division of prokaryotic cells into two identical cells
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Mitosis and Meiosis
-Mitosis- new cells with genetic material that is identical to the parent cell – 2n
-growth, development, repair, or asexual
reproduction
Meiosis- reduces the chromosome number by half
– 1n
-forms gametes for sexual reproduction
Cell Cycle –P 155
-the set of events that make up the life of a cell
a) Interphase- time between cell division
b) Cell Division
1) M phase- Mitosis
2) Cytokinesis- division of the cytoplasm
Interphase
-cells spend most of their time in interphase
a) G1 Phase- Gap 1- cells grow to full size
b) S Phase- Synthesis- copy DNA
c) G2 Phase- Gap 2 – cell prepares for division.
G0 Phase (G zero Phase)
-cells can exit the cell cycle
-cells do not copy DNA of prepare for division
-most cells may come back to the cell cycle if needed
-cells in the Central Nervous System normally do not come back to the cell cycle.
Mitosis- M phase P. 156
-division of the nucleus
a) Prophase
b) Metaphase
c) Anaphase
d) Telophase
Prophase
-DNA coils into chromosomes
-nuclear envelope and nucleolus disappear
-centrosomes (with centrioles in animals_) move to opposite poles
-spindle fibers radiate from centrosomes, forming the mitotic spindle
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Mitotic Spindle
-made of 1) Kinetochore fibers (attached to the kinetochore in the centromere) and
2) polar fibers
-will divide the chromosomes
Metaphase
-chromosomes line up at the equator
-they are held in place by kinetochore fibers
Anaphase
-chromatids separate and move to opposite poles
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Telophase
-chromosomes are at opposite poles
-spindle fibers break down
-chromosomes unwind to chromatin
-nuclear envelope and nucleolus reappear
Cytokinesis
-occurs during telophase
-animal cells- a cleavage furrow pinches the cell in half.
-plant cells- a Cell Plate (from vesicles) forms in the middle, this will from a new cell
wall
Control of cell division
-Proteins regulate cell division. A system of checkpoints controls whether cell
division will go forward or stop.
1. Cell growth (G1) checkpoint- a full grown, healthy cell will enter the S phase (copy
DNA). Cells may also rest or enter G0 phase.
2. DNA synthesis (G2) checkpoint- DNA repair enzymes will signal the next phase.
3. Mitosis checkpointCell will finish mitosis and enter G1.
Cancer
-mutations may cause regulatory proteins to malfunction
Cancer- the uncontrolled growth of cells.
-cancer cells do not respond normally to the body’s control mechanisms.
8-3 Meiosis
-produces haploid gametes (reproductive cells)-egg and sperm
-reduces the number of chromosomes by half
-cells undergo G1, S and G2 .
Meiosis I
Prophase I
-DNA coils
-Synapsis- pairing of homologous chromosomes
-forms a Tetrad
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Crossing Over (during Prophase I) P. 162
-homologous chromosomes in a tetrad are aligned
so the genes of one chromatid are adjacent to the
same genes of the other chromatid,
-portions of the chromatids twist around each other,
they may break off and exchange.
The chromosome is now a mixture of maternal and
paternal genes genetic recombination
Metaphse I
-tetrads line up at the equator
Anaphase I
-each homologous chromosome moves to a pole
-independent assortment- random separation of homologous chromosomes
genetic recombination
Telophase I
-chromosomes reach the poles, cytokinesis begins
-each cell contains 2 copies (as chromatids) of the SAME chromosome
-each cell contains half the number of original chromosomes.
Meiosis II P. 163
Prophase II
-chromosomes move towards the equator
Metaphase II
-chromosomes are at the equator
Anaphase II
-chromatids separate
Telophase II and Cytokinesis
-nuclear membrane forms
-cytoplasm divides 4 cells
-each cell contains half the original number of
chromosome
-Humans – 23 chromosomes
Spermatogenesis P. 164
-in humans, occurs in the testes
-forms 4 spermatids become 4 equal sperm cells.
-equal division of cytoplasm
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Oogenesis
-in humans, occurs in the ovary
-one mature Ova (egg cell) and 3 polar bodies
that disintegrate.
-unequal division of cytoplasm.
Sexual Reproduction
-offspring are genetically different from
parents and from each other. (except for
identical twins)
-this enables a species to adapt to changes in
the environment, because of genetic
variation.
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