Page 1 of 7. Chapter 8. Cell Reproduction. 8-1 Chromosomes. It is estimated ... -cells in the Central Nervous System normally do not come back to the ...
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
Page 1 of 7
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 Page 2 of 7
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
Page 3 of 7
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
Page 4 of 7
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 Page 5 of 7
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
Page 6 of 7
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.
