Chapter 6  Chromosomes & Cell Reproduction
I. Chromosomes  DNA & histones (organizing proteins)
A. Formation of New Cells by Cell Division
• 2 Trillion cells/day or 25 million/sec
• PURPOSE  growth & development, repair & sex cells/gametes*
• Cell Genetic Info  DNA to make proteins  enzymes (cell machinery)  metabolism
• Each new cell needs a complete set of DNA or genetic info
1. Prokaryotic Cell Reproduction  (bacteria) binary fission  1 circular chromosme
2. Eukaryotic Cell Reproduction  (rest of organisms) Mitosis & Cytokinesis
• gene  segment of DNA that codes for a protein/enzyme or RNA molecule
• Chromosome  long segment of DNA with 1000s of genes (2%) & 98% non-coding
DNA
• chromatids  “condensed” chromosomes for dividing & moving (DNA has been
duplicated)
• centromere  area that holds identical chromatids together until cell division
• Fg. 2 p. 119  organization of chromosomes
B. How Chromosomes Number & Structure Affect Development
• Humans  46 chromosomes (23 prs)
• Each chromosome contains 1000s of genes (20,000 -25,000 genes total)
1. Sets of Chromosomes
• Homologous chromosomes  chromosomes similar in size, location & genetic content
• Diploid  full set of chromosomes (½ from mother/female & ½ from dad/male)
• Haploid  half set of chromosomes for SEXUAL REPRODUCTION
• Zygote  fertilized egg with 2 haploid sets or 1 diploid set
2. Sex Chromosomes
• Sex Chromosomes  23rd pair of chromosomes that contain gender determination
• Autosomes  1st through 22nd pairs of chromosomes
3. Change in Chromosome Number
• Down Syndrome  47 chromosomes/cell (extra 21st chromosome)(Triosomy 21)
• Klinefelter’s Syndrome  47 chromosomes/cell (extra sex chromosome) (XXY)
• Turner’s Syndrome  45 chromosomes/cell (missing sex chromosome) (XO)
• Patua’s Syndrome  47 chromosomes/cell (extra 13th Chromosome)
• Edward’s Syndrome  47 chromosomes/cell (extra 18th chromosome)
• Triosomy X  47 chromosomes/cell (Extra X chromosome)
• 47 XYY  (extra sex chromosome)
4. Change in Chromosome Structure
a. Deletion  Example: Cri-du-chat  Partial Deletion of Chromosome 5
b. Duplication  Example: Fragile X  Extra portion of the X chromosome
c. Inversion  may or may not cause any harm, but may lead to a chromosome
mutation in offspring
d. Translocation  TWO chromosomes switch parts (may or may not be a
problem, but may lead to a chromosome disorder)
II. The Cell Cycle
A. The Life of a Eukaryotic Cell
• Cell division is eukaryotic cells is more complex than prokaryotic (bacteria) cells
• nucleus and other organelles must be divide into TWO functioning cells
1. The Cell Cycle  repeating sequence of events of cell division
a. Interpahse  generally 90% of cell’s life
i. G1 (First Growth) phase  cell growth and functioning
ii. S (Synthesis) phase  duplication of DNA
iii. G2 (Second Growth) phase  preparation for cell division
b. Mitosis  division of the nucleus and the chromosomes (identical number &
type)
c. Cytokinesis  division of the cell’s cytoplasm into 2 cells after mitosis
B. Control of a Eukaryotic Cell
• ensures normal and appropriate growth
• complex systems are in place to control cell growth
1. Three Checkpoints
a. G1 (Cell Growth) Checkpoint  allows cell to move on to S phase for mitosis
b. G2 (DNA Synthesis) Checkpoint  “repair” enzymes will check DNA before
mitosis
c. Mitosis Checkpoint  triggers the completion of mitosis and the beginning of
cytokinesis
2. When Control Is Lost: Cancer
• cancer  uncontrolled cell growth & division; no response to checkpoints; error in
cell systems allow cells to divide uncontrollably and disrupt metabolism
III. Mitosis & Cytokinesis
• 2 million RBCs produced every second in bone marrow
• Spindles direct movement of chromosomes to opposite sides of the cell
• Centrioles (animal cells only) are the destination of the moving chromosomes in cell division
A. Chromatid Separation in Mitosis
1. Forming the Spindle
• pathway for chromosome movement that begin to be formed in G2 phase of cell cycle
2. Separation of Chromosome by attaching Spindle Fibers
• centromeres split to allow chromatids (now chromosomes to move to opposite sides
during cell division
B. Mitosis & Cytokinesis
1. Mitosis  4 stages (track nucleus membrane, chromosomes, centrioles & spindle)
a. Prophase  nucleus breaks down & disappears, chromosomes are condensing,
centrioles divide into 2 & move to opposite sides of cell, spindle begin to form
b. Metaphase  nucleus is completely broken down, chromosomes are fully
condensed and lined-up at center of cell, centrioles are opposite sides of cell,
spindles stretch from centrioles to centromeres of chromosomes
c. Anaphase  nucleus is still gone, centromere of chromosomes split and
chromosomes begin to move to opposite sides of cell, centrioles are directing
movement, spindle is the chromosomes pathway
d. Telophase  2 nuclei begins to form, chromosomes are opposite sides of the cell
and begin to “un-condense”, centrioles are finished directing movement of
chromomes and spindle begins to break down and disappear
2. Cytokinesis  other cell organelles have duplicated and are divided into the 2 new cells
formed by cleavage furrow (animal cells only) or cell plate (plant & fungi cells with cell
walls)