Biology Chapter 6 Notes:
Chromosomes and Cell Division
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Telomere: cell reproduction "counter" and human cells divide about 50 times
Some chromosomes are circular, some are linear;
Binary fission (division of 2): asexual reproduction of prokaryotic cell division; daughter cells
inherit their DNA from a single parent cell; DNA is carried in a single, circular chromosome;
begins with replication (DNA duplicates itself); DNA molecule unwinds from its coiled-up
configuration; the DNA strand splits apart and free-floating nucleotide bases attach; the cell
elongates and pinches into two
Eukaryotic chromosome: made of more than just DNA; the long linear DNA strand is wrapped
around proteins called histones (keeps DNA from getting tangled)
Cell Cycle
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eukaryote cells: somatic cells are cells forming the body of the organism and gametes are
reproductive sex cells
Two phases to the somatic cell cycle: cells grow and prepare to divide during interphase and
divide during the mitotic phase
Interphase
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Gap 1: cell grows and performs all cellular functions (making proteins, getting rid of waste, etc);
cells that divide infrequently (neurons, heart muscle cells) spend most of their time in Gap 1
DNA synthesis: cells begin to prepare for cell division; every chromosome creates an exact
duplicate of itself by replication; before replication, each chromosome has become a pair of
identical long linear strands, held together near the centre (region where chromosomes are in
contact is called centromere)
Gap 2: cells continue to grow and prepare for cell division; differs from Gap 1 because genetic
material has now been duplicated; Gap 2 usually shorter than Gap 1
Mitosis
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nucleus of parent cell duplicates; followed by cytokinesis
DNA replication: Unwinding of coiled double-stranded DNA molecule and Rebuilding of double
stranded DNA with an enzyme connecting the complementary base pairs
 Purpose: enable existing cells to generate new, identical cells for Growth and Replacement
 Apoptosis: Cells that must be replaced die on purpose
1. Prophase: nuclear membrane breaks down; sister chromatids (replicated chromosomes)
condense; spindle fiber forms
2. Metaphase: sister chromatids line up at the centre of the cell
3. Anaphase: the sister chromatid pairs are pulled apart by the spindle fibrers; one full set of
chromosomes from each chromatid goes to each side of the cell
4. Telophase: chromosomes begin to uncoil; nuclear membrane is reassembled around them
5. Cytokinesis: cytoplasm and organelles duplicate and are divided into approximately equal parts
Out of Control Cell Division
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Cancer: unrestrained cell growth and division and can cause serious health problems
Loses their "contact inhibition"
Cancer cells can divide indefinitely
Benign vs. malignant; metastasis is the process cancer cells separate from a tumor and invade
the circulatory system
Meiosis
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Fertilization: fusion of two reproductive cells
Outcomes: 1. Reduces the amount of genetic material in gametes 2. Produces gametes that
differ from one another with respect to the combinations of alleles they carry
Gametes produced in a human's gonads; maternal and paternal copies of a chromosome are
called homologous pair
Homologues: Sister chromatids are connected by a centromere during the replication of a
homologues
Meiosis Division I
1. Prophase I: chromosomes condense and crossing over occurs; spindle is formed; homologous
pairs of sister chromatids come together and cross over; nuclear membrane disintegrates
2. Metaphase I: homologues move towards the center of the cell and line up
3. Anaphase I: homologues separate and are pulled to opposite poles; sister chromatids going to
each side are a mix of maternal and paternal genetic material
4. Telophase I and Cytokinesis: sister chromatids arrive at the cell poles and the nuclear membrane
reassembles around them; the cell pinches into two daughter cells; chromosomes may unwind
slightly
Meiosis Division II
1. Prophase II: Chromosomes in daughter cells condense
2. Metaphase II: Sister chromatids line up at the centre of the cell
3. Anaphase II: sister chromatid pairs are pulled apart by the spindle fibers toward opposite cell
poles
4. Telophase II and Cytokinesis: two daughter cells pinch into four haploid daughter cells; nuclear
membrane reassembles around the chromosomes
Female Gametes
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genetic material is divided evenly but cytoplasm goes to mainly one cell; there is only 1 gamete
at the end of the two meiosis stages
Sex of the Offspring
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Female gametes only have an X chromosome; females have XX chromosomes while males have
XY chromosomes
Birds: Female can either have W or Z chromosome gamete; male only has Z chromosome
Ants, Bees, Wasps: sex is determined by the number of chromosome sets an individual
possesses
Turtles: high tempt usually leads to females; lower tempt usually leads to males
Karyotypes
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a visual display of a complete set of chromosome: obtain some cells from the individual -->
encouraged to divide by culturing them in a test tube --> divided cells are treated with a
chemical that stops them exactly midway through cell division --> cells placed on microscope
slide and a stain is added that binds the chromosomes to make them visible --> arranged by size
and shape and displayed on a monitor or in a photograph
nondisjunction: unequal distribution of chromosomes during meiosis; results in gametes having
zero or two copies of a chromosome rather than a single copy
Disorders
1. Turner syndrome: missing sex chromosome X _; short height, web of skin between neck and
shoulders, underdeveloped ovaries, some learning difficulties
2. Klinefelter syndrome: extra X chromosome XXY; underdeveloped testes, lower testosterone
levels, development of some female features, long limbs and slightly taller than average
3. Super males: XYY; taller than average, moderate to severe acne, slightly lower than average
intelligence
4. Metafemale: XXX; may be sterile, no obvious phsyical or mental problems