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Chapter 19
Chromosomes and Cell Division
Human Chromosomes
• Homologous chromosomes: one
maternal & one paternal
• Sex chromosomes: X & Y
• Autosomes: the 22 pairs of
chromosomes other than X & Y
Chromosomes Consist of DNA
and Protein
• Genes
– 1000-5000 genes/chromosome
– Most genes code for a protein
– Mutation causes change in nucleotide
sequence of DNA
• Somatic cells (Non-germ cells) have 46
chromosomes (23 pairs)
Most people have
A.
B.
C.
D.
23 chromosomes
46 chromosomes
23 pairs of chromosomes
46 pairs of chromosomes
Most people have
A.
B.
C.
D.
23 chromosomes
46 chromosomes
23 pairs of chromosomes
46 pairs of chromosomes
Chromosomes are composed of
A.
B.
C.
D.
E.
DNA
Genes
Chromatin
All of the above
None of the above
Chromosomes are composed of
A.
B.
C.
D.
E.
DNA
Genes
Chromatin
All of the above
None of the above
During Cell Division (Mitosis) Cells
Replicate Their Chromosomes
DNA Replication (synthesis)
• Precedes cell division
• DNA polymerase (enzyme) catalyzes
DNA synthesis
• Necessary to maintain 46
chromosomes/cell
Overview of Mitosis
Cell Division
• During metaphase the chromosomes
form a line at the center of the cell
Cytokinesis Occurs Toward the
End of Mitosis
• Phase when Cytoplasm divides
• Microfilaments act like purse string
Meiosis
Meiosis Forms Haploid Gametes
• Occurs in gonads (ovary and testes)
• Two Major Accomplishments of Meiosis
– Reduce chromosome number of gametes
by ½
– Shuffle the chromosomes and genes
• This increases genetic variability in the
population
Meiosis Forms Haploid Gametes
• Meiosis:
1. Keeps the chromosome number constant
through generations and
2. Increases genetic variability in the
population
• Meiosis involves two cell divisions
Overview of Meiosis
The following is accomplished during meiosis
A.
B.
C.
D.
E.
Two identical cells are produced
Four identical cells are produced
The chromosome number is doubled
The genes are shuffled
The number of chromosomes in the
resulting cells is halved
The following is accomplished during meiosis
A.
B.
C.
D.
E.
Two identical cells are produced
Four identical cells are produced
The chromosome number is doubled
The genes are shuffled
The number of chromosomes in the
resulting cells is halved
Comparison of Spermatogenesis and
Oogenesis
Mammalian Egg With Polar Bodies
Meiosis and Mitosis
Compared
Crossing over and independent
assortment cause genetic
recombination (gene shuffling)
during meiosis
Crossing Over
Independent Assortment
Number of different gametes
produced by humans
• 23 pairs of chomosomes
– Each pair line up randomly on the
metaphase plate
• Therefore 2 different combinations =
23
– > 8,000,000 different gametes from each
person
Gene shuffling during meiosis is caused
by:
A. Crossing over of chromosomes
B. Independent assortment of
chromosomes
C. Nondisjunction of chromosomes
D. Cytokinesis
E. Condensation of chromosomes
Mistakes Sometime Happen
During Meiosis
• Failure of chromosomes to separate
during meiosis creates cells with extra
or missing chromosomes
• Called Nondisjunction of Chromosomes
Examples of Nondisjunction
– Down syndrome (three # 21 chromosomes)
– Turner syndrome (one X chromosome)
– Klinefelter syndrome (XXY)
Missing or extra sex chromosomes cause Turner syndrome and Klinefelter syndrome. (a) A
female with Turner syndrome has one rather than the normal two X chromosomes. Because the
ovaries are underdeveloped, she is usually infertile. (b) A male with Klinefelter syndrome is
XXY. Klinefelter syndrome is characterized by small testes (and sometimes breast
enlargement).