Genetic Information Storage Structures

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Chromosomes

• Genetic Information Storage Structures

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Chromosome Structure
DNA wrapped around histone proteins forming
bead-like units or nucleosomes These are
coiled forming a super coil which coils to form
a chromosome
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• Gene
• Segment of DNA on chromosome that codes for the
  making of a polypeptide chain or protein

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Chromosome Copies

• Review
• DNA is copied, creating a chromosome with two
  identical sides called
• sister chromatids
• which are connected at the centromere

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Chromosomes usually occur in pairs that are
similar to each other Homologous Chromosomes
Humans have 23 pairs How many total?
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Chromosome Number

• Diploid (2n) -
• Cells containing both members of a homologous
  pair
• Ex Somatic (Body) Cells

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Haploid (n) Cells containing one member of
a homologous pair Gametes (Reproductive
cells - sperm and egg)
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Chromosomes and Development

• Sexual Reproduction
• Fusion of two haploid cells from two parents
• What type of cell would this make???

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Zygote Diploid cell formed from sexual
reproduction (fertilized egg) Has full set of
DNA to develop into new organism
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Meiosis
Production of Gametes (One diploid cell
produces four haploid cells)
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Overview of The Process of Meiosis
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Chromosomes duplicate
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Homologous chromosomes separate
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Sister chromatids separate
Homologous pair of chromosomes in diploid parent
cell
Homologous pair of duplicated chromosomes
Sister chromatids
Interphase before meiosis
Meiosis II
Meiosis I
Figure 8.14
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Two parts

• Meiosis I
• Homologous chromosomes separate into two
• haploid cells

2. Meiosis II Chromatids of each chromosome
separate forming at total of four haploid
cells
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Interphase
Chromosomes/DNA replicate
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Prophase I
Chromosomes condense Nucleolus breaks
down Homologous chromosomes pair up forming a
tetrad (synapsis) Crossing-over
occurs (exchange of sections of chromosomes and
thus the genetic information
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Crossing Over
(Where crossing over occurs)
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Metaphase I
Homologous chromosomes move to the middle and
spindle fibers attach
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Anaphase I
Homologous chromosomes separate and begin moving
to opposite sides Nondisjunction can occur
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Nondisjunction
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Telophase I
Cytoplasm divides and nuclei form Two new cells
have ½ the number of chromosomes of the original
cell
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MEIOSIS II
JUST LIKE MITOSIS )
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Prophase II
Chromosomes condense Nucleus breaks down No
replication occurs!
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Metaphase II
Chromosomes (2 chromatids each) line up in the
center
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Anaphase II
Chromatids separate and move to opposite poles of
the cell
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Telophase II
Cytoplasm divides Nuclei form around four
haploid cells
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Formation of sperm and egg - Only one egg is
usually produced due to unequal dividing of the
cytoplasm Other three cells are called polar
bodies
However, four sperm are produced!
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Meiosis allows for Genetic Recombination- Source
of variation among organisms because of crossing
over and independent assortment of homologous
chromosomes
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Mitosis vs. Meiosis
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2
YES
NO
DIPLOID
HAPLOID
YES- ASEXUAL
YES - SEXUAL
YES
NO
2
1
NO
YES