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SC435 Genetics Seminar
• Welcome to our Unit 3 Seminar
• We will continue our discussion of
heredity with a focus on the
chromosomes
• The seminar will begin at 9:00PM ET
Unit 3
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Discussion board
Unit 3 Quiz
Unit 3 Exam
Final Project Topic Submission
• Looking Ahead
– Unit 4
• Assignment (2-3 page paper)
UNIT 3 KEY CONCEPTS
• Chromosomes in eukaryotic cells are usually present in pairs.
• The chromosomes of each pair separate in meiosis, one going
to each gamete.
• In meiosis, the chromosomes of different pairs undergo
independent assortment.
• Chromosomes consist largely of DNA combined with histone
proteins.
• In many animals, sex is determined by a special pair of
chromosomes, the X and Y.
• Irregularities in the inheritance of an X-linked gene in Drosophila
gave experimental proof of the chromosomal theory of heredity.
Chromatin Structure
• Each core particle consists of an octamere of pairs each of
histone H2A, H2B, H3, and H4; a
segment of DNA containing about 145 base pairs
Fig.
3.15a
Fig.
3.19
Chromosomes
• The chromosomes in the nuclei of somatic cells are
usually present in pairs. For example, humans have
23 pairs of chromosomes
• Cells with nuclei of this sort, containing two similar
sets of chromosomes, are called diploid
6
Chromosomes
• The chromosome complement = the complete set of
chromosomes of plants and animals
• The nucleus of each somatic cell contains a fixed
number of chromosomes typical of the particular
species
• The number of chromosomes vary tremendously
among species and have little relationship to the
complexity of the organism
Fig. 3.2
Fig. 3.3 Mitosis: Interphase, Late prophase,
Metaphase
Fig. 3.3 Mitosis: Anaphase and
Telophase
Fig. 3.3
Mitosis
Meiosis
• Meiosis is a mode of cell division in which cells are
created that contain only one member of each pair of
chromosomes
• Meiosis consists of two successive nuclear divisions
• Meiosis results in four daughter cells, each genetically
different and each containing one haploid set of
chromosomes
• Meiosis is a more complex and considerably longer
process than mitosis and usually requires days or even
weeks
Meiosis
• The oocytes form egg cells and the
spermatocytes form sperm cells
• In the females of both animals and plants, only
one of the four products develops into a
functional cell (the other three disintegrate)
Fig. 3.5
Outline of Meiosis
• Prior to the first nuclear division, the members of each
pair of chromosomes become closely associated
along their length
• The chromosomes that pair with each other are said
to be homologous chromosomes
• Each member of a pair of homologs consists of a
duplex of two sister chromatids joined at the
centromere. The pairing of the homologous
chromosomes therefore produces a four-stranded
structure
Outline of Meiosis
• At the time of pairing, the homologs can exchange
genes which results in chromosomes that consist of
segments from one homolog intermixed with segments
from the other
• In the first nuclear division, the homologous
chromosomes are separated from each other, one
member of each pair going to opposite poles of the
spindle
• Two nuclei are formed, each containing a haploid set of
duplex chromosomes
Outline of Meiosis
• The second nuclear division resembles a
mitotic division, but there is no DNA
replication.
• At metaphase, the chromosomes align on
the metaphase plate, and at anaphase, the
chromatids are separated into opposite
daughter nuclei
• The net effect of the two divisions is the
creation of four haploid nuclei, each
containing the equivalent of a single sister
chromatid from each pair of homologous
chromosomes
Mitosis vs. Meiosis
• Meiosis produces four cells: each contains one
copy of each pair of homologous chromosomes
= genetically haploid
• Mitosis produces two cells which contain both
members of each pair of homologous
chromosomes = genetically diploid
Meiosis: Prophase I
• Leptotene - the chromosomes first become visible as long,
thread-like structures
• Zygotene - synapsis of homologous chromosomes = bivalent
• Pachytene - crossing-over between homologs
• Diplotene - chromosome repulsion,
however, they remain held together
by cross-connections resulting from
crossing-over. Each crossconnection, called a chiasma is
formed by a breakage and rejoining
between nonsister chromatids
Fig. 3.9b
Meiosis II
• The second meiotic division (meiosis II) is called the
equational division because the chromosome number
remains the same in each cell before and after the
second division
• In some species, the chromosomes pass directly from
telophase I to prophase II without loss of condensation
• After a short prophase II and the formation of seconddivision spindles, the centromeres of the chromosomes
in each nucleus become aligned on the central plane of
the spindle at metaphase II
Meiosis II
• In anaphase II, the centromeres divide and the
chromatids of each chromosome move to opposite
poles of the spindle
• Once the centromere has split at anaphase II, each
chromatid is considered a separate chromosome
• Telophase II is a transition to the interphase condition
of the chromosomes in the four haploid nuclei,
accompanied by division of the cytoplasm.
Meiosis
• The chromatids of a chromosome are usually
not genetically identical because of crossingover associated with the formation of
chiasmata during prophase of the first
division
Chromatin Structure
• Nucleosomes coil to form higher order DNA structure
called the 30-nm chromatin fiber
• In the nucleus of a nondividing cell, chromatin fibers form
discrete chromosome territories
• Chromosome territories are correlated with gene
densities
• Territories of chromosome domains that are relatively
gene rich tend to be located toward the
interior of the nucleus
Fig. 3.21
(Micrograph courtesy of T.C. Hsu and Sen
Pathak)
Nondisjunction
• Experimental proof of the chromosome theory of
heredity came from nondisjunction
• Nondisjunction = chromosomes fail to separate
(disjoin) and move to opposite poles of the division
spindle, results in loss or gain of a chromosome
• Calvin Bridges demonstrated that exceptional behavior
of chromosomes is precisely paralleled by exceptional
inheritance of their genes
X-Linked Inheritance
• Special chromosomes determine sex in many
organisms
• X and Y chromosomes = sex chromosomes which
are non-identical but share some genes
• In most organisms, the Y chromosome carries few
genes other than those related to male
determination
• X-linked genes are inherited according to sex
• Hemophilia is a classic example of human Xlinked inheritance