Download What Are Chromosomes?

Fatchiyah, Ph.D.
Lab. Molecular Biology
Brawijaya University
5/23/2017
fatchiyah, JB UB
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Chromosome & DNA?
• Chromosomes, threadlike structures, first
observed by Nageli in 1842.
• Walter Flemming was the first to follow the
process of mitosis and replication of
chromosomes.
• Thomas Morgan, in his experiments with fruit
flies, described genetic recombination, and
demonstrated that traits were to inherited
together to varying degrees.
• Alfred Sturtevant extended Morgan’s ideas,
used observed recombination rates to
produce the first genetic maps.
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The nucleus is the center of cellular operations
• Surrounded by a nuclear envelope
•Perinuclear space
• Communicates with cytoplasm
through nuclear pores
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The Nucleus
Content of nucleus:
•A supportive nuclear matrix
•One or more nuclei
•Chromosomes
•Chromatin
•DNA bound to histones
Figure 3.13
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What Are Chromosomes?
• Cytogenetics is the study of
chromosomes and the related disease
states caused by abnormal chromosome
number and/or structure.
• Chromosomes are complex structures
located in the cell nucleus, they are
composed of DNA, histone and nonhistone proteins, RNA , and
polysaccharides.
• They are basically the "packages" that
contain the DNA.
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What Are Chromosomes?
• Normally chromosomes can't be seen with a
light microscope but during cell division they
become condensed enough to be easily
analyzed at 1000X.
• To collect cells with their chromosomes in
this condensed state they are exposed to a
mitotic inhibitor which blocks formation of
the spindle and arrests cell division at the
metaphase stage
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Type of chromosome structure
Metacentric
Chrom. 1
Submetacentric
Chrom. 9
Acrocentric
Chrom. 14
The ideogram is basically a "chromosome map"
showing the relationship between the short and
long arms, centromere (cen), and in the case of
acrocentric chromosomes the stalks (st) and
satellites (sa). The specific banding patterns are
also illustrated. Each band is numbered to aid
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describing rearrangements.
1. The short arm is
designated as p and
the long arm as q.
2. The centromere is the
location of spindle
attachment and is an
integral part of the
chromosome.
3. It is essential for the
normal movement and
segregation of
chromosomes during
cell division.
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Figure 3.14 Chromosome
Structure
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Figure 3.14
Organization of Genes on Human
Chromosome
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Structural Organization of the
Nucleosome
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Structural Organization of the
Nucleosome
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Nucleosome Disruption
Figure 4-34. A cyclic mechanism for nucleosome disruption and re-formation. According to this model, different chromatin remodeling complexes
disrupt and re-form nucleosomes, although, in principle, the same complex might catalyze both reactions. The DNA-binding proteins could function in
gene expression, DNA replication, or DNA repair, and in some cases their binding could lead to the dissociation of the histone core to form nucleosome5/23/2017
JBCell
UB
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free regions
of DNA like those illustrated in Figure 4-30. (Adapted fatchiyah,
from A. Travers,
96:311 314, 1999.)
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Chromosome Facts
• number of chromosomes: 22 pairs + 1 pair
sex-determining chromosomes = 46
– one chromosome of each pair donated from
each parent’s egg or sperm
– sex chromosomes: X,Y for males; X,X for
females
– largest chromosome #1 = ~263 million base
pairs (bp)
– smallest chromosome Y = ~59 million bp
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Chromosomes can be “painted” for easy
identification.
By technique of multiplex
fluorescence in situ hybridization (M-FISH)
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Mitotic
chromosomes
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G1 chromosomes
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Gene Facts
• size of human genome: 3.4 billion base
pairs (bp)
• number of human genes: ~100,000
• genes vary in length and can cover
thousands of bases
– avg. size: ~3,000 bp
• only about 5% of the human genome
contains genes
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UB
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function of much of
theJB genome
is unknown20
Chromosomes come in pairs
• Humans have 23 pairs of
chromosomes
–Total of 46 chromosomes
• Fruit flies have 4 pairs of
chromosomes
–Total of 8 chromosomes
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Normal Chromosomes
 Normal human somatic cells have 46 chromosomes: 22
pairs, or homologs, of autosomes (chromosomes 1-22)
and two sex chromosomes. This is called the diploid
number. Females carry two X chromosomes (46,XX)
while males have an X and a Y (46,XY).
 Germ cells (egg and sperm) have 23 chromosomes: one
copy of each autosome plus a single sex chromosome.
This is referred to as the haploid number.
 One chromosome from each autosomal pair plus one
sex chromosome is inherited from each parent.
 Mothers can contribute only an X chromosome to
their children while fathers can contribute either an
X or a Y.
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Chromosomes of human
Male
Female
Y-chrom
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Members of a chromosome pair
• Cells with PAIRS of chromosomes are
diploid
– di – means “two”, “double”, “twice”
– oid – “appearance”
• The two members of any chromosome pair are
called homologues, or a homologous pair
• Ex:
– The two “number 1” chromosomes are homologues
– The two “number 21” chromosomes are
homologues.
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In sexual reproduction
• new organisms (zygotes) are
formed by the joining together of
two sex cells (gametes):
– a sperm cell from a male and
–an egg cell from a female
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What would happen if…….
• a human egg cell had 46 chromosomes
AND
• a human sperm cell had 46 chromosomes?
46 chromosomes
+ 46 chromosomes
92 chromosomes
Too many for a human!
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Diploid organisms need….
• A way to produce sex cells (eggs and
sperm) that only have ONE member of
each chromosome pair
• In humans, sex cells each have 23
chromosomes
– ONE member of each homologous pair
– Half of amount of chromosome is named
genome (haploid)
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23 chromosomes
+23 chromosomes
46 chromosomes
Just right for a human!
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The different possibilities of the way chromosomes will
separate into gametes helps create genetic diversity
OR
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Effect of Crossing Over
Crossing over occurs
when homologous
chromosomes pair with
each other before the
first meiotic division
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Chromatids cross over one
another, and the crossed
sections of the chromatids
are exchanged.
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Chromosomes
Figure 4-14. Two closely related species of deer with very different
chromosome numbers. In the evolution of the Indian muntjac, initially separate
chromosomes fused, without having a major effect on the animal. These two
species have roughly the same number of genes. (Adapted from M.W. Strickberger,
Evolution, 3rd edition, 2000, Sudbury, MA: Jones & Bartlett Publishers
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The emphasis in the human
genome
•
•
•
•
•
Human
26,588 genes
The nervous system
The immune system
The blood system
Signaling and cell-cell
communication
Programmed cell
death
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Arabidopsis
25, 498 genes
Missing
Missing
Missing
Different
Missing
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Structure of Chromosome - NORs
Interphase nucleus of onion
root tip through light
microscope
nucleolus
Structure of Chromosome - NORs
electron micrograph of interphase cell from bat pancreas
nucleolus
Polytene Chomosome of Drosophila as
Giant chromosome
Polytene chromosome
Phase-contrast image of Drosophila
melanogaster polytene cromosomes.
A. The end of the X-chromosome is marked with
an arrow. Chromocentre is in the upper right
corner.
B. Shows a magnification of chromomere and
interchromomere bands
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W-chromosome
Yeast Artificial Chromosome
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Bacterial Artificial Chromosome Manipulation
by Homologous Recombinant
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(HeintZ, 2001)
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Metaphase: Chromosome in middle cell
Antibodies of a person with an
autoimmune disease stain
centromers