Download Chromosomes

Lecture 8
Dr. M. A. Fouad
• Chromatin is a mass of genetic
material composed of DNA and
proteins.
• In non dividing cells chromatin is
dispersed thorough the nucleus.
• Chromosomes can be visualized using
light microscope during cell division.
• During cell division, the chromatin
condense into visible chromosomes.
• Under light microscope,
chromosomes appear as
thread-like structures or
'colored bodies'.
• The word chromosome is
derived from the Greek words:
• Chroma = color
• Soma= body
• Each human cell contains approximately 2 meters of DNA if
stretched end-to-end;
• The nucleus of a human cell, which contains the DNA, is only
about 6 μm in diameter (1m = 1000000 um).
• The packaging of DNA into
chromosomes involves
several orders of DNA
coiling and folding.
• This allows the very long
DNA molecules to fit into
the cell nucleus.
• Chromosomes are formed of DNA which is embedded in
protein material.
• Each chromosomes is made of single Linear DNA molecule
and several types of associated proteins.
• The chromosome consists of
two chromatids (sister
chromatids)
• The chromatids are joined at
a constriction known as the
centromere.
• Sister chromatids, are two
identical (= exactly the
same) parts (Chromatids)
arising from replication of a
chromosome.
Each centromere divides the
chromosome into:
• Short arms designated p
(= petite) and
• Long arms designated q
('g' = grande), respectively.
Centromeres
are responsible
for the
movement of
chromosomes
at cell division.
• The tip of each chromosome
arm is known as the
telomere.
• Telomeres play a crucial role
in sealing the ends of
chromosomes and
maintaining their structural
integrity.
• During DNA replication,
telomeres become
progressively shorter.
• When a critical length is
reached, the cell can no
longer divide and becomes
senescent.
• In human, somatic cell nucleus contains 23 pairs
chromosomes made up of:
• 22 pairs of autosomes
• a single pair of sex chromosomes
• XX in the female and
• XY in the male.
• Gametes (sperms or ovum) contains 23
chromosomes (haploid number of
chromosomes or 1n)
• Somatic cell contains 46 chromosomes
(diploid number of chromosomes or 2n).
• The cell has 23 pairs of “homologous chromosomes”.
• Homologous chromosomes carries the same genes in the
same order, but the alleles for each trait may not be the
same.
• One chromosome of each homologous pair comes from the
mother and one comes from the father.
• Sister chromatids, are two
identical (= exactly the
same) parts (Chromatids)
arising from replication of a
chromosome.
• Homologous
chromosomes carries the
same genes in the same
order, but the alleles for
each trait may be different.
Human somatic cells are diploid  they have two
alleles of each gene.
• Homozygous: Identical alleles of a gene in an
individual (PP or pp)
• Heterozygous: Different alleles of a gene in an
individual (Pp)
• The X and Y chromosomes are
known as the sex chromosomes
because of their crucial role in sex
determination .
• In humans both the male and the
female have two sex chromosomes
• XX in the female and
• XY in the male.
• In the female each ovum carries
an X chromosome.
• In the male each sperm carries
either an X or a Y chromosome.
• If an X-bearing sperm fertilize
the ovum , this result in female
baby
• If an Y-bearing sperm fertilize
the ovum, this result in male
baby.
Quizzes
1. Fill in the spaces
1.
2.
3.
4.
5.
………………………
………………………
………………………
………………………
………………………
2. Human somatic cell has:
A. 23 chromosomes
B. 23 pairs of homologous chromosomes
C. 46 pairs of homologous chromosomes
3. Human somatic cell has:
A. 23 pairs of homologous chromosomes
B. 46 chromosomes
C. 22 pair of autosomes and one pair of
sex chromosomes
D. All of the above
4. Human Germ cell (gamete):
A. Has haploid number of chromosomes
B. Has diploid number of chromosomes
C. Has the same number of chromosomes
as the body cells
5. Once duplicated, a chromosome is composed of
two identical parts called _____.
A.
B.
C.
D.
Sister chromatids
Centromeres
Homologous chromosomes
Telomeres
6. Female has ______ sex chromosomes
A. XX
B. YY
C. XY
7. The mature egg of a human has _____
chromosomes
A. 46
B. 23
C. 2
8. Male has ______ sex chromosomes
A. XX
B. YY
C. XY