GENETICS SCRIPT
Chromosomes
So the coloured thread like bodies present in the nucleoplasm of the living cells, which helps in
the inheritance (transmission) of characters in form of Genes from generation to generation are
known as CHROMOSOMES.
E. strasburger
was one of the most admirable scientists in the field of plant biology, not just as the founder of
modern plant cell biology but in addition as an excellent teacher who strongly believed in
"education through science."
Chromosomes number
The number of chromosomes in a given species is generally constant.
All the members of the species ordinarily have definite and generally a constant somatic and
gametic chromosome number.
● Somatic chromosome number is the number of chromosomes found in somatic cells
of a species and is represented by 2n.
Generally somatic cells contain two copies of each chromosome except the sex
chromosomes.
Both the copies are ordinarily identical in morphology, gene content and gene order and
hence known as homologous chromosomes (chromosome pairs in a diploid organism
(like humans) that have the same genes at the same locations (loci), though the versions
of those genes (alleles) might differ.)
EXAMPLE: Humans have 23 pairs of homologous chromosomes, totaling 46
chromosomes. One set of 23 chromosomes comes from the mother, and the other set of
23 comes from the father.
● Gametic chromosome number is exactly half of somatic chromosome number and is
represented by n. It denotes the number of chromosomes found in gametes of a species.
CHROMOSOMES SIZE
Chromosome size is not proportional to the number of genes present on the chromosome
● Interphase: chromosome are longest & thinnest
● Prophase: there is a progressive decrease in their length accompanied with an increase
in thickness
● Anaphase: chromosomes are smallest.
● Metaphase: Chromosomes are the most easily observed and studied during metaphase
when they are very thick, quite short and well spread in the cell.
CHROMOSOME MORPHOLOGY
The outer covering or sheath of a chromosome is known as pellicle, which encloses the matrix
(it's the substance that fills the space within the chromosome, surrounding the chromonemata
(These are the spirally coiled threads within the matrix of each chromosome. They are
considered the main structural component of the chromosome) It's composed of non-genetic
material) Within the matrix lies the chromatin.
GENETICS SCRIPT
● Walther Flemming a German biologist who described chromosomes and their behavior
during cell division in the late 19th century. Alexander Fleming is famous for discovering
penicillin, the first widely effective antibiotic.
Flemming introduced the term chromatin in 1879.
Chromatin is the material that makes up chromosomes inside the nucleus of eukaryotic
cells.
The chromosome morphology changes during cell division and mitotic metaphase is the most
suitable stage for studies on chromosome morphology.
In mitotic metaphase chromosomes, the following structural features can be seen under the light
microscope.
Chromatid
Each metaphase chromosome appears to be longitudinally divided into two identical parts each
of which is called chromatid.
Chromatids of a chromosome appear to be joined together at a point known as centromere.
Two chromatids making up a chromosome are referred to as sister chromatids.
The chromatids of homologous chromosomes are known as non-sister chromatids ( are a pair of
chromosomes, one inherited from each parent, containing the same genes but potentially
different alleles) on the other hand, sister chromatid are identical copies formed during
replication.
Centromere
The region where two sister chromatids appear to be joined during mitotic metaphase is known
as centromere.
It generally appears as constriction and hence called primary constriction.
helps in the movement of the chromosomes to opposite poles during anaphase of cell division.
The centromere consists of two disk shaped bodies called kinetochores.
Normally chromosomes are monocentric having one centromere each.
Telomere
a region of repetitive nucleotide sequences at the end of a chromosome that protects the end of
the chromosome from damage and fusion.
The two ends of chromosomes are known as telomeres.
They are highly stable and do not fuse or unite with telomeres of other chromosomes due to
polarity effect.
Any broken end of a chromosome is unstable and can join with a piece of any other
chromosome.
But the telomeres impart stability to the chromosome, which retains its identity and individuality
through cell cycle and for many cell generations.
Secondary constriction
The constricted or narrow region other than that of centromere is called secondary constriction.
The chromosomes having secondary constriction are known as satellite chromosomes or sat
chromosomes.
Chromosomes may possess secondary constriction in one or both arms of it.
Chromosomal end distal to the secondary constriction is known as satellite.
Production of nucleolus is associated with secondary constriction and therefore it is also called
nucleolus organizer region.
GENETICS SCRIPT
Satellite chromosomes are often referred to as nucleolus organizer chromosomes due to their
role in nucleolus formation.
Chromomere
In some species like maize, rye etc. chromosomes in the pachytene stage of meiosis show
small bead-like structures called chromomeres.
The distribution of chromomeres in chromosomes is highly characteristic and constant.
The pattern of distribution being different for different chromosomes.
They are clearly visible as dark staining bands in the giant salivary gland chromosomes.
Chromomeres are regions of tightly folded DNA.
Chromonema
A chromosome consists of two chromatids and each chromatid consists of thread-like coiled
structures called chromonema (plural chromonemata).
The term chromonema was coined by Vejdovsky in 1912.
The chromonemata form the gene bearing portion of chromosomes.
Matrix
The mass of achromatic material which surrounds the chromonemata is called matrix.
The matrix is enclosed in a sheath which is known as pellicle.
Both matrix and pellicle are non genetic materials and appear only at metaphase, when the
nucleolus disappears.
Chromosomes can be grouped as;
a) Metacentric: Centromere is located exactly at the centre of chromosome, Such
chromosomes assume 'V' shape at anaphase.
b) Submetacentric: The centromere is located on one side of the centre. point such that one
arm is longer than the other. These chromosomes become 'J' or 'L' shaped at anaphase.
c) Acrocentric: Centromere is located close to one end of the chromosome and thus giving a
very short arm and a very long arm. These chromosomes acquire shape or rod shape during
anaphase.
d) Telocentric: Centromere is located at one end of the chromosome so that the chromosome
has only one arm. These chromosomes are rod shaped.
[I]- The chromosomes are capable of self-duplication. During duplication process the DNA
strands unwind. As unwinding starts, each template of DNA forms its complementary strand in
double-helix nature. The conversion of the old DNA molecule into two new molecules, helps in
duplicating the chromosomes.
[II]- They help in expression of different characters in an organism by synthesizing proteins in
cells. A definite protein is accumulated to produce a definite character.
[III]- As carrier of genes they transmit characters from generation to generation , i.e.
parents to offspring.
[IV]- The chromosomes control the physiological and biochemical processes in the body of
the organism.
Functions of Chromosomes
GENETICS SCRIPT
1. Carries Genetic Information
Chromosomes store the genetic code that determines how an organism grows, functions, and
looks.
● Each chromosome contains genes, which are segments of DNA.
● These genes are like instructions or “recipes” for making proteins.
● Example: A gene on chromosome 11 may code for the protein insulin.
2. Controls Cellular Activities
Genes on chromosomes tell cells what proteins to make and when to make them.
● Proteins are essential for almost every function in the body (e.g., enzymes, hormones,
structural proteins).
● If a gene is “switched on,” it tells the cell to make a specific protein.
● This controls activities like digestion, cell repair, growth, and metabolism.
3. Ensures Accurate DNA Replication and Division
Before a cell divides, chromosomes duplicate to ensure that each new cell gets a full copy of
DNA.
● During mitosis (for growth and repair) or meiosis (for reproduction), chromosomes line
up and split.
● This prevents errors that could lead to genetic disorders or cell malfunction.
4. Determines Heredity and Traits
Chromosomes are passed from parents to offspring.
● You inherit half of your chromosomes from your mother and half from your father.
● This mix of chromosomes determines your traits like eye color, blood type, or risk for
certain diseases.
GENETICS SCRIPT
5. Protects DNA from Damage
The DNA in chromosomes is tightly packed and coiled to prevent it from being damaged.
● Special protective structures called telomeres at the ends of chromosomes help prevent
DNA loss during replication.
● Without this packaging, long strands of DNA could break or become tangled.
