NPTEL – Biotechnology – Cell Biology
Module 2- Chromosome structure and organisation
This module deals with the genetic material of the cell, its structure, with details of the
human chromosome and the giant chromosomes.
Module 2 Lecture 1
Genetic material in a cell: All cells have the capability to give rise to new cells and the
encoded information in a living cell is passed from one generation to another. The
information encoding material is the genetic or hereditary material of the cell.
Prokaryotic genetic material:
The prokaryotic (bacterial) genetic material is usually concentrated in a specific clear
region of the cytoplasm called nucleiod. The bacterial chromosome is a single, circular,
double stranded DNA molecule mostly attached to the plasma membrane at one point. It
does not contain any histone protein. Escherichia coli DNA is circular molecule 4.6
million base pairs in length, containing 4288 annotated protein-coding genes (organized
into 2584 operons), seven ribosomal RNA (rRNA) operons, and 86 transfer RNA (tRNA)
genes. Certain bacteria like the Borrelia burgdorferi possess array of linear chromosome
like eukaryotes.
Besides the chromosomal DNA many bacteria may also carry extra chromosomal genetic
elements in the form of small, circular and closed DNA molecules, called plasmids. They
generally remain floated in the cytoplasm and bear different genes based on which they
have been studied. Some of the different types of plasmids are F plasmids, R plasmids,
virulent plasmids, metabolic plasmids etc. Figure 1 depicts a bacterial chromosome and
plasmid.
Figure 1: Bacterial genetic material
Joint initiative of IITs and IISc – Funded by MHRD Page 1 of 24
,NPTEL – Biotechnology – Cell Biology
Virus genetic material:
The chromosomal material of viruses is DNA or RNA which adopts different structures.
It is circular when packaged inside the virus particle.
Eukaryotic genetic material:
A Eukaryotic cell has genetic material in the form of genomic DNA enclosed within the
nucleus. Genes or the hereditary units are located on the chromosomes which exist as
chromatin network in the non dividing cell/interphase. This will be discussed in detail in
the coming sections.
Chromosome:
German biologist Walter Flemming in the early 1880s revealed that during cell division
the nuclear material organize themselves into visible thread like structures which were
named as chromosomes which stains deep with basic dyes. The term chromosome was
coined by W. Waldeyer in 1888. Chrome is coloured and soma is body, hence they mean
“colored bodies” and can be defined as higher order organized arrangement of DNA and
proteins. It contains many genes or the hereditary units, regulatory elements and other
nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve in
packaging the DNA and control its functions. Chromosomes vary both in number and
structure among organisms (Table 1) and the number of chromosomes is characteristic of
every species. Benden and Bovery in 1887 reported that the number of chromosomes in
each species is constant. W.S. Sutton and T. Boveri in 1902 suggested that chromosomes
are the physical structures which acted as messengers of heredity.
Chromosomes are tightly coiled DNA around basic histone proteins, which help in the
tight packing of DNA. During interphase, the DNA is not tightly coiled into
chromosomes, but exists as chromatin. The structure of a chromosome is given in Figure
2. In eukaryotes to fit the entire length of DNA in the nucleus it undergoes condensation
and the degree to which DNA is condensed is expressed as its packing ratio which is the
length of DNA divided by the length into which it is packaged into chromatin along with
proteins.
Joint initiative of IITs and IISc – Funded by MHRD Page 2 of 24
, NPTEL – Biotechnology – Cell Biology
Figure 2: Eukaryotic chromosome
The shortest human chromosome contains 4.6 x 107 bp of DNA. This is equivalent to
14,000 µm of extended DNA. In its most condensed state during mitosis, the
chromosome is about 2 µm long. This gives a packing ratio of 7000 (14,000/2). The
DNA is packaged stepwise into the higher order chromatin structure and this is known as
“hierarchies of chromosomal organization”. The level of DNA packaging is
schematically represented in Table 2.
Chromosome number:
There are normally two copies of each chromosome present in every somatic cell. The
number of unique chromosomes (N) in such a cell is known as its haploid number, and
the total number of chromosomes (2N) is its diploid number. The suffix ‘ploid’ refers to
chromosome ‘sets’. The haploid set of the chromosome is also known as the genome.
Structurally, eukaryotes possess large linear chromosomes unlike prokaryotes which have
circular chromosomes. In Eukaryotes other than the nucleus chromosomes are present in
mitochondria and chloroplast too. The number of chromosomes in each somatic cell is
same for all members of a given species. The organism with lowest number of
chromosome is the nematode, Ascaris megalocephalusunivalens which has only two
chromosomes in the somatic cells (2n=2).
Joint initiative of IITs and IISc – Funded by MHRD Page 3 of 24
Module 2- Chromosome structure and organisation
This module deals with the genetic material of the cell, its structure, with details of the
human chromosome and the giant chromosomes.
Module 2 Lecture 1
Genetic material in a cell: All cells have the capability to give rise to new cells and the
encoded information in a living cell is passed from one generation to another. The
information encoding material is the genetic or hereditary material of the cell.
Prokaryotic genetic material:
The prokaryotic (bacterial) genetic material is usually concentrated in a specific clear
region of the cytoplasm called nucleiod. The bacterial chromosome is a single, circular,
double stranded DNA molecule mostly attached to the plasma membrane at one point. It
does not contain any histone protein. Escherichia coli DNA is circular molecule 4.6
million base pairs in length, containing 4288 annotated protein-coding genes (organized
into 2584 operons), seven ribosomal RNA (rRNA) operons, and 86 transfer RNA (tRNA)
genes. Certain bacteria like the Borrelia burgdorferi possess array of linear chromosome
like eukaryotes.
Besides the chromosomal DNA many bacteria may also carry extra chromosomal genetic
elements in the form of small, circular and closed DNA molecules, called plasmids. They
generally remain floated in the cytoplasm and bear different genes based on which they
have been studied. Some of the different types of plasmids are F plasmids, R plasmids,
virulent plasmids, metabolic plasmids etc. Figure 1 depicts a bacterial chromosome and
plasmid.
Figure 1: Bacterial genetic material
Joint initiative of IITs and IISc – Funded by MHRD Page 1 of 24
,NPTEL – Biotechnology – Cell Biology
Virus genetic material:
The chromosomal material of viruses is DNA or RNA which adopts different structures.
It is circular when packaged inside the virus particle.
Eukaryotic genetic material:
A Eukaryotic cell has genetic material in the form of genomic DNA enclosed within the
nucleus. Genes or the hereditary units are located on the chromosomes which exist as
chromatin network in the non dividing cell/interphase. This will be discussed in detail in
the coming sections.
Chromosome:
German biologist Walter Flemming in the early 1880s revealed that during cell division
the nuclear material organize themselves into visible thread like structures which were
named as chromosomes which stains deep with basic dyes. The term chromosome was
coined by W. Waldeyer in 1888. Chrome is coloured and soma is body, hence they mean
“colored bodies” and can be defined as higher order organized arrangement of DNA and
proteins. It contains many genes or the hereditary units, regulatory elements and other
nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve in
packaging the DNA and control its functions. Chromosomes vary both in number and
structure among organisms (Table 1) and the number of chromosomes is characteristic of
every species. Benden and Bovery in 1887 reported that the number of chromosomes in
each species is constant. W.S. Sutton and T. Boveri in 1902 suggested that chromosomes
are the physical structures which acted as messengers of heredity.
Chromosomes are tightly coiled DNA around basic histone proteins, which help in the
tight packing of DNA. During interphase, the DNA is not tightly coiled into
chromosomes, but exists as chromatin. The structure of a chromosome is given in Figure
2. In eukaryotes to fit the entire length of DNA in the nucleus it undergoes condensation
and the degree to which DNA is condensed is expressed as its packing ratio which is the
length of DNA divided by the length into which it is packaged into chromatin along with
proteins.
Joint initiative of IITs and IISc – Funded by MHRD Page 2 of 24
, NPTEL – Biotechnology – Cell Biology
Figure 2: Eukaryotic chromosome
The shortest human chromosome contains 4.6 x 107 bp of DNA. This is equivalent to
14,000 µm of extended DNA. In its most condensed state during mitosis, the
chromosome is about 2 µm long. This gives a packing ratio of 7000 (14,000/2). The
DNA is packaged stepwise into the higher order chromatin structure and this is known as
“hierarchies of chromosomal organization”. The level of DNA packaging is
schematically represented in Table 2.
Chromosome number:
There are normally two copies of each chromosome present in every somatic cell. The
number of unique chromosomes (N) in such a cell is known as its haploid number, and
the total number of chromosomes (2N) is its diploid number. The suffix ‘ploid’ refers to
chromosome ‘sets’. The haploid set of the chromosome is also known as the genome.
Structurally, eukaryotes possess large linear chromosomes unlike prokaryotes which have
circular chromosomes. In Eukaryotes other than the nucleus chromosomes are present in
mitochondria and chloroplast too. The number of chromosomes in each somatic cell is
same for all members of a given species. The organism with lowest number of
chromosome is the nematode, Ascaris megalocephalusunivalens which has only two
chromosomes in the somatic cells (2n=2).
Joint initiative of IITs and IISc – Funded by MHRD Page 3 of 24
