Chromosome
Contents for Tables:-
S.R.No. | Contents | Page No. |
1 | Acknowledgement | 3-3 |
2 | Declaration | 4-4 |
3 | Introduction | 5-5 |
4 | Chromosome in Prokaryotic Cell | 6-7 |
5 | Chromosome in eukaryotic Cell | 7-9 |
6 | Chromosome and Plasmids | 9-11 |
7 | Giant Chromosome | 11-13 |
8 | Chromosome Meaning and Discovery | 13-14 |
9 | Chromosome Structure | 15-19 |
10 | Type of Chromosome | 20-37 |
11 | Function of Chromosome | 37-39 |
Introduction
A chromosome is a package of DNA with part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome forming packaging proteins; in eukaryotic cells the most important of these proteins are the histones. These proteins, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity. These chromosomes display a complex three-dimensional structure, which plays a significant role in transcriptional regulation.
Chromosomes are normally visible under a light microscope only during the metaphase of cell division (where all chromosomes are aligned in the center of the cell in their condensed form). Before this happens, each chromosome is duplicated (S phase), and both copies are joined by a centromere, resulting either in an X-shaped structure (pictured above), if the centromere is located equatorially, or a two-arm structure, if the centromere is located distally. The joined copies are now called sister chromatids. During metaphase the X-shaped structure is called a metaphase chromosome, which is highly condensed and thus easiest to distinguish and study. In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.
Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer.
Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation.
Chromosomes
Chromosomes are present in the nucleus and they carry genetic information. They play an important role in cell division, heredity, variation, growth and repair, etc. The term chromosome was coined by W. Waldeyer in 1883 to describe darkly stained material in the nucleus.
Chromosomes in prokaryotic cells
Prokaryotic cells mostly have a single circular chromosome. Some bacteria have linear chromosomes also. They are present in the cytoplasm as the nucleus is absent. DNA is not scattered all around in the cell but is present in the nucleoid region. The DNA makes large loops and is held by proteins. The organisation of chromosomes in prokaryotes is less complex compared to eukaryotes.
Bacteria often get a bad rap: they’re described as unsafe “bugs” that cause disease. Although some types of bacteria do cause disease (as you know if you’ve ever been prescribed antibiotics), many other are harmless, or even beneficial.
Bacteria are classified as prokaryotes, along with another group of single-celled organisms, the archaea. Prokaryotes are tiny, but in a very real sense, they dominate the Earth. They live nearly everywhere – on every surface, on land and in water, and even inside of our bodies.
To emphasize that last point: you probably have about the same number of prokaryotic cells in your body as human cells! That may sound gross, but many of our prokaryotic “sidekicks” play important roles in keeping us healthy.
In this article, we’ll look at what prokaryotes are and what exactly makes them different from eukaryotes (such as you, a houseplant, or a fungus). Then, we’ll take a closer look at the structures these efficient, omnipresent little organisms use to survive.
Chromosomes in eukaryotic cells
Eukaryotic chromosomes contain a linear DNA double helix associated with proteins. Chromosomes appear as long thin threads at the interphase called chromatin. DNA is wrapped around the histone octamer to form a nucleosome, which is the repeating unit of chromatin thread. Chromatin fibres coil and condense further to form chromosomes at the metaphase. The structure of chromosomes is mostly studied at mitotic metaphase.
Chromosome number, size, shape, etc. varies in different organisms. Each diploid cell contains two sets of homologous chromosomes.
Eukaryotic cells are defined as cells containing organized nucleus and organelles which are enveloped by membrane-bound organelles. Examples of eukaryotic cells are plants, animals, protists, fungi. Their genetic material is organized in chromosomes. Golgi apparatus, Mitochondria, Ribosomes, Nucleus are parts of Eukaryotic Cells. Let’s learn about the parts of eukaryotic cells in detail.

Cytoplasmic Membrane
Description: It is also called plasma membrane or cell membrane. The plasma membrane is a semi-permeable membrane that separates the inside of a cell from the outside.
Structure and Composition: In eukaryotic cells, the plasma membrane consists of proteins, carbohydrates and two layers of phospholipids (i.e. lipid with a phosphate group). These phospholipids are arranged as follows:
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- The polar, hydrophilic (water-loving) heads face the outside and inside of the cell. These heads interact with the aqueous environment outside and within a cell.
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- The non-polar, hydrophobic (water-repelling) tails are sandwiched between the heads and are protected from the aqueous environments.
Scientists Singer and Nicolson described the structure of the phospholipid bilayer as the ‘Fluid Mosaic Model’. The reason is that the bi-layer looks like a mosaic and has a semi-fluid nature that allows lateral movement of proteins within the bilayer.

Chromosome and plasmids
Most prokaryotes have a single circular chromosome, and thus a single copy of their genetic material. Eukaryotes like humans, in contrast, tend to have multiple rod-shaped chromosomes and two copies of their genetic material (on homologous chromosomes).
Also, prokaryotic genomes are generally much smaller than eukaryotic genomes. For instance, the E. coli genome is less than half the size of the genome of yeast (a simple, single-celled eukaryote), and almost times smaller than the human genome.
By definition, prokaryotes lack a membrane-bound nucleus to hold their chromosomes. Instead, the chromosome of a prokaryote is found in a part of the cytoplasm called a nucleoid.

Prokaryotes generally have a single circular chromosome that occupies a region of the cytoplasm called a nucleoid. They also may contain small rings of double-stranded extra-chromosomal DNA called plasmids.
In addition to the chromosome, many prokaryotes have plasmids, which are small rings of double-stranded extra-chromosomal (“outside the chromosome”) DNA. Plasmids carry a small number of non-essential genes and are copied independently of the chromosome inside the cell. They can be transferred to other prokaryotes in a population, sometimes spreading genes that are beneficial to survival.
For instance, some plasmids carry genes that make bacteria resistant to antibiotics. (These genes are called R genes.) When the plasmids carrying R genes are exchanged in a population, they can quickly make the population resistant to antibiotic drugs. While beneficial to the bacteria, this process can make it difficult for doctors to treat harmful bacterial infections.
Giant Chromosomes
Polytene chromosome
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- Balbiani first discovered a structure in the nuclei of secretory glands of midges
- Painter, Heitz and Bauer, rediscovered them in the salivary gland of Drosophila and recognised them as a chromosome
- Also known as Salivary gland chromosome
- These are called polytene by Kollar due to the presence of many chromonemata in them
- These are present in some cells of the larvae of Dipteran insects
- These are very large due to the presence of high DNA content
- The polytene chromosome of Drosophila’s salivary gland has 1000 DNA molecules Chironomus has 1600 DNA molecules in its each polytene chromosome
- There is a series of alternating dark and clear bands called interband
- Chromosome puffs or Balbiani rings are present, which are the swelling of bands due to DNA unfolding into open loops. These are the region of the intense transcription or mRNA formation
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- Lampbrush chromosome
- Balbiani first discovered a structure in the nuclei of secretory glands of midges

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- First discovered in the oocytes of salamander
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- The name is given due to its resemblance with a brush that is used for cleaning lamp, glass chimneys, etc.
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- They occur at the diplotene stage of oocytes in vertebrates and invertebrates
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- Lampbrush chromosomes are also found in the spermatocytes of many animals and also found in the giant nucleus of an algae Acetabularia
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- They are present as a bivalent with 4 chromatids
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- Chromosomal axis is formed from highly condensed chromatin and lateral loops extend from the row of chromomeres
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- Lateral loops of DNA are always symmetrical and formed due to intense RNA synthesis
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- In the oocytes of salamander, there are 10,000 loops present per haploid set of chromosomes
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- The centromere doesn’t bear any loops
Chromosome Meaning and Discovery
Chromosome means ‘coloured body’, that refers to its staining ability by certain dyes.
Karl Nägeli in 1842, first observed the rod-like structure present in the nucleus of the plant cell.
W. Waldeyer in 1888 coined the term ‘chromosome’.
Walter Sutton and Theodor Boveri in 1902 suggested that chromosomes are the physical carrier of genes in the eukaryotic cells.
The number of chromosomes in any species is constant for all the cells. The number of chromosomes in gametes (e.g. sperms, egg) is half of the somatic cell and known as a haploid set of chromosomes, which is the result of meiosis during sexual reproduction. Chromosome number is preserved in the mitotic division of somatic cells, which is required for an organism to grow, repair and regenerate.
Chromosome number varies in different species. A nematode species contains only 2 chromosomes in a cell, whereas a protozoan species contains as much as 1600 chromosomes in the cell. Most of the plant and animal species contain 8 to 50 number of chromosomes in its somatic cell. The number of chromosomes does not reflect the complexity of a species. A human cell contains total 23 pair of chromosomes (2n, total 23×2=46), of which 22 are autosomes and 1 sex chromosome.
Karyotyping is a technique to study the structure of chromosomes present in a species. Chromosomes are isolated, stained and photographed. This technique is useful in finding out any chromosomal abnormalities.
Chromosome Structure
The chemical composition of a chromosome is histone proteins and DNA. Each cell has a pair of each kind of chromosome known as a homologous chromosome. Chromosomes are made up of chromatin, which contains a single molecule of DNA and associated proteins. Each chromosome contains hundreds and thousands of genes that can precisely code for several proteins in the cell. Structure of a chromosome can be best seen during cell division.

Main parts of chromosomes are:
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- Chromatid: Each chromosome has two symmetrical structures called chromatids or sister chromatids which is visible in mitotic metaphase.
- Each chromatid contains a single DNA molecule
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- At the anaphase of mitotic cell division, sister chromatids separate and migrate to opposite poles
- Chromatid: Each chromosome has two symmetrical structures called chromatids or sister chromatids which is visible in mitotic metaphase.
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- Centromere and kinetochore: Sister chromatids are joined by the centromere.
- Spindle fibres during cell division are attached at the centromere
- The number and position of the centromere differs in different chromosomes
- The centromere is called primary constriction
- Centromere divides the chromosome into two parts, the shorter arm is known as ‘p’ arm and the longer arm is known as ‘q’ arm.
- The centromere contains a disc-shaped kinetochore, which has specific DNA sequence with special proteins bound to them
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- The kinetochore provides the centre for polymerisation of tubulin proteins and assembly of microtubules
- Centromere and kinetochore: Sister chromatids are joined by the centromere.
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- Secondary constriction and nucleolar organisers: Other than centromere, chromosomes possess secondary constrictions.
- Secondary constrictions can be identified from centromere at anaphase because there is bending only at the centromere (primary constriction)
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- Secondary constrictions, which contain genes to form nucleoli are known as the nucleolar organiser
- Secondary constriction and nucleolar organisers: Other than centromere, chromosomes possess secondary constrictions.
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- Telomere: Terminal part of a chromosome is known as a telomere.
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- Telomeres are polar, which prevents the fusion of chromosomal segments
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- Telomere: Terminal part of a chromosome is known as a telomere.
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- Satellite: It is an elongated segment that is sometimes present on a chromosome at the secondary constriction.
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- The chromosomes with satellite are known as sat-chromosome
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- Satellite: It is an elongated segment that is sometimes present on a chromosome at the secondary constriction.
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- Chromatin: Chromosome is made up of chromatin. Chromatin is made up of DNA, RNA and proteins. At interphase, chromosomes are visible as thin chromatin fibres present in the nucleoplasm. During cell division, the chromatin fibres condense and chromosomes are visible with distinct features.
- The darkly stained, condensed region of chromatin is known as heterochromatin. It contains tightly packed DNA, which is genetically inactive
- The light stained, diffused region of chromatin is known as euchromatin. It contains genetically active and loosely packed DNA
- At prophase, the chromosomal material is visible as thin filaments known as chromonemata
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- At interphase, bead-like structures are visible, which are an accumulation of chromatin material called chromomere. Chromatin with chromomere looks like a necklace with beads
- Chromatin: Chromosome is made up of chromatin. Chromatin is made up of DNA, RNA and proteins. At interphase, chromosomes are visible as thin chromatin fibres present in the nucleoplasm. During cell division, the chromatin fibres condense and chromosomes are visible with distinct features.
Labelled diagram of chromosome is given below.

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