What is Chromosome
A chromosome is a thread-like structure made up of DNA (deoxyribonucleic acid) and proteins, found in the nucleus of most living cells. It carries genetic information in the form of genes, which determine an organism's traits and biological functions. Chromosomes play a key role in the processes of cell division (mitosis and meiosis), ensuring that DNA is accurately replicated and distributed to new cells.
DNA and Genes
Chromosomes are composed of long strands of DNA. Genes, which are segments of DNA, are located on the chromosomes and provide the instructions for building proteins and regulating the body's processes.
Chromosome Numbers in Humans
In humans, there are typically 46 chromosomes arranged in 23 pairs. One set of 23 comes from the mother, and the other set comes from the father. Of these, 22 pairs are called autosomes, and the remaining pair are the sex chromosomes (XX for females and XY for males).
Mitosis and Meiosis
During cell division, chromosomes ensure that genetic material is copied and distributed equally. In mitosis, the division occurs to produce identical body cells, while in meiosis, chromosome numbers are halved to form gametes (sperm and egg cells), which is crucial for sexual reproduction.
Chromosomal Disorders
Abnormalities in chromosome number or structure can lead to genetic disorders. For example, Down syndrome occurs due to an extra copy of chromosome 21 (trisomy 21), and Turner syndrome results from a missing X chromosome in females.
Chromosomes are vital for inheritance, genetic diversity, and proper cellular function.
History of Discovery of Chromosome
The discovery of chromosomes is attributed to several scientists over the 19th century, but Walther Flemming, a German biologist, is widely credited with making major contributions to this discovery.
Key Figures in the Discovery of Chromosomes include:
Walther Flemming (1879)
Flemming is considered the first to observe and describe chromosomes in detail. He studied the process of cell division, which he called mitosis, and used special dyes to stain the thread-like structures (later known as chromosomes) within the cell nucleus. He did not initially understand the genetic significance of these structures, but his work laid the foundation for future discoveries.
Karl Wilhelm von Nägeli (1842)
Swiss botanist von Nägeli observed thread-like structures in plant cells, which were the first observations of what would later be identified as chromosomes. He described them as “transitory cytoblasts”, but did not grasp their significance. The term “chromosome” was coined by Heinrich Wilhelm Waldeyer in 1888, building on Flemming’s discoveries.
Edouard Van Beneden (1883)
A Belgian scientist, Van Beneden observed chromosomes in roundworm eggs and noted that they exist in pairs. His work helped to establish the concept of chromosome pairing during sexual reproduction.
Wilhelm Roux (1883)
Roux proposed that chromosomes carry hereditary information, suggesting that their distribution during cell division plays a crucial role in inheritance.
Types of Chromosomes
Chromosomes can be classified into various types based on different criteria such as their shape, function, and the type of organism they are found in. The main types of chromosomes are:
1. Based on Centromere Position and Shape
Chromosomes are classified by the position of the centromere. The types include
a. Telocentric Chromosomes
Root words: "Telo-": from Greek "telos," meaning end. "Centric": from Greek "kentron," meaning center.
Definition: In a telocentric chromosome, the centromere is located at the very end of the chromosome, making it appear as if the chromosome has only one arm. Essentially, one side of the centromere is almost non-existent. They are found in some organisms but not in humans.
Shape: The chromosome looks i-shaped during cell division.
b. Acrocentric Chromosomes
Root words: "Acro-": from Greek "akron," meaning extreme or tip. "Centric": meaning center.
Definition: In an acrocentric chromosome, the centromere is located close to the end, but not at the very tip. This results in one very short arm (the p arm) and one long arm (the q arm). In humans, chromosomes 13, 14, 15, 21, and 22 are acrocentric.
Shape: The chromosome looks j-shaped during cell division.
c. Submetacentric Chromosomes
Root words: "Sub-": from Latin, meaning below or near. "Meta-": from Greek "meta," meaning middle. "Centric": meaning center.
Definition: In a submetacentric chromosome, the centromere is positioned slightly off-center, resulting in one arm being longer than the other, but not as extreme as in acrocentric chromosomes. Most human chromosomes fall into this category.
d. Metacentric Chromosomes
Root words: "Meta-": meaning middle. "Centric": meaning center.
Definition: In a metacentric chromosome, the centromere is located exactly in the middle, giving the chromosome two arms of equal length. Human chromosomes 1 and 3 are examples of metacentric chromosomes.
Shape: The chromosome looks v-shaped during cell division.
2. Based on Function
Chromosomes are also classified based on their role in the organism.
a. Autosomes
These are non-sex chromosomes that carry the bulk of an organism's genetic information, including traits like hair color, height, etc. Humans have 22 pairs of autosomes.
b. Sex Chromosomes
These determine the sex of an organism. In humans, there are two types of sex chromosomes.
X chromosome: They are found in both males and females (XX in females, XY in males).
Y chromosome: They are found only in males. The presence of the Y chromosome determines maleness.
3. Based on Type of Organism
Chromosomes are also categorized based on the type of cells or organisms they are found in.
a. Prokaryotic Chromosomes
They are found in prokaryotes (bacteria and archaea). Prokaryotic chromosomes are usually circular and contain fewer proteins compared to eukaryotic chromosomes. Bacteria typically have a single, circular chromosome.
b. Eukaryotic Chromosomes
They are found in eukaryotic cells (animals, plants, fungi, and protists). Eukaryotic chromosomes are linear and found inside the nucleus. They are made of DNA and histone proteins. The number of chromosomes varies across species. For example, humans have 46 chromosomes (23 pairs).
4. Special Types of Chromosomes
a. Polytene Chromosomes
These are giant chromosomes found in some insects (like fruit flies). They are formed by repeated rounds of DNA replication without cell division, resulting in thick, visible bands.
b. B Chromosomes
These are extra, non-essential chromosomes found in some species. They don't carry critical genetic information for survival and can vary in number between individuals within the same species.
c. Lampbrush Chromosomes
They are found in the oocytes (egg cells) of some vertebrates (like amphibians), these are highly extended chromosomes that resemble a brush, used during high transcriptional activity.
5. Artificial Chromosomes
a. Yeast Artificial Chromosomes (YACs)
These are artificially constructed chromosomes used in genetic research. They are created by inserting large fragments of DNA into yeast cells.
b. Bacterial Artificial Chromosomes (BACs)
Similar to YACs, BACs are used to clone and study large sequences of DNA in bacteria.
These classifications of chromosomes help scientists understand their structure, function, and behavior in different organisms.
Some Questions and Answers
1. What is the function of chromosomes?
A. Chromosomes carry genetic information in the form of genes that control traits and biological functions.
2. What is a telocentric chromosome?
A. A telocentric chromosome has its centromere at the end, resulting in a chromosome with only one arm.
3. How many chromosomes do humans have?
A. Humans have 46 chromosomes, arranged in 23 pairs.
4. Which chromosomes determine the sex of a human?
A. The sex chromosomes, X and Y, determine sex. Females have XX, and males have XY.
5. What is the difference between metacentric and acrocentric chromosomes?
A. A metacentric chromosome has the centromere in the middle, with equal arms, while an acrocentric chromosome has the centromere near the end, resulting in one short arm and one long arm.
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