Chromosomes-featured

Chromosomes – Discovery, Structure, Types & Functions of Chromosomes

Overview

The chromosome is a thread-like structure resulting from the organization of chromatin materials during cell division.

  • Under the microscope, they appear to be made of arms (chromatids) and a centromere.
  • A centromere is a place where chromosomes and kinetochores attach with spindle fibres during cell division.
  • The DNA is organized with proteins to form chromosomes.
  • The chromosomes condense before the cell division and become thick. Now they can be seen as separate structures.
  • Each eukaryotic species has a characteristic number of chromosomes.
  • A human cell has 46 chromosomes in its nucleus. But the gametes – egg and sperm have only 23 chromosomes in humans.
Chromosomes

Chromosomes are thread-like structures present in the nucleus, which brings hereditary information from one generation to another. They play an important role in cellular division, heredity, variation, repair, regeneration and mutation.

In Eukaryotic cells, the genetic product exists in the nucleus in chromosomes, which is made up of extremely organized DNA molecules with histone proteins supporting its structure. Chromosome implies ‘coloured body’, that describes its staining capability by particular dyes.

History of Discovery

Karl Nägeli in 1842, very 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 recommended that chromosomes are the physical carrier of genes in the eukaryotic cells. The number of human chromosomes was published in 1923 by Theophilus Painter. By inspection through the microscopic lens, he counted 24 sets, which would indicate 48 chromosomes. His mistake was copied by others and it was not until 1956 that the true number, 46, was identified by Indonesia-born cytogeneticist Joe Hin Tjio.

Chromosomes-Discovery

Chromosomes in different organisms

Chromosome number varies in different species. A nematode species includes only 2 chromosomes in a cell, whereas a protozoan species includes as much as 1600 chromosomes in the cell. Most of the plant and animal species include 8 to 50 number of chromosomes in its somatic cell. The variety of chromosomes does not show the intricacy of a species. A human cell consists of a total 23 pair of chromosomes (2n, overall, 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 separated, stained and photographed. This strategy is useful in finding out any chromosomal abnormalities.

Chromosomes-organisms

 

Structure of Chromosome

Each cell has a set of each kind of chromosome called a homologous chromosome. Chromosomes are made up of chromatin, which contains a single molecule of DNA and associated proteins. Each chromosome includes hundreds and thousands of genes that can specifically code for a number of proteins in the cell. Structure of a chromosome can be best seen throughout the cellular division.

Main parts of chromosomes are:

Chromatid: Each chromosome has two symmetrical structures called chromatids or sister chromatids which is visible in mitotic metaphase. Each chromatid consists of a single DNA molecule. At the anaphase of mitotic cellular division, sister chromatids separate and move to opposite poles.

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 differ in various chromosomes. The centromere is called primary constriction.

Centromere divides the chromosome into 2 parts, the much shorter arm is called ‘p’ arm and the longer arm is known as ‘q’ arm. The centromere includes a disc-shaped kinetochore, which has specific DNA sequence with special proteins bound to them. The kinetochore supplies the centre for polymerization of tubulin proteins and assembly of microtubules.

Secondary constriction and nucleolar organizers: Besides centromere, chromosomes have secondary constriction. Secondary constriction can be identified from centromere at anaphase due to the fact that there is flexing just at the centromere (primary constriction). Secondary constriction, which includes genes to form nucleoli are called the nucleolar organizer.

Telomere: Terminal part of a chromosome is referred to as a telomere. Telomeres are polar, which prevents the fusion of chromosomal sections.

Satellite: It is an extended segment that is sometimes present on a chromosome at the secondary constriction. The chromosomes with satellite are called sat-chromosome.

Chromosomes-Structure

Chromatin:

The chromosome is comprised 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 cellular division, the chromatin fibres condense and chromosomes are visible with unique features. The darkly stained, condensed region of chromatin is called heterochromatin.

It contains securely loaded DNA, which is genetically non-active. The light stained; diffused area of chromatin is called euchromatin. It contains genetically active and loosely packed DNA. At prophase, the chromosomal material is visible as thin filaments referred to as chromonemata. At interphase, bead-like structures are visible, which are an accumulation of chromatin material called chromomere. Chromatin with chromomere appears like a string with beads.

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Types of Chromosomes
A.Autosomes and Sex Chromosomes

Human chromosomes are of two types autosomes and sex chromosomes. Hereditary traits that are connected to the sex of the person are handed down through the sex chromosomes. The rest of the genetic info exists in the autosomes. Human beings have 23 pairs of chromosomes in their cells, of which 22 pairs are autosomes and one set of sex chromosomes, making an overall of 46 chromosomes in each cell.

B.On the Basis of Number of Centromeres
  • Monocentric with one centromere.
  • Dicentric with 2 centromeres
  • Polycentric with more than 2 centromeres.
  • Acentric without centromere. Such chromosomes represent freshly broken segments of chromosomes which do not make it through for long.

Diffused or non-located with indistinct centromere diffused throughout the length of the chromosome.

C.On the Basis of Area of Centromere
  • Telocentric are rod-shaped chromosomes with centromere inhabiting the terminal position so that the chromosome has just one arm.
  • Acrocentric are likewise rod-shaped chromosomes with centromere inhabiting a sub-terminal position. One arm is long and the other is short.
  • Sub-metacentric chromosomes are with centromere a little far from the mid-point so that the two arms are unequal.
  • Metacentric are V-shaped chromosomes in which centromere depends on the middle of the chromosome so that the two arms are practically equivalent.

Centromere

Function and Significance of Chromosomes

The number of chromosomes is constant for a particular species. Therefore, these are of great importance in the determination of the phylogeny and taxonomy of the species.

  1. Genetic Code Storage:

Chromosome includes the genetic product that is needed by the organism to develop and grow. DNA particles are made from a chain of systems called genes. Genes are those sections of the DNA which code for specific proteins required by the cell for its appropriate performance.

  1. Sex Determination:

Humans have 23 sets of chromosomes out of which one set is the sex chromosome. Females have 2 X chromosomes and males have one X and one Y chromosome. The sex of the child is identified by the chromosome passed down by the male. If X chromosome is passed out of the XY chromosome, the child will be a female and if a Y chromosome is passed, a male child develops.

Sex-Determination

  1. Control of Cellular Division:

Chromosomes check successful division of cells throughout the procedure of mitosis. The chromosomes of the parent cells ensure that the right information is passed on to the child cells needed by the cell to grow and develop correctly.

  1. Development of Proteins and Storage:

The chromosomes instruct the sequences of proteins formed in our body and also keep the order of DNA. The proteins are likewise kept in the coiled structure of the chromosomes. These proteins bound to the DNA assistance in proper packaging of the DNA.

Multiple Choice Questions (MCQs) with Answers:

  1. What is the primary role of chromosomes in cells?
  2. What is a centromere’s function during cell division?
    • A. Synthesizing DNA
    • B. Anchoring microtubules
    • C. Controlling cell metabolism
    • D. Producing ATP
    • Answer: B
  3. Who first observed the rod-like structures in the nucleus of plant cells?
    • A. Walter Sutton
    • B. Karl Nägeli
    • C. Theophilus Painter
    • D. Joe Hin Tjio
    • Answer: B
  4. What is karyotyping used for in studying chromosomes?
    • A. Determining DNA sequence
    • B. Identifying chromosomal abnormalities
    • C. Studying protein structures
    • D. Analyzing RNA composition
    • Answer: B
  5. What is the characteristic number of chromosomes in a human cell nucleus?
    • A. 23
    • B. 46
    • C. 48
    • D. 22
    • Answer: B
  6. What are the main components of chromosomes?
    • A. DNA and RNA
    • B. Proteins and lipids
    • C. Carbohydrates and nucleotides
    • D. Enzymes and vitamins
    • Answer: A
  7. What are sister chromatids connected by?
    • A. Centriole
    • B. Centromere
    • C. Kinetochore
    • D. Nucleolus
    • Answer: B
  8. What is the terminal part of a chromosome called?
    • A. Centromere
    • B. Kinetochore
    • C. Telomere
    • D. Chromatid
    • Answer: C
  9. What is euchromatin?
    • A. Genetically inactive, condensed DNA
    • B. Genetically active, condensed DNA
    • C. Genetically inactive, diffused DNA
    • D. Genetically active, diffused DNA
    • Answer: D
  10. How many pairs of chromosomes are autosomes in humans?
  • A. 22
  • B. 23
  • C. 44
  • D. 46
  • Answer: A
  1. Which chromosomes determine the sex of an individual?
    • A. Autosomes
    • B. Telocentric
    • C. Acrocentric
    • D. Sex chromosomes
    • Answer: D
  2. What is the function of chromosomes in sex determination?
    • A. Control of cellular division
    • B. Storage of genetic code
    • C. Determination of species phylogeny
    • D. Determination of sex traits
    • Answer: D
  3. What is the primary role of centromere during cell division?
    • A. Synthesizing proteins
    • B. Anchoring microtubules
    • C. Controlling cellular respiration
    • D. Producing ATP
    • Answer: B
  4. Which scientist coined the term ‘chromosome’?
    • A. Karl Nägeli
    • B. W. Waldeyer
    • C. Theophilus Painter
    • D. Joe Hin Tjio
    • Answer: B
  5. How is karyotyping useful in studying chromosomes?
    • A. Identifying chromosomal abnormalities
    • B. Studying protein structures
    • C. Analyzing RNA composition
    • D. Determining DNA sequence
    • Answer: A
  6. What is the function of a telomere?
    • A. Anchoring microtubules
    • B. Preventing chromosomal fusion
    • C. Controlling cellular respiration
    • D. Synthesizing DNA
    • Answer: B
  7. What is the significance of the number of chromosomes in a species?
    • A. Determination of DNA sequence
    • B. Control of cellular division
    • C. Phylogeny and taxonomy determination
    • D. Synthesis of proteins
    • Answer: C
  8. What does euchromatin contain?
    • A. Genetically inactive, condensed DNA
    • B. Genetically active, condensed DNA
    • C. Genetically inactive, diffused DNA
    • D. Genetically active, diffused DNA
    • Answer: D
  9. How are chromosomes visible during interphase?
    • A. As thin chromatin fibers
    • B. As condensed structures
    • C. As individual genes
    • D. As nucleoli
    • Answer: A
  10. What do acrocentric chromosomes look like?
    • A. Rod-shaped with centromere at the terminal position
    • B. Rod-shaped with centromere at a sub-terminal position
    • C. V-shaped with centromere at the middle
    • D. Diffused with indistinct centromere
    • Answer: B
  11. What is the function of kinetochore during cell division?
    • A. Synthesizing proteins
    • B. Anchoring microtubules
    • C. Controlling cellular respiration
    • D. Producing ATP
    • Answer: B
  12. How many chromosomes are present in a human gamete (egg or sperm)?
    • A. 23
    • B. 46
    • C. 22
    • D. 44
    • Answer: A
  13. What is the role of chromosomes in sex determination in humans?
    • A. Synthesis of proteins
    • B. Control of cellular division
    • C. Determination of sex traits
    • D. Storage of genetic code
    • Answer: C
  14. What is the primary role of chromosomes during mitosis?
    • A. Synthesizing DNA
    • B. Anchoring microtubules
    • C. Controlling cellular respiration
    • D. Ensuring successful cell division
    • Answer: D
  15. What is the significance of chromatin during interphase?
    • A. Synthesis of proteins
    • B. Packaging of DNA
    • C. Control of cellular division
    • D. Determination of species phylogeny
    • Answer: B
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Frequently Asked Questions (FAQs):

  1. What is a chromosome, and how is it formed during cell division?
    • A chromosome is a thread-like structure formed from the organization of chromatin materials during cell division. It consists of arms (chromatids) and a centromere.
  2. Where does the attachment between chromosomes and spindle fibers occur during cell division?
    • The attachment occurs at the centromere, where the chromosome and kinetochore connect with spindle fibers during cell division.
  3. How is the DNA organized to form chromosomes, and what happens to them before cell division?
    • DNA is organized with proteins to form chromosomes. Before cell division, chromosomes condense and become thick, appearing as separate structures.
  4. What is the characteristic number of chromosomes in a human cell, and how does it differ in gametes?
    • A human cell has 46 chromosomes in its nucleus. Gametes, such as eggs and sperm, have only 23 chromosomes in humans.
  5. What role do chromosomes play in cellular processes and biological functions?
    • Chromosomes play crucial roles in cellular division, heredity, variation, repair, regeneration, and mutation.
  6. Who are the scientists credited with the discovery and early understanding of chromosomes?
    • Karl Nägeli observed the rod-like structure in plant cells in 1842, and W. Waldeyer coined the term ‘chromosome’ in 1888. Walter Sutton and Theodor Boveri in 1902 suggested chromosomes as the physical carriers of genes.
  7. How does the number of chromosomes vary among different organisms?
    • The chromosome number varies, ranging from 2 in nematode species to as many as 1600 in protozoan species. Most plant and animal species have 8 to 50 chromosomes in somatic cells.
  8. What is karyotyping, and how is it useful in studying chromosomes?
    • Karyotyping is a technique to study the structure of chromosomes. It involves separating, staining, and photographing chromosomes, aiding in the detection of chromosomal abnormalities.
  9. What are the main components of a chromosome’s structure, and what are their functions during cell division?
    • Chromosomes consist of chromatids, centromere, kinetochore, secondary constriction, nucleolar organizers, telomere, and satellite. These components play roles in segregation, attachment to spindle fibers, and genetic information storage.
  10. Explain the terms heterochromatin and euchromatin in relation to chromatin and chromosomes.
    • Heterochromatin is the darkly stained, condensed region of chromatin containing inactive DNA. Euchromatin is the light-stained, diffused area with active and loosely packed DNA.
  11. What are the types of chromosomes based on centromere number and position?
    • Types include monocentric, dicentric, polycentric, acentric based on centromere number, and telocentric, acrocentric, sub-metacentric, metacentric based on centromere position.
  12. How do autosomes and sex chromosomes differ, and what role do they play in heredity?
    • Autosomes and sex chromosomes are two types of human chromosomes. Autosomes carry non-sex-related genetic information, while sex chromosomes determine the sex of an individual.
  13. What is the significance of chromosomes in determining species phylogeny and taxonomy?
    • The constant number of chromosomes in a species is essential for determining phylogeny and taxonomy.
  14. How do chromosomes store genetic information and contribute to the development of an organism?
    • Chromosomes store genetic material needed for an organism’s development. Genes, coded by DNA, provide instructions for protein synthesis and proper cellular functioning.
  15. How does chromosome number play a role in sex determination, especially in humans?
    • Humans have 23 pairs of chromosomes, including one pair of sex chromosomes. The combination of X and Y chromosomes determines the sex of the offspring.
  16. How do chromosomes control cellular division, and what happens during mitosis?
    • Chromosomes ensure successful cellular division during mitosis by passing on the right genetic information to daughter cells.
  17. What is the function of chromosomes in the synthesis and storage of proteins in the body?
    • Chromosomes instruct the sequence of proteins formed in the body and contribute to the storage and proper packaging of DNA.
  18. Explain the roles of telomeres, satellites, and nucleolar organizers in chromosome structure.
    • Telomeres prevent the fusion of chromosomal sections, satellites are extended segments, and nucleolar organizers are involved in the formation of nucleoli.
  19. How do chromosomes play a role in variation, repair, and regeneration in organisms?
    • Chromosomes contribute to genetic variation, repair damaged DNA, and play a role in the regeneration of cells and tissues.
  20. Can chromosomal abnormalities be detected through karyotyping, and why is it important?
    • Yes, karyotyping can detect chromosomal abnormalities, providing valuable information about genetic disorders and potential health risks.
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Summary: Chromosomes – Discovery, Structure, Types & Functions

The tutorial on chromosomes covers a comprehensive understanding of these crucial cellular components. Here’s a brief summary of key points:

  1. Overview:
    • Chromosomes are thread-like structures formed during cell division, comprising arms (chromatids) and a centromere. They play a vital role in heredity and cellular processes.
  2. Chromosomes:
    • Thread-like structures in the nucleus carrying hereditary information. They are made up of highly organized DNA molecules with supporting histone proteins.
  3. History of Discovery:
    • Karl Nägeli in 1842 observed rod-like structures, and W. Waldeyer coined the term ‘chromosome’ in 1888. Sutton and Boveri proposed chromosomes as carriers of genes in 1902. The true human chromosome count (46) was identified by Joe Hin Tjio in 1956.
  4. Chromosomes in Different Organisms:
    • Chromosome number varies across species. Humans have 46 chromosomes, with 23 pairs (22 autosomes, 1 sex chromosome). Karyotyping is used to study chromosome structure.
  5. Structure of Chromosome:
    • Chromosomes consist of homologous chromosomes, chromatids, centromere, kinetochore, secondary constriction, nucleolar organizers, telomere, and satellite. Chromatin, made of DNA, RNA, and proteins, condenses during cell division.
  6. Types of Chromosomes:
    • Autosomes and sex chromosomes are distinguished based on their role in heredity. Chromosomes can also be classified by the number and position of centromeres.
  7. Function and Significance of Chromosomes:
    • Chromosomes maintain a constant number for a species, aiding in phylogeny and taxonomy. They store genetic information, determine sex, control cellular division, and play a role in protein synthesis and DNA storage.
  8. Genetic Code Storage:
    • Genes in chromosomes code for proteins essential for an organism’s development and growth.
  9. Sex Determination:
    • Humans have 23 pairs of chromosomes, with sex determined by the combination of X and Y chromosomes.
  10. Control of Cellular Division:
    • Chromosomes ensure accurate cellular division during mitosis, passing on the right genetic information.
  11. Development of Proteins and Storage:
    • Chromosomes instruct protein synthesis and contribute to the proper packaging of DNA.

In conclusion, chromosomes are integral to the fundamental processes of life, storing genetic information, determining traits, and ensuring the proper functioning and growth of organisms. Understanding their discovery, structure, types, and functions is key to unraveling the mysteries of genetics and cellular biology.