Chromosomes - Structure, Functions and Properties

Aug 03, 2023, 16:45 IST

The word "chromosome" was first used by W. Waldeyer in 1888. A chromosome is a DNA molecule containing an organism's genetic code. The DNA of the organism and unique proteins make up the chromosomal structure, which has a thick, coil-like structure. The regulation of transcription and cellular replication and division depend heavily on the tertiary structure of the chromosome. Development requires sexual chromosomes. The Y chromosome's presence or absence in the embryo's cells will determine which sexual organs grow. The embryo will develop into a boy if it has a Y chromosome, while a girl if it has two X chromosomes. In this article, let us discuss in detail chromosomes, their types, and their importance.

What is Chromosome

The DNA is firmly wound up into chromosome-like structures inside the nucleus. Histone proteins surround the DNA and give the necessary structural support. The centromere, which separates the p (short) arm and the q (long) arm of each chromosome, is known as the centromere. The constriction point of the cell, which may or may not be the chromosome's core, is where the centromere is found. 46 chromosomes, comprising 22 pairs of so-called autosomes, are organized in 23 pairs in humans. For reference, autosomes are numbered 1 through 22. One chromosome is inherited from the mother and one from the father makes up each chromosomal pair. These chromosomes possess genes that carry hereditary information from the parents and help in the continuation of species.

Also Check - Chromosomal Theory of Inheritance

Deoxyribonucleic acid (DNA) and Chromosome Formation

• The biological substance that cells employ to store their genetic material is called DNA.
• The nitrogenous bases cytosine and thymine, which are pyrimidines, and adenine and guanine, which are purines, make up the DNA molecule.
• These DNA strands couple up to form a double helix structure, in which the strands are arranged antiparallel to one another.
• The strands' antiparallel orientation, which is required for the eukaryotic DNA replication mechanism, places the initial 5' end of one strand's orientation opposite the 3' end of the second strand.
• If the quantity or makeup of chromosomes in newly formed cells changes, serious problems may result. One type of leukemia and a few other cancers, for instance, are brought on by faulty chromosomes, which are made up of linked fragments of damaged chromosomes in humans.
• Furthermore, it is essential that reproductive cells, such as eggs and sperm, have the appropriate amount of chromosomes and the right kind of chromosome structure. If not, the offspring won't grow up properly. For instance, chromosome 21 has three copies in persons with Down syndrome rather than the two present in other people.
• Then, these DNA strands encircle histone proteins. There are two of each of the following histone proteins among the eight: H2A, H2B, H3, and H4.
• A nucleosome is this structure of the DNA encircling the eight histone proteins.
• To further compact the DNA material and produce a more complex chromatin structure known as a solenoid, another histone protein called H1 connects these nucleosome complexes.
• The chromosome is formed by specialized protein-protein interactions and intermolecular forces and is made possible by these solenoids going on to construct more complicated structures by coiling into supercoils.
• The chromosome's center is constrained by a region known as the centromere.

Also Check - Chromatin - Analysis, Structure and Functions

Genome

The Human Genome Project, an international research initiative, was started in 1990 to determine the human genome's sequence and catalog its genes. The Project also sequenced the genomes of several additional, crucial medical research creatures, such as the fruit fly and the mouse. An organism's entire genetic code is included in its genome. The genetic instructions for creating and maintaining life, as well as those for reproduction, are all contained in a genome. Like all other cellular life forms, humans have nuclear and mitochondrial DNA as part of their genomes. In contrast, the genome of RNA viruses is made up entirely of ribonucleic acid (RNA).

Significance and Importance

• Histone-like proteins, which resemble spools, keep DNA firmly wound around them thanks to the peculiar shape of chromosomes. Without this packing, DNA molecules would be too large to fit inside cells. For example, if all the DNA molecules in a single human cell were put end-to-end and released from their histones, the total length would be 6 feet.
• For an organism to develop and operate correctly, its cells must continually divide to produce new ones and replace damaged or old ones. During cell division, DNA must remain intact and be dispersed evenly throughout cells. DNA is accurately replicated and distributed by chromosomes in the vast majority of cell divisions. Nevertheless, mistakes do occasionally occur.
• If the quantity or makeup of chromosomes in newly formed cells changes, serious problems may result. One type of leukemia and a few other cancers, for instance, are brought on by faulty chromosomes, which are made up of linked fragments of damaged chromosomes in humans.
• Furthermore, it is essential that reproductive cells, such as eggs and sperm, have the appropriate amount of chromosomes and the right kind of chromosome structure. If not, the offspring won't grow up properly. For instance, chromosome 21 has three copies in persons with Down syndrome rather than the two present in other people.

Chromosomal Disorders

• A karyotype is a collection of chromosomes that may be observed under a microscope.
• A chromosomal anomaly is any departure from the typical karyotype.
• While some chromosome anomalies are unimportant, others are connected to medical conditions.
• Chromosome abnormalities are the cause of fifty percent of spontaneous abortions.

Chromosomal disorders can be briefly divided into the following types:

Numerical anomalies

The most severe chromosome abnormalities, which can affect hundreds or even thousands of genes and are typically deadly, are brought on by the loss or gain of entire chromosomes.

The chromosome is short and/or contains relatively few genes, or there is a built-in mechanism to help regulate gene dosage, so a few numerical aberrations support development to term.

Structural anomalies

This occurs when substantial amounts of DNA are either added to or deleted from a chromosome.

There are many different types of structural anomalies.

• Deletion is a mutation that results in a missing portion of a chromosome.
• Inversion is a mutation that causes a segment of a chromosome to have the opposite orientation (inverted).
• Ring mutation is a chromosome that has broken off in the shape of a circle or ring.

Causes of Chromosomal Disorders

• Chromosome abnormalities typically happen when a cell divides incorrectly, leaving cells with either too few or too many copies of a certain chromosome.
• The majority of chromosome abnormalities begin in the egg or sperm (gametes), although others arise in the embryo or are passed down from a parent.
• Every cell in the body has chromosomal abnormalities that start in the gametes.
• In a process known as meiosis, the two pairs of chromosomes—one from the mother and one from the father—normally split during the creation of gametes. As a result, the gametes only contain one copy of each chromosome (as opposed to the two copies found in the other cells of the body).
• The creation of gametes with incomplete sets of chromosomes or additional entire or partial sets of chromosomes results from errors in this separation process leading to meiotic nondisjunction.
• The prevalence of meiotic nondisjunction rises with maternal age, and environmental variables including exposure to particular medicines can further raise the likelihood of chromosome abnormalities.
• Multiple genes can be impacted by even minute modifications in chromosome shape, which can have profound implications.

Chromosomes: FAQs

Q1. Do all organisms have chromosomes?

Ans. Except for viruses and prokaryotes, all eukaryotic organisms have their DNA arranged in the form of chromosomes. This genetic compaction helps in preserving the DNA and protects it from enzymatic degradation.

Q2. What is the number of chromosomes in humans?

Ans. In a normal human cell, there are 23 pairs of chromosomes(46 in total) that are located inside the nucleus. Any change in the number of chromosomes can indicate the presence of genetic abnormality.

Q3. What are chromosomes made of?

Ans. Chromosomes are formed by the arrangement of genetic material which is the DNA and histone proteins which are attracted to each other due to their opposite charges. Each unit of chromosomes is called a nucleotide.

Q4. Do Chromosomes play a role in gender?

Ans. Yes, male and female characters are determined by the type of sex chromosome present in the organism. In humans, the presence of the Y chromosome results in the development of a male child while the presence of the X chromosome results in the development of a female child.

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