A chromosome is a structure that exists within cells and which possesses the cell’s genetic material. That genetic material, which regulates how an organism develops, is a molecule of deoxyribonucleic acid (DNA). A molecule of DNA is an extremely long, coiled arrangement that bears numerous identifiable subunits referred to as genes.
In prokaryotes, or cells without a nucleus, the chromosome is simply a circle of DNA. In eukaryotes, or cells with a separate nucleus, chromosomes are much more composite in structure.
In the nucleus of every cell, the DNA molecule is packaged into thread-like structures known as chromosomes. Every chromosome is made up of DNA firmly coiled a lot of times around proteins known as histones that support its structure.
Chromosomes are not noticeable in the cell’s nucleus—not even under a microscope—when the cell is not undergoing division. Nevertheless, the DNA that constitutes chromosomes becomes more closely packed during cell division and is then visible under a microscope.
The majority of what researchers know about chromosomes was discovered by observing chromosomes during cell division.
Every chromosome has a constriction point known as the centromere, which divides the chromosome into two parts, or “hands.” The short hand of the chromosome is labeled the “p hand.” The long hard of the chromosome is labeled the “q hand.”
The location of the centromere on every chromosome offers the chromosome its characteristic shape, and can be employed to assist with the location of definite genes.
DNA and histone proteins are packaged into structures known as chromosomes.
How many chromosomes do people have?
In humans, every cell usually contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, known as autosomes, look similar in both males and females. The 23rd pair, the sex chromosomes, varies between males and females.
Females possess two copies of the X chromosome, while males possess one X and oneY chromosome.
The 22 autosomes are numbered by magnitude. The other two chromosomes, X and Y, are the sex chromosomes. This picture of the human chromosomes lined up in pairs is known as a karyotype.
Chromosome: a very long DNA molecule and linked proteins, that carry portions of the hereditary information of an organism.
Structure of a chromosome (Typical metaphase chromosome):
A chromosome is formed from a single DNA molecule that contains a lot of genes.
A chromosomal DNA molecule contains three definite nucleotide sequences which are necessary for replication: a DNA replication origin; a centromere to affix the DNA to the mitotic spindle.; a telomerelocated at each end of the linear chromosome.
The DNA molecule is highly condensed. The human DNA helixes take up a lot of space in the cell. Small proteins are accountable for packing the DNA into units known as nucleosomes.
Stained chromosomes:
Chromosomes are discolored with A-T (G bands) and G-C (R bands) base pair specific dyes. When they are stained, the mitotic chromosomes possess a banded structure that unmistakably identifies every chromosome of a karyotype.
Each band posseses millions of DNA nucleotide pairs which do not match up to any functional structure.
Karyotype of a male:
The human haploid genome possess 3,000,000,000 DNA nucleotide pairs, shared between twenty two (22) pairs of autosomes and one pair of sex chromosomes.
Biological background
The terms chromosome and gene were made use of long before biologists actually understood what these structures were.
When the Austrian monk and biologist Gregor Mendel (1822–1884) came up with the fundamentary ideas of heredity, he assumed that genetic traits were one way or another transferred from parents to offspring in some kind of minute “package.”
That package was afterward given the name “gene.” When the term was first recommended, no one had any idea as to what a gene might look like.
The term was employed merely to express the idea that traits are transmitted from one generation to the other in particular discrete units.
The term “chromosome” was first recommended in 1888 by the German anatomist Heinrich Wilhelm Gottfried von Waldeyer-Hartz (1836–1921). Waldeyer-Hartz made use of the term to explain particular structures that develop during the process of cell division (reproduction).
One of the top most breakthroughs in the history of biology happened in 1953 when American biologist James Watson (1928– ) and English chemist Francis Crick (1916– ) revealed the chemical structure of a class of compounds referred to as deoxyribonucleic acids (DNA).
The Watson and Crick invention made it possible to communicate biological concepts (like the gene) and structures (like the chromosome) in actual chemical terms.
The structure of chromosomes and genes
Today we know that a chromosome contains a distinct molecule of DNA along with quite a few kinds of proteins.
A molecule of DNA, in turn, is made up of thousands and thousands of subunits, referred to as nucleotides, connected to one another in extremely long chains.
A lone molecule of DNA within a chromosome may be as long as 8.5 centimeters (3.3 inches). To fit within a chromosome, the DNA molecule has to be twisted and folded into a very composite shape.
What is DNA?
DNA, or deoxyribonucleic acid, is the hereditary material in humans and roughly all other organisms. Virtually every cell in a person’s body has the same DNA.
The majority of DNA is situated in the cell nucleus (where it is known as nuclear DNA), but a minute amount of DNA can as well be discovered in the mitochondria (where it is known as mitochondrial DNA or mtDNA).
The information in DNA is stored as a code consisting of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and above 99 percent of those bases are the similar in all people.
The order, or sequence, of these bases determines the information accessible for building and maintaining an organism, comparable to the way in which letters of the alphabet appear in a particular order to form words and sentences.
DNA bases pair up with one another, A with T and C with G, to form units known as base pairs. Every base is as well attached to a sugar molecule and a phosphate molecule. Jointly, a base, sugar, and phosphate are known as a nucleotide.
Nucleotides are organized in two long strands that form a spiral known as a double helix. The structure of the double helix is rather like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.
A significant characteristic of DNA is that it can duplicate, or make copies of itself. Every strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is significant when cells break up because every fresh cell requires to possess a precise copy of the DNA present in the old cell.
DNA is a double helix formed by base pairs affixed to a sugar-phosphate backbone.
Words to remember:
Deoxyribonucleic acid (DNA): The genetic material in the nucleus of cells that contains information for an organism’s development.
Eukaryote: A cell with a distinct nucleus.
Nucleotide: The building blocks of nucleic acids.
Prokaryote: A cell without a nucleus.
Protein: Large molecules that are necessary to the structure and functioning of all living cells.
Assume that a DNA molecule is represented by a formula such as this:
-[-N1-N4-N2-N2-N2-N1-N3-N2-N3-N4-N1-N2-N3-N3-N1-N1-N2-N3-N4-]
In this formula, the abbreviations N1, N2, N3, and N4 stand for the four different nucleotides used in making DNA.
The brackets at the beginning and end of the formula indicate that the authentic formula goes on and on.
A typical molecule of DNA contains up to three billion nucleotides. The unit made known above, therefore, is no more than a small portion of the whole DNA molecule.
Every molecule of DNA can be subdivided into smaller segments consisting of a few thousand or a few tens of thousands of nucleotides. Each of these subunits is a gene. Another way to represent a DNA molecule, then, is as follows:
-[-G-D-N-E-Y-D-A-B-W-Q-X-C-R-K-S-]-
where each different letter stands for a different gene.
The function of genes and chromosomes
Every gene in a DNA molecule carries the instructions for making a single kind of protein. Proteins are highly essential molecules that carry out a lot of vital functions in living organisms.
For instance, they act as hormones, carrying messages from one part of the body to another part; they act as enzymes, making possible chemical reactions that keep the cell alive; and they function as structural materials from which cells can be made.
Each cell has definite specific functions to carry out. The purpose of a bone cell, for example, is to make more bone. The purpose of a pancreas cell, on the other hand, might be to make the compound insulin, which aids in the manufacture of glucose (blood sugar).
The job of genes in a DNA molecule, consequently, is to tell cells how to manufacture all the dissimilar chemical compounds (proteins) they require to build in order to function correctly.
The way in which they perform this function is reasonably straightforward. At one point in the cell’s life, its chromosomes develop into untangled and open up to expose their genes.
The genes act as a pattern from which proteins can be built. The proteins that are constructed in the cell are determined, as pointed out above, by the instructions built into the gene.
When the proteins are constructed, they are released into the cell itself or into the environment outside the cell. They are then able to carry out the functions for which they were made.
Chromosome numbers and Xs and Ys
Every species possesses a diverse number of chromosomes in their nuclei. The mosquito, for example, has 6 chromosomes. Lilies possess 24, earthworms 36, chimps 48, and horses 64.
The biggest number of chromosomes is created in the Adder’s tongue fern, which has more than 1,000 chromosomes.
The majority of species possess, on average, 10 to 50 chromosomes. With 46 chromosomes, human beings fall well within this average.
Sex determination:
The 46 human chromosomes are prearranged in 23 pairs. One pair of the 23 constitutes the sex hormones, known as the X and Y chromosomes.
Males have both an X and a Y chromosome, while females have two X chromosomes.
If a father passes on a Y chromosome, then his child will be male. If he passes on an X chromosome, then the child will be female.
The X chromosome is three times the size of the Y chromosome and carries 100 times the genetic information.
Nevertheless, in 2000, scientists announced that the X and Y chromosomes were once a pair of indistinguishable twins. These identical chromosomes were discovered a few 300 million years ago in reptiles, long before mammals arose.
The genes in these creatures did not decide sex on their own. They reacted to a few environmental cues like temperature. That still happens today in the eggs of turtles and crocodiles.
But in one animal at that time long ago, a mutation happened on one of the pair of identical chromosomes, resulting to what scientists know today as the Y chromosome—a gene that when present always produces a male.
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