Genes and Chromosomes, Biology tutorial


Biological instructions are encoded in the DNA that is a substance of genes; the units of inheritance which transmit information from parents to offspring to make sure continuity of life.

Introduction to Genes:

The classical principles of genetics were proposed by Gregor Mendel in the year 1865 on the basis of the outcomes of breeding experiments with peas. Mendel studied the inheritance of a number of well-defined features like plant height and was capable to state general rules for their transmission. In all the cases, Mendel could properly interpret the observed patterns of inheritance by supposing that each feature is determined by a pair of inherited 'factors', which are now termed as genes.

The word gene was introduced by W. Johannsen in the year 1909 to refer to the hereditary factors of Mendel.

The gene can be stated as the functional unit of heredity which occupies a specific place (that is, locus) on a chromosome, is able of reproducing itself precisely at each cell division and directs the formation of an enzyme or other protein.

Cytologic and genetic studies exhibit that genes are the basic units of inheritance regarded as the indivisible units of the chromosomes on which they are positioned such as 'beads on a string'. Genes as the main functional genetic unit, find out the fundamental architecture of each cell, the nature and life of the cell, the particular protein synthesis, the enzyme formation and the self-reproduction of the cell.

Genes are molecular patterns which can maintain their identities for numerous generations can be self-duplicated in each generation and can regulate cell processes by letting their specificities to be occupied. Genes can mutate, can be classified, can be shuffled in various combinations, thus, genes are regarded as the base for modern interpretation of evolution.

1) A Cistron: A gene can be termed to as a unit of function termed as a cistron which is the smallest functional area on a chromosome. This idea substituted the unitary concept of the physical gene having the concept of an operational gene comprised of one or more functional components (that is, mutant alleles) which embraces an array of mutant sites, which behaves in a Mendelian fashion. The word cistron was proposed by Seymour Benzer and is regarded as a part of DNA specifying a single polypeptide chain. Each and every cistron is accountable for coding one messenger RNA molecule, which in turn take part in the making of a polypeptide chain. It has been discovered that hundreds of units of mutation (or mutons) and recombination (or recons) exist in each cistron. Cistrons, thus, occupy a much greater chromosomal length than mutons or recons.

2) Muton: There are lots of positions or sites in a cistron where mutations can take place. A muton is the smallest genetic unit or length of DNA which can mutate that is, a change in muton could outcome in mutation to generate a phenotypic effect. A muton might comprise of a single nucleotide or lots of nucleotides.

3) Recon: At times crossing over or recombination takes place in a cistron to give another sub-divisional concept of the cistron, the recon. The recon is the smallest genetic unit which can undergo crossing over (that is, exchange of genetic material), or recombination that is, the smallest unit in the DNA capable of being independently comprised in recombination.


Thomas Morgan after observing Hugo De Vries theories of mutations, bred the fruit fly Drosophila melanogaster for a year without mutations, however a fly appeared having white eye rather than red eyes, in another year. 40 various types of mutations had been observed. In trying to describe how organisms inherit features Morgan discovered that the different mutations of the flies were related with 4 pairs of chromosome possessed by Drosophila.

Thomas Morgan's work verify the chromosomal theory of inheritance which illustrates that chromosome are the elements which transmits inheritable features, as well discovered that chromosome are the carriers of genes that cause the expression of individual features. 

Chromosomes are filamentous thread-like or rod-like gene bearing bodies found in the nucleus throughout cell division. Each and every nucleus has information coded in the form of DNA and organized into groups termed as genes. Genes are arranged on the chromosome and each gene has adequate information for the productions of one protein which can encompass some effects on the individual chromosome differ broadly between different organisms.

The chromosome molecule might be circular or linear, usually eukaryotic cells (that is, cells having nuclei) have big linear chromosomes and prokaryotic cells (cells with no defined nuclei) encompass circular chromosomes. Chromosomes are the necessary unit for cellular division and should be replicated, divided and passed successfully to their daughter cells so as to make sure the genetic diversity and survival of their progeny.

Chromosome Structure

Chromosome in a cell take place in matched pairs termed as homologous xomes, joined at the centre through a centromse. Each chromosome includes numerous genes and each gene is positioned at a specific site on the chromosome, termed as the locus. Similar to chromosome, genes usually occur in pairs. A gene found on one chromosome in a pair generally consists of the similar locus as other gene in the other chromosome of the pair, and these two genes are termed as alleles. Alleles are alternate forms of the similar gene.

In organisms which employ sexual reproduction, offspring inherit one-half of their genes from each parent and then mix the two sets of genes altogether. This generates new combinations of genes; so that each individual is exclusive however still possesses the similar genes as its parents.

Chromosome Number:

In the cells of many organisms which produce sexually, chromosomes take place in pairs; one chromosome is inherited from the female parent; and one is inherited from the male parent. The two chromosomes of each and every pair contain genes which correspond to the similar inherited features. Each pair of chromosomes is distinct from every other pair of chromosome in the similar cell.

The number of chromosome pairs in an organism differs based on the species. The number of chromosome feature of a specific organism is termed as the diploid number. Dogs, for illustration, have 39 pairs of chromosomes and a diploid number of 78 whereas tomato plants encompass 12 pairs of chromosomes and a diploid number of 24.

Gametes or sex cells (that is, eggs and sperm) have only half the number of chromosomes found in the other cells of an organism. This reduced number of chromosomes in the gametes is termed as the haploid number. Throughout fertilization the gametes join to form a cell termed as a zygote having the diploid number of chromosomes features of the species.

Sex Chromosomes:

Most of the organisms have complete sets of matching chromosomal pairs, termed as autosomes. In mammals, birds and some other organisms, one pair of chromosomes is not similar termed as the sex chromosomes, this pair play a dominant role in finding out the sex of an organism. Females encompass two copies of the X chromosome whereas males have one Y chromosome and one X chromosome. Both females and males inherit one sex chromosome from the mother (for all time an X chromosome) and one sex chromosome from the father (an X chromosome in female offspring and a Y chromosome in the male offspring). The presence of the Y chromosome finds out that a zygote will build up into a male. 

Human Chromosomes and Genetic Disorders:

Humans have 23 pairs of chromosomes, having a diploid number of 46 numbered according to their size. The largest is chromosome 1 and the smallest is chromosome 23. Chemical and Physical meiosis (formation of gametes) can harm chromosomes or change their number in a cell, to give rise to embryos having more of less genetic material, at times resultant in developmental disabilities or health problems. In a procedure termed as non disjunction, paired members of chromosomes fail to separate from one other throughout meiosis. Non disjunction can lead to a condition termed as Down syndrome, in which a person inherits three copies of chromosome 21. The other condition which might outcome from non disjunction is Turner Syndrome, a disorder in which a female inherits merely a single X chromosome.

Breakage of a chromosome can lead to four kinds of changes in the structure of chromosome. A deletion takes place if a chromosome fragment lacking a Centromere is lost throughout cell division. In some situations, the fragment might join to the homologous chromosome to produce duplication. It might reattach to the original chromosome however in a reverse orientation, producing an inversion; the fragment can join a non-homologous chromosome a rearrangement termed as translocation.

Nucleic Acids:

Nucleic acids are very complex molecules generated by living cells and viruses, to pass on hereditary features from one generation to the next, and to trigger the manufacture of specific proteins. The name nucleic acids come from their initial isolation from the nucleic of living cells. Though, certain nucleic acids are found not in the cell nucleus however in cell cytoplasm.

Nucleic acid molecules are much large chains of repeating nucleotide units linked in numerous sequences. Therefore, nucleic acids are high polymers having very high molecular weights. A nucleotide is a molecular unit or a nucleic acid molecule which comprises of three subunits:

a) A phosphate group.

b) A pentose sugar (that is, ribose or deoxyribose).

c) A nitrogen base (that is, purine or pyrimidine).

Type of Nucleic Acids:

There are mainly two types of nucleic acids: DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid); both are chemical relatives which are universally present in all living cells and they make the chemical basis of life. Both RNA and DNA comprise the purines: Adenine (A) and Guanine (G) and the pyrimidine cytosine (C). The second type of pyrimidine in DNA is Thymine (T) where as it is Uracil (U) in RNA. Thus a unique pyrimidine differentiates DNA from RNA.


The DNA is a polymer which is made up of repeating units of mononucleotides carrying the genetic material of all the cellular organisms and mostly viruses. DNA carries the information required to direct protein synthesis ad replication. Protein synthesis is the production of the proteins required by the cell or virus for its activities and growth. Replication is the procedure of which DNA copies itself for each and every descendant cell or virus, passing on the information required for protein synthesis. In most of the cellular organisms, DNA is organized on chromosomes positioned in the nucleus of the cell.

DNA Structure:

The DNA is a spiral ladder having the nucleotides making the side pieces and the steps composed of a combination of purine and pyrimidine that attach the deoxyribose sugars in the side pieces to hold them altogether. The purines are of two kinds Adenine (A) and Guanine (G), the pyrimidines too are of two kinds Cytosine (C) and Thymine (T). In making the steps of the ladder G might join C, or C might join G, and T might join A, or A might join T, generally by hydrogen bond, by the pairings G-C, C-G, A-T or T-A occurring all through the length of the DNA molecule. The combination of the sugar molecule, phosphate group and a nitrogenous base completes the fundamental structure of a nucleotide.

By the purine linked to a pyrimidine precisely adenine (A) for all time pairing by thymine (T) and guanine (G) pairing by the cytosine (C), a mirror image of the nucleotide is added to produce the double nucleotide chain that will twist to produce the α - helix. 


Ribonucleic acid (RNA) - In cellular organisms is the molecule which directs the middle steps of protein production and the genetic material of certain viruses. In cellular organisms, the DNA carries the information which finds out the protein structure. However DNA can't act alone and relies on RNA to transfer this crucial information (that is, translate) throughout protein synthesis -production of the proteins required by the cell for its activities and growth. In RNA viruses, the RNA directs two processes-protein synthesis (that is, production of the virus's protein coat) and replication (that is, the process by which RNA copies it).

RNA Structure:

The structure of RNA is alike to that of DNA and it is composed of a single string of ribonucleotides, each of which is comprised of:

a) A pentose sugar (that is, ribose sugar)

b) A phosphate group

c) A nitrogenous base (that is, one of the two bases: adenine, guanine, uracil and cytosine)

These components are joined altogether in the similar manner as in the DNA molecule. However RNA distinct chemically from DNA by being single stranded, having a D-ribose sugar rather of Deoxyribose sugar and having uracil as nitrogenous base rather than thymine. These nitrogenous bases can take place in any sequence.

Types of RNA:

There are three kinds of RNA categorized based on their molecular size. The smallest kind of RNA is termed as transfer-RNA (or tRNA) that carries amino acids to the ribosomes for incorporation to a protein. Each amino acid consists of different classes of tRNA that read the codes of mRNA, thus comprised in protein synthesis. The tRNA receives information from mRNA, via pairing of their bases and accordingly chooses specific amino acids and pass to the ribosome.

The second kind of RNA is the ribosomal-RNA (or rRNA), this is bigger than tRNA and composes the ribosomes in the cytoplasm, the specialized structures which are the sites of protein synthesis.

The largest kind of RNA is the messenger-RNA (or mRNA). Messenger-RNA is a strand of RNA which is complementary to the DNA sequence for a gene and carries the genetic blueprint copied from the sequence of bases in the cell's DNA. This blue print states the sequence of amino acids in a protein. All kinds of RNA are formed as required, using particular sections of the cell's DNA as template.

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