TRANSFER OF GENETIC INFORMATION

BYASMA AMIN & BUSHRA SOHAIL

(Department of Environmental Sciences)

TABLE OF CONTENTS1 .GENETICS

2 .HISTORY OF GENETICS

3 .FEATURES FOR INHERITENCE

4 .MOLECULAR BASIS FOR INHERITENCE

5 .RECOMBINATION AND LINKAGE

6 .GENE EXPRESSION

7 .HORIZENTAL GENE TRANSFER

8 .REFERENCES

Genetics:

The word genetic, derived from the Greek word “genno” means to give birth.

Definition:

A discipline of biology is the science of heredity and variation in living organisms in which we study transfer of characters from parents to their offspring.

History of genetics The modern science of genetics traces its roots to the observations made by Gregor Johann Mendel, a German-Czech Augustinian monk and scientist who made detailed studies of the nature of inheritance in plants. In his paper ("Experiments on Plant Hybridization"), presented in 1865 to the Brunn Natural History Society, Gregor Mendel traced the inheritance patterns of certain traits in pea plants and showed that they could be described mathematically. Although not all features show these patterns of Mendelian inheritance, his work suggested the utility of the application of statistics to the study of inheritance.

CONTINUE…The word genetics itself was coined in 1905 by William Bateson, a significant proponent of Mendel's work, in a letter to Adam Sedgwick. (The adjective genetic, derived from the Greek word genno: to give birth, predates the noun and was first used in a biological sense in 1860).

In 1910 Thomas Hunt Morgan argued that genes reside on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies. In 1913 his student Alfred Sturtevant used the phenomenon of genetic linkage and the associated recombination rates to demonstrate and map the linear arrangement of genes upon the chromosome.

Features of inheritance:

At its most fundamental level, inheritance in organisms occurs by means of discrete traits, called genes. This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in pea plants. In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white and never an intermediate between the two colors. These different, discrete versions of the same gene are called alleles.

Molecular basis for inheritance:

The molecular structure of DNA. Bases pair through the arrangement of hydrogen bonding between the strands.The molecular basis for genes is deoxyribonucleic acid (DNA). DNA is composed of a chain of nucleotides, of which there are four types: adenine (A), cytosine (C), guanine (G), and thymine (T). Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain. Viruses are the only exception to this rule—sometimes viruses use the very similar molecule RNA instead of DNA as their genetic material .

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DNA normally exists as a double-stranded molecule, coiled into the shape of a double-helix.

Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand.

This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a

template for synthesis of a new partner strand . Genes are arranged linearly along long chains of DNA

sequence, called chromosomes. In bacteria, each cell has a single circular chromosome.

Continue…while a eukaryotic organisms (which includes plants and animals) have their DNA arranged in multiple linear chromosomesIn eukaryotes chromatin is usually composed of nucleosomes, repeating units of DNA wound around a core of histone proteins. The full set of hereditary material in an organism (usually the combined DNA sequences of all chromosomes) is called the genome.

The two alleles for a gene are located on identical loci of sister chromatids, each

allele inherited from a different parent .

Diagram of eukaryotic cell division. Chromosomes are copied, condensed, and organized. Then, as the cell divides, chromosome

copies separate into the daughter cells.

Recombination and linkage:

The diploid nature of chromosomes allows for genes on different chromosomes to assort independently during sexual reproduction, recombining to form new combinations

of genes .

Genes on the same chromosome would theoretically never recombine, however, were it not for the process of chromosomal crossover.

During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes. This process of chromosomal crossover generally occurs during meiosis.

Gene expression:The DNA, through a messenger RNA intermediate, codes for protein with a triplet code. Genes generally express their functional effect through the production of proteins, which are complex molecules responsible for most functions in the cell.

Proteins are chains of amino acids, and the DNA sequence of a gene is used to produce a specific protein sequence. This process begins with the production of an RNA molecule with a sequence matching the gene's DNA sequence, a process called transcription.

This messenger RNA molecule is then used to produce a corresponding amino acid sequence through a process

called translation.

Each group of three nucleotides in the sequence, called a codon, corresponds to one of the twenty possible amino acids in protein this correspondence is called the genetic code. The flow of information is unidirectional: information is transferred from nucleotide sequences into the amino acid sequence of proteins, but never from protein back into the sequence of DNA a phenomenon Francis Crick called the central dogma of molecular biology.

Horizontal gene transfer:

Horizontal gene transfer (HGT), also Lateral gene transfer (LGT), is any process in which an organism transfers genetic material to another cell that is not its offspring. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g. its parent or a species from which it evolved. Most thinking in genetics has focused on the more prevalent vertical transfer, but there is a recent awareness that horizontal gene transfer is a significant phenomenon.

Prokaryotes:Horizontal gene transfer is common among bacteria, even very distantly-related ones. This process is thought to be a significant cause of increased drug resistance; when one bacterial cell acquires resistance, it can quickly transfer the resistance genes to many species. Enteric bacteria appear to exchange genetic material with each other within the gut in which they live. There are three common mechanisms for horizontal gene transfer.

1 .Transformation:The genetic alteration of a cell resulting from the introduction, uptake and expression of foreign genetic material (DNA or RNA). This process is relatively common in bacteria, but less common in eukaryotes. Transformation is often used to insert novel genes into bacteria for experiments, or for industrial or medical applications .

Factors affecting transformation:a. DNA size state:

Double stranded DNA of at least 5 X 105 daltons works best. Thus, transformation is sensitive to nucleases in the environment.b. Competence of the recipient:

Some bacteria are able to take up DNA naturally. However, these bacteria only take up DNA a particular time in their growth cycle when they produce a specific protein called a competence factor. At this stage the bacteria are said to be competent. Other bacteria are not able to take up DNA naturally. However, in these bacteria competence can be

induced in vitro by treatment with chemicals (e.g. CaCl2) .

Steps in transformation:

a. Uptake of DNA:

Uptake of DNA by Gram+ and Gram- bacteria differs. In Gram + bacteria the DNA is taken up as a single stranded molecule and the complementary strand is made in the recipient. In contrast, Gram- bacteria take up double stranded DNA.

b. Legitimate/Homologous/General Recombination : After the donor DNA is taken up, a reciprocal recombination event occurs between the chromosome and the donor DNA. This recombination requires homology between the donor DNA and the chromosome and results in the substitution of DNA between the recipient and the donor.

2 .Transduction: The process in which bacterial DNA is moved from one

bacterium to another by a bacterial virus (a bacteriophage, commonly called a phage).

Types of Transduction:

Generalized Transduction: Generalized transduction is transduction in which potentially any bacterial gene from the donor can be transferred to the recipient.Specialized transduction:

Specialized transduction is transduction in which only certain donor genes can be transferred to the recipient. Different phages may transfer different genes but an

individual phage can only transfer certain genes.

3 .Conjugation:Transfer of DNA from a donor to a recipient by direct physical contact between the cells. In bacteria there are two mating types a donor (male) and a recipient (female) and the direction of transfer of genetic material is one way; DNA is transferred from a donor to a recipient. Mating types in bacteria:

Donor: The ability of a bacterium to be a donor is a consequence of

the presence in the cell of an extra piece of DNA called the F factor or fertility factor. The F factor is a circular piece of DNA that can replicate autonomously in the cell; Extra chromosomal pieces of DNA that can replicate autonomously are given the general name of plasmids.

Recipient:The ability to act as a recipient is a consequence of the lack of the F factor.

Bacterial conjugation:

Bacterial conjugation: is the transfer of genetic material between bacteria through

direct cell-to-cell contact. It is discovered in 1946 by Joshua Lederberg and Edward Tatum, conjugation is a mechanism of horizontal gene transfer as are transformation and transduction although these mechanisms do not involve cell-to-cell contact.

It is merely the transfer of genetic information from a donor cell to a recipient. In order to perform conjugation, one of the bacteria, the donor, must play host to a conjugative or mobilizable genetic element, most often a conjugative or mobilizable plasmid or transposon. Most conjugative plasmids have systems ensuring that the recipient cell does not already contain a similar element.

Mechanism:

Inter-kingdom transfer:The nitrogen fixing Rhizobia are an interesting case, wherein conjugative elements naturally engage in inter-kingdom conjugation. Such elements as the Agrobacterium Ti or Ri plasmids contain elements that can transfer to plant cells. Transferred genes enter the plant cell nucleus and effectively transform the plant cells into factories for the production of opines, which the bacteria use as carbon and energy sources. Infected plant cells form crown gall or root tumors. The Ti and Ri plasmids are thus endosymbionts of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.

References

1 .www.wikipedia.com

2 .An Introduction to genetic analysis by Griffiths AJF (1999).

3 .Exchange of Genetic Information by Holmes RK, Jobling MG (1996)

4 .DNA Replication Fidelity by Kunkel TA (2004).

5. www.encarta.msn.com

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