gene structure

Gene’s structure and localization

The science of genetics began with the work of Gregor Mendel. His goal was to

determine the principles, if such existed, governing the transmission of inherited characteristics.

Classical genetics concerned itself with the rules governing the transmission of genetic

characteristics and relationship between genes and chromosomes. His approach to the study of

inheritance involved the analysis of hybrids. (A hybrid is the offspring of a cross between

inherently unlike individuals). After several years of careful study Mendel drew a number of

conditions.

The characteristics of organisms were governed by factors (or units) of inheritance that

were later termed genes. In classical vision each gene assure manifestation of one character.

In the modern point of view the gene represents the hereditary

unit that occupies a fixed chromosomal locus, contain the

information for a polypeptide chain, tRNA molecule, or rRNA

molecule encoded in a specific nucleotide sequence, and can

mutate to various forms.

Human genome contains 3164.7 million base pairs. The human genome is

estimated to contain 30,000 to 40,000 genes. About 2% of the genome

encodes instructions for the synthesis of proteins. Repeated sequences that do not code for

proteins (“junk DNA”) make up at least 50% of the human genome. The order of almost all

(99.9%) nucleotide bases is exactly the same in all people. The average gene consists of 3000

bases, but sizes vary greatly, with the largest known human gene being dystrophin at 2.4 million

bases.

The genes consist of

transcribed region and

regulatory regions. The

regulatory regions are

represented by promoter and

terminator. A sequence of

DNA that is transcribed from

a single promoter is called

transcription unit, that in

human cells are

monocistronic. The promoter

of the structural genes

recognized by RNA-

polymerase II, contains at the distance of 20 –30 nucleotides from the initiation sites (+1) the

TATA box and in position –75 – CAAT-box. The transcribed regions of genes consist of

sequences that a represented in proteins – exons and non-coding region – introns. The number

of exons and introns is various and depends on the complexity of the encoding protein.

Some distant sequences can also participate in the process of transcription. They may

facilitate the recognition of promoter by

RNA-polymerase (enhancer) or interfere in

this process (silencer). There are no physical

frontiers between genes; only informational

sequences separate genes: promoters and

terminators.

The most of human genes are

characterized by alternative splicing: in

different tissues under different condition

distinct types of mRNA are obtained from the same gene. Alternatively, one gene may contain

Structure of a II-nd

class gene


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some promoters. In fact, one gene may contain instructions about synthesis of some proteins. For

example, there are 6 types of actine in different tissues: 1 – in skeletal muscles, 1 – in heart

muscle, 2 – in smooth muscles, 2 – in cytoskeleton.

The gene’s proprieties

- Replication – the duplication of DNA. During cell division copies of genes are

transmitted to the daughter cells;

- Repair – restoration of correct base sequence in DNA;

- Discreteness – each gene represents a separate unit; each gene is responsible for one

(or some) established founctions;

- Specificity – each gene works properly only under specific conditions;

- Stability – due to replication and repair usually the genes maintain their structure and

function

- Mutability – during replication or under environmental factors the gene’s structure

may change

- Polyallelisms - large number of genes are present in many allelic forms in the

population (e.g. there are 3 main alleles for ABO blood group system: A, B and 0)

- Dosage effect - the effect of the number of copies of a gene in a cell. Each gene

assures synthesis of an appropriate quantity of mRNA and protein. If the number of

genes in the cell is changed, the cell activity is modified to (e.g. in trisomy 21

increase the quantity of superoxiddismutase – an enzyme encoded by a gene located

in 21 chromosome. In fact, a lot of abnormalities are appear in Down syndrome);

- Linkage - loci that tend to be inherited together more often than would be expected by

chance are said to show linkage

- Penetrance - a figure (usually given as a percentage) that expresses the probability

that an abnormal gene will exert at least one of its phenotypic effects in an individual

inheriting that gene

- Pleiotropy - the case in which a mutation in a single gene produces a multiplicity of

different effects.

Gene classification

Housekeeping genes are genes, which encode polypeptides or RNAs whose function is

required by all cell types in a multicellular organism, for example cytoskeletal proteins such as

actin and tubulin, RNA polymerases and ribosomal proteins. These genes are usually


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constitutively expressed and may number several thousands within a single cell type in higher

eukaryotes. The specific genes usually are active in some tissues under appropriate conditions.

Some genes – early genes – are active only during embryonic development, later they are

inactivated. Other genes – late genes – are active only in different periods of adult’s

development.

The genes, which encode for polypeptides are divided in some classes, depending on

functions:

The position of a gene on a chromosome is called locus (pl. – loci). A group of loci, which can be shown to segregate together with predictable

frequency, represent a linkage group. This genetic linkage is a direct result of

their physical location on the same stretch of DNA. The number of linkage groups is the same as

the haploid chromosome numbers. There are 24 linkage groups in human: 22 – for autosomes, X,

Y and one linkage group in mitochondria.

Each person posed a diploid number of chromosomes: so there are two complete sets of

homologous chromosomes, which are inherited from both parents, and have the same genetic

loci and structure. In homologous chromosomes each locus may be occupied by identical

sequences or by different variants of the same gene – alleles. Allele is one of an array of different forms of a given gene, which always have the

same position in a chromosome, assure expression of different

variants of the same trait (e.g. Rh+/Rh

-), and is resulted from

mutation(s) in an ancestral gene. Some times there are many alleles for a given

- Enzymes – 32,2%

- Proteins that assure stability, compactisation – 13,6%

- Receptors

- Transcriptional factors

- Intracellular matrix

- Transmembrane transporters

- Proteins-canales

- Hormones

- Ig

- Signal polypeptides

- Extracellular transporters

44,8%


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gene (multiple allelism), but each diploid cell may contain concomitant not more than 2

different alleles.

A diploid cell or organism in which the two alleles at a given locus are identical is called

homozygote (or

homozygous). A diploid cell

or organism in which the two

alleles at a given locus are

different is called

heterozygote (or

heterozygous).

During meiosis the

members of a pair of alleles

segregate in different gametes.

Each gamete contains a haploid

number of chromosomes (a

single set of chromosomes) and

as result - a single allele for

each gene.

The genes, which are

located in different loci, are

called nonallelic genes. Usually

they encode for different

products and are inherited

independently. Generally the

segregation of alleles at one

locus is quite independent of the

segregation of alleles at any

other, unless the loci concerned

are very close together on the

chromosome. A number of

closely linked loci, which are

usually inherited as a unit a

called haplotype. Each

haplotype defines the sequence

of alleles along one of the

homologous chromosomes.

During meiosis the process of the physical exchange between homologous chromosomes may

take place – crossing-over. This reciprocal exchange is responsible for genetic recombination,

which results in a reassortment of alleles between the homologues. The frequency with which

recombination occurs between two genes on a single chromosome can be used as a means of

estimating the distance between them, as the further apart the two genes are, the greater the

chance of a cross-over

occurring between

them. Crossing-over

can be used to order

genes on a

chromosome, thereby

generating a linkage

map or genetic map. Multiple cross-overs between an homologous chromosome pair can occur

during a given meiotic event (seen as chiasmata) although the occurrence of a cross-over in one

The inheritance of the A and a alleles explained in light of meiosis


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region suppresses the occurrence of a second crossing-over within that region, a phenomenon

known as interference.

The genes, which are located

on different chromosomes, are

inherited separately. This rule is

called The principle of independent

assortment: Segregation of the

members of a pair of alleles id

independent of the segregation of

other pairs during processes leading to

formation of the reproductive cells

(meiosis).

The inheritance of two traits on different chromosomes can be explained

by meiosis

gene structure

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