Genes 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
-nd
of the structural genes
Structure of a II class gene
recognized
by
RNApolymerase 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

Genes structure and localization

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 genes 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 genes 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

Genes structure and localization

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 adults
development.
The genes, which encode for polypeptides are divided in some classes, depending on
functions:
- Enzymes 32,2%
- Proteins that assure stability, compactisation 13,6%
- Receptors
- Transcriptional factors
- Intracellular matrix
- Transmembrane transporters
- Proteins-canales
44,8%
- Hormones
- Ig
- Signal polypeptides
- Extracellular transporters

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

Genes structure and localization

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

Genes structure and localization

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