Gene structure and its characteristics: structure of DNA, structure by watson and crick double helix structure, dominant and recessive gene, homologous and heterozygous state, translation, transcription, characteristics of gene.

1. GENES AND ITS
CHARACTERISTICS
Presented by:
Ms. Manisha Thakur
Nursing Tutor

4. GENES AND DNA
 The genes are segment of DNA(deoxyribonucleic
acid). DNA has its own nitrogenous base, which are :
A, C, G, T. These letters in different combinations
code for different functions. Each letter represents a
different base of a molecule called nucleotide. Each
gene is a specific sequence of nucleotide.
 The A stands for adenine, the C stands for Cystosine,
the G stands for Guanine and T stands for thymine.
 Adenine and guanine are double ring base called
purines.
 Cystonine and thymine are single ring base called

6. WATSON AND CRICK DNA
DOUBLE HELIX
 Watson and crick (1953) proposed
a DNA exists in the form of double
helix in which the two
polynucleotide chains are coiled
abut one another like a spiral
ladder.
 Two strands of alternating
phosphate groups and deoxyribose
sugar form the upright the ladder
and have opposite chemical
polarity.
 The base pairs always match up
according to these rule:
1. Adenine is always form hydrogen
bond with thymine.
2. Cystosine is always form
hydrogen bond with guanine.

7.  Diameter of DNA is 20 A0, a
pitch of spiral is 34A0,
distance between adjacent
nucleotide pairs is 3.4A0 and
there are 10 base pairs per
turn.

8. STRUCTURE OF A TYPICAL HUMAN GENE
 Most human genes are split genes, in which
message of gene is interrupted by some non coding
intervening sequence called introns.
 The coding sequence are called exons. At start of
each gene is a promoter region where RNA
polymerase enzyme bind during the process of
transcription. At the end of gene is terminator region
where transcription stops and RNA polymerase
dissociates from DNA.

9. CONT….
 Genes are chemically composed of DNA and are
situated on the chromosomes. They are
determination of inherited characters.
 There are two types of genes:
1. Structural genes: synthesis the specific protein.
2. Control genes: control genes are responsible for
regulation of activity of structural genes.

10. DOMINANT AND RECESSIVE GENE
 The genes are present on 23 pairs of
chromosomes. Each pair of chromosomes
comprises of 2 allele which carry genetic material
specific to that gene. Each gene in a pair has the
same overall function but may have different
version. Different versions of a gene are called
alleles.
 Dominant allele : it produces a dominant phenotype
in individuals who have one copy of the allele,
which can come from just one parent.
 Recessive allele: it produces a recessive
phenotype, the individual must have two copies,
one from each parent.

11. CONT….
 For eg. Take stature of the man.
 There are two allele for stature. One allele carries
genes for tallness, while the other allele carries for
shortness.
 An allele, which get expressed even if one copy is
partnered is called dominant which is partnered
with allele for shortness called recessive genes.
Since the single gene for tallness has expressed,
therefore , he/she will be tall. A person also would
have tall stature if both the alleles were identical
and are coded for tallness.
 For this reason , a person to have short stature,
both the alleles should be coded for short stature.

13. HOMOZYGOUS AND HETEROZYGOUS STATES
 Homozygous state: if a person has two identical
allele(TT) for a particular trait(i.e tallness), that
person is called homozygous for that trait. His
genotype is TT.
 Heterozygous state: when a person has two non
identical alleles for a trait for example: one is for tall
stature (T) and other is for short stature (t), then
that person is heterozygous for that trait. His
genotype is Tt.

14. CONT….
 A person can be homozygous dominant or
homozygous recessive.
 Homozygous person having two dominant alleles
(for eg. TT for tallness) is called homozygous
dominant.
 When a person has two recessive allele (tt for short
stature) then it is called homozygous recessive
state for that trait.

15. DNA AND PROTEIN SYNTHESIS
 Francis crick (1957) proposed that genetic
information is passed from DNA to DNA by the
process of replication from DNA to RNA by the
process of transcription and from RNA to protein by
the process of translation. It is called central dogma
of molecular genetics.
replication DNA mRNA Protien
transcription translation

16. Transcription: making of mRNA from DNA
 Moving along the poly nucleotide strand in a 5’ to 3’ direction, the enzymes
can recognize short DNA sequences of As, Gs and Ts in a particular region.
The sequence is called promoter sequence region. It is called promoter
region. This promoter region on DNA acts as an enhancer for gene reading
process. The enzyme RNA polymerase recognize the promoter region and
then bind to it.
 The enzyme starts transcribing a copy of DNA sequence into single
stranded molecule called RNA.
 The end result of DNA polymerase reaction is messenger RNA (mRNA)
which produces a particular peptide.

17.  RNA splicing : transcription is followed by RNA splicing,
which takes place in nucleus of the cell.
 In this process, the coding sequences (exons) are cut
by the enzymes and spliced together leaving out the
intervening non-coding introns. The final mRNA is thus
assembled and passed into cytoplasm.

18. CONT….
 Translation: it means the translation of genetic
information carried by mRNA into the protein
synthesis. This takes place in the cytoplasm by
organelles called ribosomes which read the RNA
sequence from 5’ end to 3’ end by attaching to
mRNA. The nucleotides that code the amino acids
constitute the coding regions that lie in the middle
of mRNA, while the first and the last nucleotides
that do not code for amino acids, are regulatory
sequences. These are called 5’ untranslated region
(5’UTR) and 3’ untranslated regions (3’UTR)

20. CHARACTERISTIC OF GENES
 Genes are the functional unit of heredity.
 The genetic information of each gene of the organism is
tranismitted faithfully from one cell to another and parents to
progeny. This funtion of genes is called genotype function
or replication.
 Genes are situated in chromosomes. Every gene occupies
a fixed position in a chromosome.
 A unit of genetic material can undergo mutations and
evolution.
 Genes control the functions of DNA and RNA.
 Genes consist of a particular set of instructions or specific
functions. For example, the globin gene was instructed to
produce haemoglobin. Haemoglobin is a protein that helps to
carry oxygen in the blood.

21. CONT…
 Every person has two copies of each genes, one
is heritened from each parent.
 Genes consists of long strand of DNA that contain
promoter, which control the activity of a gene and
coding and non coding sequence.
 The genes control the development of the
phenotype of the organisms.
 Total set of genes in a person is called genome.
 Gene is basically an instruction for human body.
Each gene has specific function and every single
function of the human body is coded in one or more
genes.

22. CONT….
 Dominant allele : it produces a dominant phenotype in
individuals who have one copy of the allele, which can
come from just one parent.
 Recessive allele: it produces a recessive phenotype,
the individual must have two copies, one from each
parent.
 Homozygous state: if a person has two identical
allele(TT) for a particular trait(i.e tallness), that person is
called homozygous for that trait. His genotype is TT.
 Heterozygous state: when a person has two non
identical alleles for a trait for example: one is for tall
stature (T) and other is for short stature (t), then that
person is heterozygous for that trait. His genotype is Tt.