INTRODUCTION
DR.SUYASHREE PALKAR
CONSULTANT FETAL MEDICINE
ZULEKHA HOSPITAL DUBAI & SHARJAH
JAMES WATSON
FRANCIS CRICK
Structure of DNA
• DNA is a polymer which is made up of
double stranded polynucleotide chains
held together by weak thermodynamic
forces
• Each chain is made up of sub-units called
nucleotides
• Nucleotides = 5 carbon sugar (
deoxyribose ) + nitrogen containing base +
phosphate group
5’
3’
3’
5’
3’
Deoxyribose Sugar
• It contains 5 carbons and 3 oxygen
molecules.
• The 3 ‘ ( 3 rd carbon ) of a sugar molecule
is connected through a phosphate group
to the 5’ ( 5 th carbon ) of the next sugar
through phospho-diester bonds
Nitrogenous bases
•
•
•
•
There are four nitrogenous bases
PURINES : Adenine and Guanine
PYRIMIDINES : Cytosine and Thymine
The base is covalently attached to the C 1
position of the sugar.
• The two strands form a helical spiral winding
around a helix axis in a right handed spiral
• The two polynucleotide chains run in opposite
directions ( like the railing of a spiral staircase )
• All strands are read from the 5 ‘ to the 3 ‘ end in
a direction , known as polarity
• All bases of the individual nucleotides are on the
inside of the helix stacked on top of each other
like the steps of a spiral staircase.
• The most important function of the DNA is to
carry genes, which regulate when , in what
type of cells and in what quantity each protein
of the cell is to be made.
• Each human cell contains approx. 2 metres
of DNA , yet in the nucleus of the cell , it is
compacted to 6 microns in diameter. The
packaging is done in such a way that the
DNA sequences are still available for gene
expression , replication and repair
The complex packaging of DNA and protein
( histones ) is called chromatin
DNA is wound around the histone
core particle -nucleosome
• A gene is defined as a segment of DNA
that contains instructions for making a
particular protein. A chromosome is
formed from a single , long DNA molecule
that contains a linear array of many genes.
• All the genes of the body ( 25,000 genes )
and DNA of each cell is contained within
the 46 chromosomes present in the
nucleus of each cell
46 chromosomes of the cell
As seen in the metaphase phase of the cell cycle
where the chromosomes are condensed and
compacted and easily seen
DNA consists introns , exons and DNA
regulatory sequencing genes
Conserved regions : exons and
regulatory sequences
Introns : non-useful
DNA
DNA sequencing genes : replication,
telomere and centromere
Replication origin : the location at which
duplication of the DNA begins . There may be
many origins of replications to ensure that the
entire chromosome is replicated.
Centromere : allows one copy of each
duplicated and condensed chromosome to be
pulled into each daughter cell when the cell
divides
Telomeres : ends of chromosomes
along with the adjoining regions
protect the end of the
chromosome from being
recognised as broken and in need
of repair
CHROMATID
Examination of chromosomes
• Traditional method : to stain them with dyes that produce
a striking and reliable pattern of bands along each
mitotic chromosome which are unique to each
chromosome ( Giemsa
staining )
DNA hybridization
• Can be used to distinguish chromosomes by
• “painting “ each chromosme a different colour. Typically
done at the stage of the cell cycle when the
chromosomes are compacted and easy to visualize (
METAPHASE )
A picture or chromosome map of all the 46 chromosomes is called a
KARYOTYPE
The chromosomes are arranged in pairs in decreasing order of length . The
first 22 pairs of chromosomes are called autosomes followed by the two sex
chromosomes .
A karyotype is written as 46 XX ( normal male ) and 46 XY ( normal female )
• A locus is the specific location of a gene or DNA sequence
on a chromosome .Same allele ( same version ) :
homozygous ;
different alleles ( different version of the same gene ) :
heteorozygous
• Each chromosome arm is divided into regions.
• The regions immediately adjacent to the centromere are
designated as region 1 ( p 1 and q 1 )
• Region numbers increase distally to the centromere
• Regions are divided into bands and bands into sub-bands
• For example : Locus of a gene : X p 22.3 is read as
X chromosome , short arm region 2 , band 2 , sub – band 3
and is read as X p two – two point 3 and not X p twenty two
pic
point three
A karyotype report
CELL CYCLE
There are two kinds of cell divisions : MITOSIS AND MEIOSIS
MITOSIS : One cell with 46 chromosomes becomes two cells with 46 chromosomes
each. This kind of cell division occurs throughout the body except in reproductive
organs.
MEIOSIS : results in cells with half the number of chromosomes ( 23 ) . These are eggs
and sperms