Dentistry college - first class
Medical biology- Lec. 9
Dr. Hanan Selman H.
The genetics is a branch of biology concerned with the study of heredity and
variation , that is mean it is define and analyzes heredity and changes in vast array of
physiologic functions that form properties of organisms , the unite of heredity is a gene
, a segment of DNA that carries in its nucleotide sequence information for a specific
biochemical or physiologic property .
Within the cell DNA is organized into long structures called chromosomes , these
chromosomes are duplicated before cell divide in a process called DNA replication ,
gnomic DNA is tightly and orderly packed in a process called DNA condensation to fit
the small available volumes of cell . DNA usually occurs as linear chromosomes in
eukaryotes and circular chromosomes in prokaryotic cells . The set of chromosome in
a cell makes up its genome for ex. in human genome has approximately 3 billions base
pairs of DNA arranged into 46 ( 23 pairs ) . Eukaryotic organisms store most of their
DNA inside of cell nucleus and some of their DNA in mitochondria or chloroplast , in
contrast prokaryotic cells store their DNA only in cytoplasm .
Fig. chromosome structure in eukaryotic cell
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The chemical composition of DNA and RNA
Both DNA and RNA are macromolecules , polymeric molecules made up of
different monomeric units called nucleotides , each nucleotide consists of three
distinct parts : 1- pentose ( 5- carbon ) sugar , 2- nitrogenous ( N2 – containing)
base , 3- phosphate group , because they can be isolated from nuclei and
because they are acidic , these macromolecules are called nucleic acids .
For DNA , the pentose sugar is deoxyribose and for RNA it is ribose , the two
sugars differ by the chemical groups attached to the 2′ carbon ; a hydrogen atom
( H) in deoxyribose and a hydroxyl group ( OH) in ribose ( the carbon atoms in
pentose sugar are numbered 1′ to 5′ to distinguish them from the numbered carbon
and nitrogen atoms in the rings of the bases ) . There are two classes of bases , the
purines and pyrimidines , in DNA the purines are adenine (A) and guanine ( G) , the
pyrimidines are thymine ( T) and cytosine ( C). RNA also contains adenine,guanine
and cytosine but the pyrimidine uracil ( U) replaces thymine .
In DNA and RNA , bases are always attached to the 1′ carbon of pentose sugar by a
covelent bond . The purine bases are bonded at the 9 nitrogen , while the pyrimidines
bond at the 1 nitrogen . The phosphate group (PO4 ) is attached to 5′ carbon of sugar
in both DNA and RNA . The nucleotide , a basic building block of DNA and RNA
molecules , consist of sugar , base and a phosphate group . The sugar and base only is
called a nucleoside .
Fig. Chemical structure of DNA strand
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Fig. Chemical structure of RNA strand
Watson – Crick‫ۥ‬s double helical model of DNA
The nucleic acids are informational molecules because their primary structure
contains a code by which they can duplicate themselves and guide the synthesis of
proteins. The synthesis of proteins - most of which are enzymes - ultimately governs
the metabolic activities of the cell . In 1953, Watson, an American biologist, and
Crick, an English biologist, proposed the double helix structure for DNA. This
development set the stage for a new and continuing era of chemical and biological
investigation.
Watson and Crick‫ۥ‬s helical model of DNA has main features as fellow :
1- The DNA molecule consist of two polynucleotide chains wound around each other
in a right – handed double helix , that is the two strands wind around each other in a
clockwise ( right – handed ) fashion .
2- The external diameter of the helix is 2 nm .
3- The two chains are antiparallel that is , the two strands are oriented in opposite
directions , with one strand oriented in the 5′ to 3′ way , while the other strand
oriented 3′ to 5′ , antiparallel means that the head of one chain is against the tail
of the other chain .
4- The two sugar – phosphate backbones of the double helix are not equally
spaced along the helical axis , this result in grooves of unequal size between the
backbones called the major ( wider ) groove and the minor ( narrower ) groove,
both of these grooves are large enough to allow protein molecules to make contact
with bases .
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5- The bases of opposite strands are bonded together by hydrogen bonds , which
are relatively weak chemical bonds . The specific pairings observed are A with T
( two hydrogen bonds) and G with C ( three hydrogen bonds ) . The hydrogen
bonds make it relatively easy to separate the two strands of the DNA . The specific
A – T and G – C pairs are called complementary base pairs , so the nucleotide
sequence in one stand dictates the nucleotide sequence of the other , for instance ,
if one chain has the sequence 5′- TATTCCGA- 3′ , then the opposite antiparallel
chain must bear the 3′- ATAAGGCT - 5′ .
6- The sugar – phosphate backbones are on the outsides of the double helix ,
while the bases oriented toward the central axis .
Fig. sequences of bases in DNA strand
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DNA replication
Replication involves the separation of the DNA strands to synthesis of a
complementary copy of new DNA on each of the two parent strands . As a result of
this process two DNA molecules of which half of each is derived from the parent DNA
molecule and the other half is newly synthesized .
Replication begins when the hydrogen bonds that hold the purine – pyrimidine bases
together break apart , the separation starts at one end and proceeds along DNA
molecule each half acts as a template for reconstructing the missing half .
There is information that the DNA polymerase catalyzes DNA replication and also
share in the repair of DNA in vivo . This requires the presence of all four
deoxyribonucleoside triphosphates , d ATP, d GTP , d TTP , d CTP and Mg2 .
The information carried by DNA is held in the sequence of pieces of DNA is called
genes , transmission of genetic information in genes is achieved via complementary
base pairing in a process called transcription, when a cell uses the information in
a gene, the DNA sequence is copied into a complementary RNA sequence through
the attraction between the DNA and the correct RNA nucleotides .
Fig. DNA replication process
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In eukaryotic mRNA is then exported from the nucleus to the cytoplasm,
where it is bound to ribosome and translated into its corresponding protein form
with the help of transfer RNA (tRNA) is a small RNA chain of about 80
nucleotides that transfers a specific amino acid to a growing polypeptide chain at
the ribosomal site of protein synthesis during translation .
Fig. the replication of DNA and translation of information into protein
Base: a building block of DNA and RNA , there are five different bases: Adenine, Thymine,
Guanine, Cytosine, and Uracil which is found only in RNA and replaces Thymine in DNA.
Amino acids: Polypeptide building blocks.
Polypeptides: chains of amino acids form the proteins that made up of several
polypeptides
Proteins: the organic compounds that make up cells and organ structures that carries
out reactions throughout the body, from breaking down food to fighting off disease.
Genome : all the genes contained in a single set of chromosome .
Genotype : The genetic composition of an organisms .
Phenotype : the collective structural and physiologic properties of an organism such as
eye color in a human or resistance to an antibiotics in a bacterium .
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