Download The Genetic Code

THE GENETIC CODE
Dr. Sinan Bahjat
M.B.Ch.B., M.Sc., F.I.B.M.S.
Lec. 3
It is the sequence of
nucleotides in DNA or
RNA that determines
the amino acid sequence
in the proteins made. It
is the biochemical basis
of heredity and is nearly
universal
in
all
organisms.
The genetic code is a triplet code
(i.e. composed of 3 bases), these
triplets are called codons
Genetic code consists of 64 triplets
of nucleotides. These codons encode for the
20 amino acids used in the synthesis of
proteins. Each triplet codon specifies only
one amino acid, but an individual amino
acid may be specified by more than one
codon.
A start codon, AUG, sets the
reading frame, and signals the start of
translation of the genetic code. the
code is read in a 5' to 3' direction.
THE
CODON,
FUNCTIONS;
 it
AUG
SERVES TWO RELATED
signals the start of translation.
 and it codes for the incorporation of the
amino acid methionine (Met) into the
growing polypeptide chain
Translation continues in a nonoverlapping fashion until a stop codon
(UAA, UAG, or UGA) is found in frame. The
nucleotides between the start and stop
codons encode for the subsequent protein
produced.
The genetic code can be
expressed as either RNA codons or
DNA codons.
RNA codons occur in messenger
RNA (mRNA) and are the codons that
are actually "read" during the
synthesis of polypeptides in the
process of translation.
PROPERTIES OF GENETIC CODES
SEVERAL PROPERTIES OF THE GENETIC
CODE WERE APPARENT, AS FOLLOWS:
The genetic code is composed of nucleotide
triplets.
 The code is non- overlapping.
 Each codon specifies a particular amino acid, and
only one amino acid.
 Each amino acid can be specified by more than
one codon.
 The code is nearly universal.

DNA REPLICATION
DNA replication is a biological
process that occurs in all living
organisms and copies their DNA; it is
the basis for biological inheritance.
The process starts when one
double-stranded
DNA
molecule
produces two identical copies of the
molecule.
Each strand of the original
double-stranded DNA molecule serves
as template for the production of the
complementary strand.
DNA replication begins at
specific locations in the genome, called
"origins". Unwinding of DNA at the
origin, and synthesis of new strands,
forms a replication fork.
The DNA polymerase is an enzyme that is
used by the cell to synthesize the new DNA
by adding nucleotides matched to the
template strand.
A DNA polymerase can only
extend an existing DNA strand paired
with a template strand; it cannot
begin the synthesis of a new strand.
To begin synthesis, a short
fragment of DNA or RNA, called
a primer, must be created and paired
with the template DNA strand.
DNA REPLICATION
PROCEEDS IN
THREE COORDINATED STEPS:
 Initiation
 Elongation
 Termination
Eukaryotic cells can only initiate
DNA replication at a specific point in
the cell cycle, the beginning of (S)
phase.
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