Lecture 9:
Genetics: Molecular
Overview
Watson & Crick discovery
Structure and Replication of DNA
Structure of RNA
Transcription and Translation
Control of Expression
Prepared by Mayssa Ghannoum
Overview
 DNA,
the substance of inheritance, is the most celebrated
molecule of our time.
 Mendel’s
heritable factors and genes on chromosomes are,
in fact, composed of DNA.
 The
genetic endowment is the DNA contained in the 46
chromosomes we inherit from our parents.
 Nucleic
acids are unique in their ability to direct their own
replication from monomers.
Overview
 Hereditary
information is encoded in the chemical
language of DNA and reproduced in all the cells of our
body.
 It
is this DNA program that directs the development of
our biochemical, anatomical, physiological, and to some
extent, behavioral traits.
 Once
it was showed that genes are located along
chromosomes, the two chemical components of
chromosomes- DNA and protein- became the candidates
for the genetic material.
Watson & Crick Discovery
 The American
James Watson and the Englishman
Francis Crick, were the first biologists to come up
with the correct answer about how the structure
of DNA could account for its role in inheritance
after being convinced that:
DNA is the genetic material.
Structure of DNA
DNA consists of two molecules that are arranged into a
ladder-like structure called a Double Helix.
 A molecule of DNA is made up of millions of tiny subunits
called Nucleotides.
 Each nucleotide consists of:
1. Phosphate group
2. Pentose sugar
3. Nitrogenous base

Nucleotides
 The
phosphate and sugar form the backbone of the
DNA molecule.
 There
are four types of Nitrogenous base: A, C, T, G
Nucleotides
 Each
base will only bond with one other specific base.

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)
Form a base pair
Form a base pair
Nucleotides
Watson & Crick reasoned that there
must be additional specificity of
pairing dictated by the structure of
the bases.
 Each base has chemical side groups
that can form hydrogen bonds with
its appropriate partner:
Adenine can form two hydrogen
bonds with thymine
( A pairs with T).
Guanine forms three hydrogen bonds
with cytosine ( G pairs with C).

Nucleotides

Because of this complementary base pairing, the
order of the bases in one strand determines the order
of the bases in the other strand.

Wherever one strand of a DNA molecule has an A,
the partner strand has a T.
And a G in one strand is always paired with a C in
the complementary strand.
 Therefore, in
the DNA of any organism, the amount
of adenine equals the amount of thymine, and the
amount of guanine equals the amount of cytosine.
DNA structure
A chromosome consists of DNA molecule
packed together with proteins
 Bacterial
chromosome: is a circular DNA molecule
Associated with a small amount of proteins.
 Eukaryotic
chromosomes: each is a linear DNA molecule
associated with large amounts of proteins (this complex
between DNA and protein is called chromatin)
DNA(deoxyribose nucleic acid)
 DNA contains
the instructions for making
proteins within the cell
 Contained
 Arranged
in the nucleus
in 22 chromosomes, plus two sex
chromosomes
Molecular Basis
 Nucleic
acid and protein serve as the molecular basis
of genetics
 Nucleic
 Trinity
Protein
acid : DNA and RNA.
of molecular genetics : DNA , RNA and
- DNA makes RNA that makes Proteins
- The final product of any gene is a protein.
Replication of DNA
 The replication of DNA molecule begins at special sites called
origins of replication, short stretches of DNA having a specific
sequence of nucleotides.

Proteins that initiate DNA replication attach to it, separating the two
strands and opening a replication “bubble”.

Replication of DNA then proceeds in both directions until the entire
molecule is copied.

In contrast to a bacterial chromosome, a eukaryotic chromosome
may have hundreds or even a few thousand replication origins.
Multiple replication bubbles form and eventually fuse, speeding up
the copying of the very long DNA molecules.
Replication of DNA
 At each end of a replication bubble is a replication fork,
a Y-shaped region where the parental strands of DNA are being
unwound and new DNA strands are elongating.

Helicases are enzymes that untwist the double helix at the
replication forks, separating the two parental strands and making
them available as template strands for the synthesis of new
complementary DNA strands.

After parental strands separation, a single-strand binding
proteins bind to the unpaired DNA strands, stabilizing them.
Replication of DNA

The enzymes that synthesize the DNA cannot initiate the
synthesis of a polynucleotide; they can only add nucleotides to the
end of an already existing chain that is base-paired with the
template strand.

The initial nucleotide strand is a short RNA called a primer and
is synthesized by the enzyme primase.

The primer is short (5–10 nucleotides long), and its 3 end serves
as the starting point for the new DNA strand.

Enzymes called DNA polymerases catalyze the elongation of
new DNA at a replication fork, a new DNA strand can elongate
only in the 5 to 3direction.
Origin of replication
3
5
RNA primer
5
DNA pol III
3
Parental DNA
5
3
5
5
3
Figure 16.15 Synthesis of the leading
5 strand during DNA replication
DNA Replication
Fig.16.9 p:311
DNA Replication…followed
Fig.16.9 p:311
Structure of RNA
 The
chemical structure of RNA is very similar to that
of DNA, but differs in three main ways:
1. Unlike double-stranded DNA, RNA is a singlestranded molecule in many of its biological roles and
has a much shorter chain of nucleotides.
2. While DNA contains deoxyribose, RNA
contains ribose.
RNA is less stable than DNA because it is more
prone to hydrolysis.
3. The complementary base to adenine is not thymine,
as it is in DNA, but rather uracil.
Structure of RNA

Each nucleotide in RNA contains:
- ribose sugar,
- A base which can
be: adenine (A), cytosine (C), guanine (G), or uracil (U).
- A phosphate group.

The bases form hydrogen bonds between cytosine and
guanine, and between adenine and uracil

There are many biologically active forms of RNA:
- mRNA: messenger RNA
- tRNA: transfer RNA
- rRNA: ribosomal RNA
Structure of RNA
 Some
RNA molecules play an active role within
cells by catalyzing biological reactions,
controlling gene expression, or sensing and
communicating responses to cellular signals.
 One
of these active processes is protein synthesis, a
universal function whereby:
mRNA molecules direct the assembly of proteins
on ribosomes.
This process uses tRNA molecules to deliver amino
acids to the ribosome, where rRNA links amino
acids together to form proteins.
Transcription and Translation
 The
DNA inherited by an organism leads to specific
traits by dictating the synthesis of proteins and of RNA
molecules involved in protein synthesis.
 In
other words, proteins are the link between genotype
and phenotype.
 Gene
expression is the process by which DNA directs the
synthesis of proteins.
 The
expression of genes that code for proteins include
two stages: Transcription and Translation.
Transcription: is the DNA–directed synthesis of
RNA
The 3 stages of transcription are:
a- Initiation: after RNA Polymerase bind to the promoter
,the DNA strands unwind ,and the polymerase initiates
RNA synthesis at the start point on the template strand.
Promoter: is the DNA sequence where RNA polymerase
attaches and initiates transcription
b- Elongation: the polymerase moves downstream the DNA
& untwists the double helix at 3’→5’.
c- Termination: RNA transcript (mRNA) is released & the
polymerase detaches from the DNA.
The stages of
transcription:
initiation,
elongation,
and
termination.
Figure 17.7,
p. 332.
Translation: is the RNA–directed synthesis of
polypeptides

Transfer RNA (tRNA) is transcribed from DNA inside
the nucleus & travels to the cytoplasm. It transfers amino
acids from cytoplasm to ribosomes
 The cell keeps its cytoplasm filled with the 20 amino
acids either by making them from other compounds ,or by
taking them up from the surrounding solution.
 Each tRNA molecule translates a particular mRNA codon
into a particular amino acid.
 tRNA carries an amino acid at one end & its anticodon (a
nucleotide triplet) at the other end which base pairs with
complementary codon on mRNA.
 Ribosomes facilitate
the specific coupling
of tRNA anticodon
with mRNA codon
during protein
synthesis.
Figure17.13, p.337.
 Ribosome is made
up of 2 subunits
(large & small), they
are build up of
proteins and RNA
molecule named
ribosomal RNA
(rRNA) in the
nucleolus, p.337
Translation: is the RNA–directed synthesis of polypeptides
Translation has 3 stages:
1-Initiation: brings together mRNA ,a tRNA bearing the first amino acid
of the polypeptide, and the ribosomes.
The starting a.a is Methionine, Met (AUG).
2- Elongation: addition of a.a one by one to the preceding amino acid
chain.
→ Elongation requires energy which is provided by GTP (Guanine
triphosphate) hydrolysis.
3-Termination: the final stage where translation stops at a stop codons
(UAG,UAA and UGA) that do not code for amino acids, but act as
signals to stop translation.
Mutations: changes in the nucleotides sequence of an organism’s DNA,
creating genetic diversity, meaning that mutations are responsible for the
production of new genes
Protein Synthesis

In a cell, inherited information flows from DNA to RNA to protein.

DNA directs RNA synthesis, and RNA controls protein synthesis.

DNA → RNA → Proteins

RNA messenger carries messages from DNA to ribosomes to produce
proteins.
Proteins are required to implement genetic programs.

Gene Expression: is the process by which DNA direct the synthesis of
proteins.

Gene expression occurs in two stages:
1- Transcription
2- Translation
The Genetic Code

The genetic instructions for a polypeptide chain are written in the
DNA as a series of non overlapping three-nucleotides words.

There are only 4 nucleotide bases to specify 20 amino acids,
Adenine (A), Guanine (G), Cytosine (C), Thymine (T). Go to figure
17.5, p. 330, The dictionary of the genetic code.

Codons: are triplets of bases.
Such as: AGT (codon)→ Serine (an amino acid (a.a))
AUG → Methionine (Met), an a.a that acts as a start signal,
where ribosomes start translating the mRNA.

For each gene only one of the two DNA strands is transcribed and
this strand is called the Template(pattern) strand.
The Triplet Code