The document provides an overview of key concepts in molecular biology including: - DNA and RNA structure, including nucleotides, bases, sugars, and single vs double stranded forms. - Key cellular components like genes, chromosomes, and genomes of prokaryotes and eukaryotes. - Central processes like transcription, translation, and the central dogma. - Differences between prokaryotic and eukaryotic cells, including bacterial vs human DNA organization and composition. It also includes diagrams of DNA structure, the genetic code, and tRNA structure to illustrate these concepts. The document concludes with sample review questions.

1. Molecular Diagnostics 4th year
1 Ms.Omer Yahia Elhussein
Molecular Biology
Molecular biology:
Is the branch of biology that study gene structure and function at the
molecular level.
Genes:
It is a segment of DNA containing the information for a single
polypeptide or RNA molecule. It is the unit of inheritance that governs the
character of a particular trait.
DNA:
It is a polymer of nucleotides (deoxyribose sugar, phosphate and one of
the four nitrogenous bases (Guanine, thymine, cytosine and adenine). It is the
genetic material of all cellular organisms and most viruses.
Transcription:
It is the formation of a complementary RNA from a DNA template.
Translation:
It is synthesis of proteins in the cytoplasm using the information
encoded by mRNA.
Central Dogma:
It is a concept stated that information in a cell always flowed from DNA
to RNA to Protein. This concept was overturned by the discovery of reverse
transcriptase enzyme which produces DNA from RNA.

2. Molecular Diagnostics 4th year
2 Ms.Omer Yahia Elhussein
Eukaryote
Prokaryote

3. Molecular Diagnostics 4th year
3 Ms.Omer Yahia Elhussein
 Bacteria contain only one circular DNA molecule contained in the
cytoplasm and Plasmid; small, circular, double stranded DNA separated
from bacterial chromosome.
 Human cells contain two sources of DNA: Nuclear and Mitochondrial DNA.
 Nuclear DNA arranged in 23 pairs of chromosomes as follows”
22 pairs (autosomes).
1 pair (sex chromosome); XX for female and XY for male
Mitochondrial DNA is a circular, protein and intron free DNA and It is inherited
maternally.
Diploid:
Containing both member of each pair of homologous chromosomes
e.g. All cells except germinal cells and mature RBCs
Haploid:
Containing only one member of each pair of homologous chromosomes.
Haploid cells are produced during Meiosis.
e.g. sperm and ova
Features held in common by Eukaryote and Prokaryote:
1. Genetic information encoded in DNA using identical genetic code
(G, C, A, T).
2. Similar mechanisms for transcription and translation of genetic
information.
Features of Eukaryotic cells not found in Prokaryotes:
1. The cell is divided in to nucleus and cytoplasm, separated by a
nuclear envelope containing complex pore structure.
2. Complex chromosomes composed of DNA and associated
proteins.
3. Cell division using a microtubule – containing mitotic spindle that
separate chromosomes.
4. Presence of two copies of genes per cell (Diploid) one from each
parent.
5. Presence of three different RNA synthesizing enzymes (RNA
Polymerases).
Genome:
The complement of genetic information unique to each species of
organism. It is equivalent the DNA of Haploid set of chromosomes from that
species.

4. Molecular Diagnostics 4th year
4 Ms.Omer Yahia Elhussein
DNA STRUCTURE
 3.2 billion Base pair (BP) of DNA in Human genome. The total size of DNA
when we consider the two copies is 6.4 billion BP. 640,000,0000 bp
 Size/length unit isBp (Base Pair).
Kb (Kilo base). 1000 bp
Mb (Mega base). 1000000 bp

5. Molecular Diagnostics 4th year
5 Ms.Omer Yahia Elhussein
 The complete set of metaphase
chromosomes in a eukaryotic cell is
called its karyotype.
 Humans, which are diploid (2N)
organisms, have 46 chromosomes
(two genomes), with one haploid (N)
set of chromosomes (23
chromosomes: one genome) coming
from the egg and another haploid set
coming from the sperm.
 Chromatin: is the stainable material
in a cell nucleus: DNA and proteins. A
complex nucleoprotein material that
makes up the chromosomes of
eukaryotes. The term is commonly
used in descriptions of chromosome
structure and function.
 Chromatin that remains compacted
during interphase is called
heterochromatin to distinguish it from
euchromatin, which returns to a
dispersed state.
 Histones and nonhistones are two
major types of proteins associated
with DNA in chromatin. Both types of
proteins play an important role in
determining the physical structure of
the chromosome. The histones are
the most abundant proteins in
chromatin.
 They are small basic proteins with a
net positive charge that facilitates
their binding to the negatively charged
DNA.
 Five main types of histones are
associated with eukaryotic nuclear
DNA: H1, H2A, H2B, H3, and H4.
Weight for weight, there is an equal
amount of histone and DNA in
chromatin.
 Nonhistones are all the proteins
associated with DNA, apart from the
histones. Nonhistones include
proteins that play a role in the
processes of DNA replication, DNA
repair, transcription (including gene

6. Molecular Diagnostics 4th year
6 Ms.Omer Yahia Elhussein
regulation), and recombination.
Chemical structure and types of nucleic acids:
 Organisms contain genetic material
that governs an individual’s
characteristics and that is
transferred from parent to progeny.
 Deoxyribonucleic acid (DNA) is the
genetic material of all living
organisms and some viruses.
 Ribonucleic Acid (RNA) is the
genetic material only of certain
Viruses .In prokaryotes and
eukaryotes; the DNA is always
double-stranded, whereas in viruses
the genetic material may be double-
or single-stranded DNA or RNA,
depending on the virus.
 DNA and RNA are macromolecules
composed of smaller building blocks
called nucleotides. Each nucleotide
consists of a five-carbon sugars
(deoxyribose in DNA, ribose in
RNA) to which are attached a
nitrogenous base and a phosphate
group. In DNA, the four possible
bases are adenine, guanine,
cytosine, and thymine; in RNA, the
four possible bases are adenine,
guanine, cytosine, and uracil.

7. Molecular Diagnostics 4th year
7 Ms.Omer Yahia Elhussein
Nitrogenous bases:
Ribonucleic acid (RNA) and deoxy Ribonucleic acid (DNA):
Sugar + Phosphate + Nitrogenous base = Nucleic Acid
2’-deoxy ribose + Phosphate + adenine, guanine, = DNA
Cytosine or Thymine
Ribose + Phosphate + adenine, guanine, = RNA
Cytosine or Uracil

8. Molecular Diagnostics 4th year
8 Ms.Omer Yahia Elhussein

9. Molecular Diagnostics 4th year
9 Ms.Omer Yahia Elhussein
 According to Watson and Crick’s model, the DNA molecule consists of two
polynucleotide (polymers of nucleotides) chains joined by hydrogen bonds
between pairs of bases—adenine (A) and thymine (T); and guanine (G)
and cytosine (C)—in a double helix.
 The genetic material of viruses may be linear or circular Double-stranded
DNA, single-stranded DNA, double-stranded RNA, or single-stranded
RNA, depending on the virus. The genomes of some viruses are
organized into a single chromosome, whereas others have a segmented
genome.
 The genetic material of prokaryotes is double-stranded DNA localized into
one or a few chromosomes. Typically prokaryotic chromosomes are
circular, but linear Chromosomes are found in a number of species.
 A bacterial chromosome is compacted into the nucleoid region by the
supercoiling of the DNA helix and the formation of looped domains of
supercoiled DNA.

10. Molecular Diagnostics 4th year
10 Ms.Omer Yahia Elhussein
 The eukaryotic genome is distributed among
several linear chromosomes. The complete set
of metaphase chromosomes in a eukaryotic cell
is called its karyotype.
 The nuclear chromosomes of eukaryotes are
complexes of DNA and histone and nonhistone
chromosomal proteins. Each unduplicated
chromosome consists of one linear, unbroken,
double-stranded DNA molecule running
throughout its length; the DNA is variously
coiled and folded. The histones are constant
from cell to cell within an organism, whereas the
Nonhistones vary significantly between cell
types.
 The large amount of DNA present in the
eukaryotic chromosome is compacted by its
association with histones in nucleosomes and
by higher levels of folding of the nucleosomes
into chromatin fibers.
Properties of DNA:
1. The two chains are antiparallel (show opposite
polarity); that is, the two strands are oriented in
opposite directions, with one strand oriented in
the 5’–to–3’way and the other strand oriented
3’–to–5’.
2. The sugar–phosphate backbones are on the
outsides of the double helix, with the bases
oriented toward the central axis.
3. The bases in each of the two polynucleotide
chains are bonded together by hydrogen
bonds, which are relatively weak chemical
bonds. The specific pairings observed are A
bonded with T (two hydrogen bond) and G
bonded with C (three hydrogen bond). 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 strand dictates the nucleotide
sequence of the other. For instance, if one chain
has the sequence 5’-TATTCCGA-3’, the
opposite, antiparallel chain must bear the
sequence 3’-ATAAGGCT-5’.

11. Molecular Diagnostics 4th year
11 Ms.Omer Yahia Elhussein
4. The DNA molecule consists of two polynucleotide
chains wound around each other in a right-
handed double helix.
5. Because of the way the bases bond with each
other, the two sugar–phosphate backbones of the
double helix are not equally spaced from one
another along the helical axis. This unequal
spacing results in grooves of unequal size between
the backbones; one groove is called the major
(wider) groove, the other the minor (narrower)
groove.
6. Coiling of DNA: that is, the double helix is twisted
in space about its own axis.
Types of hydrogen bonding

12. Molecular Diagnostics 4th year
12 Ms.Omer Yahia Elhussein
Properties of RNA:
1. In the cell, the functional forms of RNA such as messenger RNA (mRNA),
transfer RNA (tRNA), ribosomal RNA (rRNA), small nuclear RNA(snRNA),
and micro RNA (miRNA) are single-stranded molecules.
2. A single-stranded RNA molecule may fold up on itself to produce regions
of antiparallel double-stranded RNA separated by segments of unpaired
RNA. This configuration is called the secondary structure of the molecule
3. Single-stranded RNA and double-stranded RNA molecules are the
genomes of certain viruses.
RNA: is a single stranded; the pyrimidine base uracil (U) replaces thymine and
ribose sugar replaces deoxyribose.
Messenger RNA/ mRNA
 Transcripts of structural genes.
 Encode all the information necessary for the synthesis of a polypeptide of
protein.
 The 5' terminus is capped by 7 methylguanosine triphosphate.
 Synthesis of the poly (A) tail involves cleavage of its 3' end and then the
addition of about 200 adenine residues.
 Intermediate carrier of genetic information; deliver genetic information to
the cytoplasm.

13. Molecular Diagnostics 4th year
13 Ms.Omer Yahia Elhussein
Transfer RNA/ tRNA:
 All the tRNAs share a common secondary structure resembles a
cloverleaf: They have four base- paired stems defining three stem-loops
(the D loop, anticodon loop, and T loop) and the acceptor stem.
 tRNA carry correct amino acids to their position along the mRNA template
to be added to the growing polypeptide chain.

14. Molecular Diagnostics 4th year
14 Ms.Omer Yahia Elhussein
Ribosomal RNA/ rRNA
 The central component of the ribosome.
 Ribosome; factory for protein synthesis; composed of ribosomal RNA and
ribosomal proteins (known as a Ribonucleoprotein or RNP).
 rRNA provides a mechanism for decoding mRNA into amino acids.
Questions:
1. Extra chromosomal double stranded, circular DNA molecule
present in bacteria which is widely used as vector is
…………………………….
2. What are the differences between bacterial and Human DNA?
3. What are the constituents of Human Genome and Bacterial
Genome?
4. What do we mean by CENTRAL DOGMA?
5. What are the constituents of Gene?
6. What do we mean by :
Nucleotide?
Nucleoside?
Deoxyribose?
Antiparallel?
7. G ?
8. A ?
References:
 Peter J. Russell, i Genetics: Copyright © Pearson Education.
 Cell and Molecular Biology concepts and experiments 6
edition.