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Chapter 10 Part - 1
Molecular Biology of the Gene DNA Structure and Replication
DNA Structure and Function
James Watson and
Francis Crick
– discovered the
chemical structure of
DNA in 1953.
– It allowed the
researchers to
understand the
fundamental
processes in genetics.
DNA Structure and Function
Watson’s and Crick’s
research
– was aided by the
work of others,
including Rosalind
Franklin, who was
using X- ray
Crystallography to
learn about DNA’s
structure.
Nucleic Acids:
- They are a special group
of chemicals in cells concerned with
the transmission of inherited
information
- They have the capacity to store the
information that controls cellular
activity
Kinds of nucleic acids
There are two kinds of
nucleic acids:
• Deoxyribonucleic
acid (DNA) – Is a
major component of
chromosomes and in
found in the nucleus
• Ribonucleic acid
(RNA) – is involved in
the “reading” of the
DNA information
Nucleotides
All nucleic acids
made up of simple
repeating units
(monomers) called
Nucleotides, link
together to form
chains or strands,
often of great length
Nucleotides
The strands vary in the
sequence of the bases
found on each
nucleotide. It is this
sequence which
provides the “genetic
code”.
• Each nucleotide
– is made up of a 5 –
carbon sugar
(monosaccharide), a
phosphate group,
and four types of
nitrogenous bases.
Nitrogenous Bases
Nitrogenous bases
– received their name
because they all contain
nitrogen
The principal bases of DNA
- Purines
Adenine (A)
Guanine (G)
- Pyrimidines
Cytosine (C)
Thymine (T)
Nitrogenous Bases
• The principal bases of RNA
– Purines
• Adenine (A)
• Guanine (G)
– Pyrimidines
• Cytosine (C)
• Uracil (U)
• Please note that in RNA Thymine (T) is replaced
by Uracil (U)
Base pairing rules
Nucleotides link according to strict base pairing
rules;
• A = T A always pairs with T)
• G = C (G always pairs with C)
Base pairs are joined to each other in the middle
by hydrogen bonds.
The Molecular Structure of DNA
• In 1953, Watson and Crick figured out that DNA
molecule has two strands twisted into a
double helix (corkscrew shape) each made of a
series of repeating subunit nucleotides.
• A DNA molecule looks like a twisted ladder or
spiral staircase
The Molecular Structure of DNA
• The sides of the ladder or the “handrails” or
“backbone” are made up of sugar and
phosphate groups.
• The steps, or “rungs” of the ladder are formed
by nitrogenous bases.
• Each rung is made up of a complimentary base
pair (either A-T or G-C) held together by
hydrogen bonding.
RNA Structure and Function
• Structure
– a single stranded structure with the base
Uracil instead of Thymine
– contains the 5-carbon sugar (Ribose instead
of the Deoxyribose as in the case of DNA)
• Function
- The primary function of RNA is to carry the
instructions from DNA to the cytoplasm
and to direct the synthesis of protein.
DNA and RNA Similarities and
Differences
Similarities –
•
•
•
•
Both are nucleic acids
Both composed of nucleotides
Both have a sugar- phosphate backbone
Both have four different types of nitrogen bases.
Differences
DNA
1. Found in nucleus
2. The genetic material
3. Sugar is a 5-carbon Deoxyribose
4. Bases are A,, C, G and T
5. Double stranded (chain)
6. Is transcribed (to give mRNA)
7. One type
RNA
1. Found in nucleolus,
nucleus and Cytoplasm
2. Helper to DNA
3. 5-carbon sugar is Ribose
4. Bases are A, C, G and U
5. Single stranded (chain)
6. Is translated (to give
proteins)
7. Three types: - mRNA
- tRNA
- rRNA
Types of RNA
• mRNA
– Carries genetic information from chromosomes
to the ribosomes and directs the synthesis of
protein (in cytoplasm)
• tRNA
– Transfer amino acids to the ribosomes, where they are
joined (in cytoplasm)
• rRNA
– Is found within the ribosomes (in cytoplasm). It is a
structural component of ribosomes
DNA Replication
• For the information in DNA to be passed on, it
must first be copied. This coping of DNA is
known as DNA replication (it is making a copy
of itself)
• The duplication of DNA, occurs during the
Interphase stage of the cell cycle
• Replication of DNA is the initial step for cell
division (both mitosis and meiosis)
• This process creates two sister chromatids
that are found in chromosomes that are held
together by a common centromere
Steps in DNA Replication
• DNA replication
requires energy and
enzymes
• Each of the two
original strands
serves as a template
(or pattern) for
creation of a new
matching strand
Step 1
• Unwind the DNA molecule
– The hydrogen bonds between the paired
bases break
– The double helix unwinds or “unzips” a short
segment of the DNA, exposing its base by a
enzyme called helicase.
– Nucleotides on each of two single strands are
now available to form base pair with their
compliment from a new free- floating
nucleotide.
Step 2
• Make new strands
– DNA polymerase “reads” the exposed
strand, and assembles complementary bases
across from them.
– The addition and matching of new bases as
catalyzed by DNA polymerase.
– DNA polymerase is also responsible for
editing out any mismatches.
Step 3
• Enzyme DNA Ligase seals ‘sugar
Phosphate’ backbone of the small
DNA segmented pieces.
• The new helices are composed of
half old (original) and half newly
joined nucleotides (Semi-conservative
model)
DNA Replication Process
Mutation
• Mutation – Permanent alteration in cell’s DNA
•
•
•
•
base sequence
A permanent change in a gene may cause a
change in phenotype
Error rate minimized by DNA polymerase proof
reading
Mutations often have no ill effect on the
organism.
Point Mutation – a permanent mistake in a
particular location in the genome.
Mutation
Chromosomal Mutations
• Mutation
– may lead to the proliferation of cells resulting
in cancerous growth.
• Mutation rate is low, but after decades of
accumulated mutations, cells can become
malignant
• Heritable mutation
– occur in germ-cells (cells that divides to make
sperm and egg)
– also create genetic diversity