Chapter 12:DNA
and RNA
(Molecular Genetics)
1. In recent years, biochemists
have found that the DNA of
chromosomes is the genetic
material that is passed form
generation to generation
2. Genes- are sections of DNA
molecules
(I) DNA Structure
• A very large molecule consisting of
thousands of smaller, repeating units
known as nucleotides
• DNA is found in the nucleus of the cell
(A) DNA Nucleotide
• A DNA nucleotide is composed of three parts:
1. A phosphate group
2. A deoxyribose (5-carbon sugar) molecule
3. A nitrogenous base of either adenine,
thymine, guanine, or cytosine
(B) Watson-Crick Model
• Watson and Crick developed a model of the
DNA molecule
• In this model, the DNA molecule consists of
two complimentary chains of nucleotides in
a “ladder” type organization
• The four nitrogenous bases of the DNA
molecule bond together in only one way:
adenine (A) with thymine (T)
cytosine (C) with guanine (G)
(B) Watson-Crick Model
For DNA:
adenine (A) with
thymine (T)
cytosine (C) with
guanine (G)
(B) Watson-Crick Model
For RNA:
Uracil (U) with
thymine (T)
cytosine (C) with
guanine (G)
(B) Watson-Crick Model
http://www.chem.pitt.edu/~grabowski/Chem_BORIS/dna-rot.mov
Double-helix Structure of DNA
• Each “step” of the
ladder consists of
nitrogenous bases
bonded together by
weak hydrogen bonds
• The two chains of the
DNA molecule are
twisted to form a
spiral, or double-helix
http://www.chem.pitt.edu/~grabowski/Chem_BORIS/dna-rot.mov
Double-helix Structure of DNA
http://www.chem.pitt.edu/~grabowski/Chem_BORIS/dna-rot.mov
Double-helix Structure of DNA
(II)
1.
2.
3.
DNA Replication
DNA, unlike any other
chemical compound, can
make exact copies of itself by
a process known as
replication
In replication, the doublestranded DNA helix
unwinds; the two strands
then separate, or unzip, by
the breaking of the hydrogen
bonds between pairs of bases
Free nucleotides in the
nucleus then bond to the
complimentary bases of the
DNA strands
Replication produces two identical DNA molecules
that are exact copies of the original molecule
Replication produces two identical DNA molecules
that are exact copies of the original molecule
(III)
RNA (Ribonucleic acid)
• RNA is a nucleic acid, like DNA, composed
of nucleotide building blocks
• There are three major differences between
the structure of DNA and RNA:
1. In RNA, ribose is substituted for
deoxyribose
2. uracil (U) is substituted for thymine (T)
3. RNA consists of only a single strand of
nucleotides
RNA Nucleotide
• A RNA nucleotide is composed of three parts:
1. A phosphate group
2. A ribose (5-carbon sugar) molecule
3. A nitrogenous base of either uracil, thymine,
guanine, or cytosine
(III)
RNA (Ribonucleic acid)
• 1. In RNA, ribose is substituted for deoxyribose
2. uracil (U) is substituted for thymine (T)
3. RNA consists of only a single strand of
nucleotides
Genetic Code
• A genetic code contains the information for
the sequence of amino acids in a particular
protein
• This code is present in mRNA molecules
and is three bases long. This is known as a
codon
Ex: UAG - is a codon
mRNA
Genetic codes
http://www.csi-net.net/suzanne/page15.htm
mRNA
Genetic codes
Messenger RNA (mRNA)
1. When portions of DNA molecules unwind
and separate, RNA nucleotides pair with
complimentary bases on the DNA strand.
This forms a mRNA that is complimentary
to the DNA strand
2. The sequence of nucleotides in the mRNA
contain the genetic code
3. The genetic code for each amino acid is a
sequence of three nucleotides forming a
codon
(IV)
Protein Synthesis
1. In the cytoplasm, the mRNA becomes associated with
a ribosome
2. Amino acids in the cytoplasm are “picked-up” by
molecules of transfer RNA (tRNA)
3. Each codon on the mRNA bonds with a
corresponding anticodon on a tRNA, which carries a
specific amino acid
4. The resulting chain of amino acids is a polypeptide