Principles of Biology
By
Frank H. Osborne, Ph. D.
Molecular Genetics
Introduction
•The topic of Molecular Genetics deals with the
DNA of the cell and the process that is used to
decode its genetic code and use the information
to make proteins.
•So, DNA contains the hereditary
information that is passed from one
generation to the next in humans.
•The expression of DNA is protein.
Introduction
•The term given for making a protein is called
"protein synthesis." This requires DNA to
provide the coded genetic information, the three
types of RNA, and the amino acids that are the
components of the protein.
•Protein synthesis is similar to making a car. The
car is made of different parts brought together on
the assembly line.
Deoxyribonucleic Acid is DNA
•DNA is a long polymer
consisting of phosphate
groups alternating with
sugars.
•Nucleotides are the subunits
of nucleic acids.
•A nucleotide consists of a
base, a sugar and a
phosphate.
Deoxyribonucleic Acid is DNA
•The sugar in DNA is called deoxyribose.
•Each sugar has a base attached to it. The bases
are made of carbon and nitrogen and are called
nitrogenous bases.
•There are two kinds of nitrogenous bases called
purine bases and pyrimidine bases.
•The purine bases found in DNA are adenine (A)
and guanine (G). The pyrimidine bases found in
DNA are cytosine (C) and thymine (T).
Ribonucleic Acid is RNA
•RNA is a long polymer consisting of phosphate
groups alternating with sugars. The sugar in
RNA is called ribose.
•Each sugar has a base attached to it.
•The purine bases found in RNA are adenine (A)
and guanine (G). The pyrimidine bases found in
RNA are cytosine (C) and uracil (U).
•Both molecules are similar. DNA has
deoxyribose and thymine, RNA has ribose and
uracil.
Structure of DNA
•DNA has two strands, each with sugars
alternating with phosphates, and with a base
attached to each sugar.
•The bases pair between the DNA strands.
•Adenine always pairs with Thymine. If there is
an Adenine on the first strand, there will be a
Thymine opposite it.
•Also, a Thymine on the first strand will be
matched by an Adenine on the other.
Structure of DNA
•Similarly, Guanine pairs with Cytosine.
•A Guanine on the first strand will be paired with
a Cytosine on the other strand. Also, a Cytosine
on the first strand will be paired with a Guanine
on the other.
•The base pairs of DNA are held together by
weak attractions known as hydrogen bonds.
Structure of DNA
Replication of DNA
•The two DNA strands unzip at the
hydrogen bonds and each acts as a
template.
•The template is a pattern that will be replicated
by enzymes synthesizing the new DNA strands.
After the DNA strands are unzipped, the enzyme
DNA-dependent DNA polymerase comes and makes
a new strand matching each base with its correct
partner.
Replication of DNA
•Wherever the template strand has an A, the new
strand will receive a T; and wherever there is T,
the new strand will receive an A.
•Similarly, wherever the template strand has a G,
the new strand will receive a C; and wherever
there is a C, the new strand will receive a G.
•As a result, the new strand will be an exact copy
of the original strand. The process is called
semiconservative replication.
Gene Expression
Protein Synthesis
•The DNA causes a protein to be made as a
result of a series of steps.
•These steps are known as transcription and
translation.
Transcription
•The DNA template is used to make messenger
RNA (mRNA). The mRNA is the
transcribed copy of the DNA molecule
so it contains the genetic message
encoded in the DNA. The mRNA
travels to the endoplasmic reticulum
where ribosomes attach to it.
•The ribosomes decode the coded genetic message
and translate it to make a protein molecule. The
code that the mRNA contains was broken in the
1960s.
Transcription
•The genetic code is read from the mRNA
molecule in units of three bases known as
codons. In order to use the DNA code, look up
the first base, then the second base, then the
third base. For example if the codon is UUU,
the amino acid is phe (phenylalanine).
Translation
•Translation of the coded message involves the
ribosomes. Ribosomes are structures made of
ribosomal RNA (rRNA) and protein molecules.
• Each ribosome contains two components. The
large component serves as the workspace where
the protein is synthesized while the small
component serves as the mechanism hat helps to
join the amino acids together.
Translation
•Bringing the amino acids of the
protein to the ribosome is the job of
transfer RNA (tRNA). Each tRNA molecule
carries a specific amino acid.
•The system knows which one to use because the
tRNA molecule has a specific anticodon that
exactly matches the codon of the mRNA. So, if
the mRNA codon was UUU, the anticodon would
be AAA. Adenine and Uracil are
complementary, so they would match exactly.
Translation
•As its tRNA brings each amino acid to the
ribosome, the chain of amino acids grows in
length by one amino acid. Enzymes on the
ribosome remove the incoming amino acid
from its tRNA and add it to the growing
polypeptide chain using a condensation
reaction.
Mutations
•Mutations are changes in genes. They can be
transmitted to the offspring.
•A point mutation results from the change in a
single base in the DNA molecule. An example of a
disease resulting from a point mutation is sicklecell anemia.
•The hemoglobin molecule in sickle-cell patients
differs in structure at only one place from the
normal hemoglobin molecule.
Mutations
Frame-shift mutation
•The code of the bases in DNA is translated to
RNA. Transfer RNA decodes the mRNA
molecule in groups of three. Each group of three
is a reading frame.
•Deletions of bases, duplications of bases, or other
abnormalities can result in an alteration of the
reading frame. Such an alteration is called a
frame-shift mutation.
Mutations
Frame-shift mutation
•Frame-shift mutations cause changes in the
structures of proteins so that they will not
function correctly.
Problem 15 Revealed
The Sequence of DNA
- AAA G AT TA C C AT G G G C C G G C T -
Problem 15 Revealed
(a) Sequence of mRNA
- AAA G AT TA C C AT G G G C C G G C T - U U U C U AA U G G U A C C C G G C C G A -
Problem 15 Revealed
(b) Sequence of Amino Acids
- AAA G AT TA C C AT G G G C C G G C T - U U U C U AA U G G U A C C C G G C C G A - phe - leu - met - val - pro - gly - arg -
Problem 15 Revealed
(c) Misread transcription
T
- AAA G AT TA C C AT G G G C C G G C T A
- U U U C U AA U G G U A C C C G G C C G A - phe - leu - met - val - pro - gly - arg his
Problem 15 Revealed
(d) Frameshift mutation
- AAA G AT TA C C AT G G G C C G G C T - U U U C U AA U G G U A C C C G G C C G A - phe - leu - met - val - pro - gly - arg - ser - asn - gly - thr gly - arg -
The End
Principles of Biology
Molecular Genetics