Download Molecular Genetics

Molecular Genetics
Introduction
The topic ofMolecular Genetics deals with the DNA oflbe ceD and the process
that is used to decode its genetic code and use the information to make proteins. Genes
are made ofDNA. The expression ofDNA is protein.
The term given for making a protein is called "protein synthesis." This requires
DNA to provide the coded genetic infonnabOD, the three types of RNA, and the amino
acids that are the componen1s of the protein. Protein synthesis is similar in some ways to
manufacturing a car. The car is made up ofdifferent parts that are brought together on
the assembly line.
Deoxyribonucleic acid is DNA
DNA is a long polymer consisting ofphosphate groups altematiDg with sugars.
Nucleotides are the subunits ofnucleic acids. A nucleotide consists of a base, a sugar and
a phosphate. The sugar in DNA is called deoryri~se. Each sugar bas a base attached to
it. The bases are made of carbon and nitrogen and are called nitroge1lOflS bases. There
are two kinds ofnitrogenous bases kDown as purine bases and pyrimidine bases. The
purine baSes found in DNA are adenine (A) and guanine (a). The pyrimidine bases
fOlind in DNA are cylosine (C) and thymine (1").
Ribonucleic acid is RNA
RNA is also a long polymer consisting ofphosphate groups alternating with
sugars. The sugar in RNA is called ribose. Each sugar bas a base attached to it. The
purine bases found in RNA are adenine (A) and guanine (0).. The pyrimidine bases
found in RNA are cytosine (C) and lD'acil (U). Thus, three bases are the same as in DNA. .
Both molecules are similar. There are two basic di1ferences between them. DNA
bas deoxyribose and thymine. RNA bas ribose and uracil.
.Structure of DNA
DNA bas two strands, each with the sugars altema1iDs with the phosphates, IDd
with a base attached to each sugar. The bases pair between the DNA strands. Adenine
always pairs with Thymine. If there is an Adenine OD the first strand ofONA, there will
be a Thymine opposite it. Also, a Thymine on the first stnmd will be matched by an
Adenine on the other.
.
. .
Similarly, Guanine pairs with Cytosine. A Guanine on the first strand will be
paired with a Cytosine on the other stJ'IlDd. Also, a Cytosine on the first strand will be
paired with a Guanine on the other.
20-1
20-2
The base pairs ofDNA are held together by weak attractions known as hydrogen
bonds. Hydrogen bonds are weaker than the covalent bonds that hold two carbon atoms
together. Howevert there are a lot ofbydrogen bonds holding a DNA molecule together.
These bonds serve to hold the strands together under normal temperatun: conditions.
Replication of DNA
The two DNA strand unzip at the hydrogen bonds and each acts as a template.
The template is a pattern that will be replicated by the enzymes synthesizing the new
DNA strands. After the DNA strands are unzipped, the enzyme DNA-dependent DNA
polymerQ&e comes and makes a new strand matching each base with its correct partner.
This enzyme is called DNA polymerase because it produces a DNA polymer as its
product. It is described as DNA -depemJelll because it relies on the pe-existiDg strand of
DNA to tell it what the pattern is. Wherever the template strand has an At the new strand
will receive a T; and wherever there is a T, the new strand will receive an A. SimilarlYt
wherever the template strand has a G, the new strand will receive a C; and wherever there
is a C, the new strand will re<:eive a G. As a result, the new strand will be an exact copy
of the original complementary strand. This process is called semiconservative
replication.
GENE EXPRESSION
Protein synthesis
The DNA causes a protein to be produced 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 and so it contains the genetic message encoded in .
the DNA. The mRNA travels to the endoplasmic reticulum where ribosomes attach to it
The n"bosomes decode the coded genetic message and translate it to make a protein
molecule. The code that the mRNA contains was broken in the 19605. It is shown in
Figure 20-1.
.The genetic code is read from the mRNA molecule in units of three bases known
as Cod01lS. In order to use the DNA code, lookup the first base, then the second base,
then the third base. For example if the codon is UUU, the amino acid is phe
(phenylalBDine).
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20-3
Second base of codon
e
u
u
UUU) ph.
uue
UUA) leu
UUG
UCU
uee
UeG
UAU)tyr
UAC
UAA··stoP
UAG··stoP
CUU
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cee
CAU)h1S
CAC
i
cuc
';
CUA
CUG
le
u
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•
A
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Sir
UCA
CCA
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val
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GCe
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GCG
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AAC
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GAC
ala
GAA)gIu
. GAG
G
UGU )cys u
C
UGC
UGA -stop A
G
UGG -try
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C "!
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G
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gIy
C
A
G
Figure 20-1. Decoding messenger RNA codoDs.
Translation .
Translation of the coded message involves the n1Josomes. Ribosomes arc
structures made ofribosomal RNA (rRNA) and protein molecules. Each noosome
contains two components. The large component serves IS the worksp8ce where the
protein is synthesized while the smaJl component serves IS the mechanism that helps to
join the ammo acids together.
Bringing the amino acids of tile' protein to the noosome is the job ofttansfer RNA
(tRNA). Each tRNA molecule cames a specific amino acid. The system knows which
one to use because the tRNA molecule has 8 specific anticodon that exaetJy matches the
codon of the mRNA. So, ifthe mRNA codon was UUU, the an1icodon for the tRNA
carrying phe would be AAA. Adenine BDd Uracil 8I"e complementary so the AAA
anticodoD would exactly match the UUU codoD.
,
.
20-4
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 10 1he growing polypeptide chain using a
condensatioD mlC'tion.