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. Genes
are made of DNA. The expression of DNA is protein.
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 in some ways to
manufacturing a car. The car is made up of different parts that are brought together on
the assembly line. In the case of DNA and RNA, they are made of nucleotides, which are
the subunits. A nucleotide consists of a base, a sugar and a phosphate.
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. 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 known as 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 also 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). Thus, three bases are the
same as in DNA.
Both molecules are similar. There are two basic differences between them. DNA
has deoxyribose and thymine. RNA has ribose and uracil.
Structure of DNA
DNA has two strands, each with the sugars alternating with the 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 of DNA, there will
be a Thymine opposite it. Also, a Thymine on the first strand 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 strand. Also, a Cytosine on the first strand will be
paired with a Guanine on the other.
20-1
20-2
The base pairs of DNA are held together by weak attractions known as hydrogen
bonds. Hydrogen bonds are weaker than the covalent bonds that hold two carbon atoms
together. However, there are a lot of hydrogen bonds holding a DNA molecule together.
These bonds serve to hold the strands together under normal temperature 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
polymerase 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-dependent because it relies on the pre-existing strand of
DNA to tell it what the pattern is. Wherever the template strand has an A, the new strand
will receive a T; and wherever there is a 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 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 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. It is shown in
Figure 20-1.
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).
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Figure 20-1. Decoding messenger RNA codons.
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 that helps to
join the amino acids together.
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 for the tRNA
carrying phe would be AAA. Adenine and Uracil are complementary so the AAA
anticodon would exactly match the UUU codon.
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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.