Modern Concept of Genetics
Modern genetics has made great strides beyond the theories of classic Mendelian
inheritance, especially in biochemistry. The modern investigator attempts to
answer “how” and “why” questions using all of the tools of modern technology.
The secrets of cell functions are contained in the biochemical events that take
place in the cell nucleus and cytoplasm. Much scientific knowledge and
technological skill has been applied to learn about the function of the cell’s
organelles and its fine structure. In addition, modern geneticists devote
investigative effort to the correction of general flaws by using techniques known
as genetic engineering.
Cistron – a complete genetic code; a sequence of a DNA that provides code for a
Double Helix – the shape of DNA
Inducer – a small molecule that initiates the synthesis of mRNA
Operator – that part of a nucleotide where mRNA synthesis begins
Nucleotide – a compound consisting of a phosphate group linked to a 5-carbon
sugar linked to a nitrogenous base
Polyribosomes – a group of ribosomes that work together to build polypeptides
Replication – the duplication of DNA
Repressor – an inhibitor gene
Sense Strand – that portion of DNA that is copied during mRNA synthesis
Transcription – production of complementary strands of mRNA from DNA
Translocation – the movement of a tRNA from one attachment site on one
ribosome to another site during protein synthesis
Triplet Codon – three nucleotides in a nucleic acid code unit
Translation – the conversion of the genetic information carried by the tRNA
molecule into the amino acid sequence of a polypeptide.
NUCLEIC ACIDS AND PROTEIN SYNTHESIS
The DNA Molecule
The genetic material, located in the chromosomes, is a combination of nucleic
acid and histones (short chained proteins). Deoxyribonucleic acid (DNA) is made
up of units called nucleotides. Each nucleotide contains three components: the
five – carbon sugar, a deoxyribose; a phosphate group; and a protein base. The
protein base may be a double-ringed purine-adenine (A) or guanine (G) –a single-
ringed pyrimidine-thymine (T) or cytosine (C). These four bases permit the
formation of four different nucleotides.
Each DNA molecule is composed of two strands of nucleotides. Study
Figure 5.1. Notice that a phosphate group is joined to a sugar and the sugar to a
protein base, forming a nucleotide. A nucleotide is linked with its complement
through the protein bases. Figure 5.2 shows that the two strands of DNA
resemble a ladder. The sugar –phosphate backbones form the sides of the ladder
while the protein base pairs form the rungs of the ladder. The diagram shows
that when nucleotides are joined together in a strand of DNA, the phosphate group
attached to the 5’ (five prime) carbon of the sugar of one nucleotide links up with
the 3’(three prime) carbon of the sugar in the adjacent nucleotide. This type of
linkage in the sugar – phosphate backbone causes the sides of the DNA “ladder”
to be uneven. The protein bases stick out to one side of the sugar –phosphate
Fig 5.1 and 5.2
The protein bases of the nucleotides are joined together in a specific way.
Adenine links only with thymine requiring two hydrogen bonds. Guanine will join
only with cystine. This linkage requires three hydrogen bonds. In each DNA
molecule, the number of adenine molecules always equals the numbers of
thymines and the number of guanine molecules equals the number of cytosines.
In 1962, James Watson and Francis Crick were awarded the Nobel Prize
for working out the model of DNA structure. The DNA molecule is referred to as
a double helix, two strands wound around each other. The two strands are
antiparallel, extending in opposite directions. One has a 5’ phosphate group
attachment at one end; the other has a 3’ attachment. The asymmetrical backbones
cause the twisting with ten nucleotide pairs per turn. Figure 5.1 shows the
Watson-crick model of DNA structure. In 1952, Rosalind Franklin showed
through x-ray crystallography that DNA is a double helix. Using her work,
Watson and Crick worked out the model that is now commonly accepted.
Just before cell division begins, DNA makes an exact copy of itself. This process
of DNA duplication is known as replication. The hydrogen bonds between the
base pairs of the two nucleotide strands weaken and the two strands break apart.
Each strand acts as a template forming a new nucleotide. This is accomplished by
the action of two sets of enzymes: DNA helicase enzymes and DNA polymerase
enzymes. The DNA helicase is working and where replication is taking place is
known as the DNA fork. The polymerase enzyme helps the protein base in a
given nucleotide to pick up and attach to a complementary base at the 3’ end.
DNA is replicated in short segments known as Okazaki fragments. DNA
ligase is the enzyme that joins these fragments together into a chromosome-length
Fig 5.3 Replication of DNA
ABOUT RNA MOLECULES
Ribonucleic acid (RNA) molecules are made of nucleotide subunits similar to
those that are present in DNA. Like the nucleotides in DNA, each of the RNA
nucleotides is made up of a phosphate group, a sugar, and a protein base.
However, the sugar in RNA is ribose, a sugar that contains one more oxygen atom
than the deoxyribose sugar contained in DNA. RNA differs from DNA in other
ways also. RNA usually consists of a single strand of nucleotides, although it can
form double-stranded sections. DNA is always double-stranded. The protein bases
that compose RNA nucleotides are adenine, uracil, guanine and cytosine. RNA
does not contain the amino acid thymine; adenine pairs with uracil instead.
There are three kinds of RNA. Messenger RNA (mRNA) carries the code
that specifies the sequence of amino acids in a polypeptide chain. The code
carried by mRNA is copied from DNA; mRNA carries the genetic code for a
protein from DNA to the ribosomes, where protein synthesis takes place. Transfer
RNA (tRNA) ferries amino acids to the ribosomes and fits them into the correct
place in the polypeptide chain. Each kind of amino acid is serviced by its own
tRNA. Ribosomal (rRNA) is present in large quantities in the ribosomes, but its
exact function is not known.
DNA and TRANSCRIPTION
As you have just read, specific pairing of the protein bases is the rule in double-
stranded DNA. Adenine binds to thymine and guanine binds to cytosine. Each set
of three nucleotides in linear sequence represents a code for an amino acid. This
three nucleotide sequence is known as a triplet codon, or simply a codon. The
entire amino acid sequ4ence in a protein is coded in a DNA molecule. The codon
is the part of the genetic code that specifies a particular amino acid that is to be
put into a polypeptide chain.
DNA is not directly involved in protein synthesis. It serves as a template
directing the production of messenger ribonucleic acid (mRNA). The production
of the complementary single-stranded mRNA from DNA is called transcription.
Messenger RNA carries to the ribosomes the code for the sequence of amino
acids in the protein under construction.
Transcription begins when that portion of a DNA molecule containing the
code unwinds and is “recognized” by an enzyme known as RNA polymerase. The
following series of events then occurs: RNA polymerase becomes attached to a
linear group of nucleotides on DNA called the promoter. Then the two strands of
DNA become separated and only one strand of DNA is copied. The strand of
DNA that is copied is known as the sense strand. What determines the selection of
a sense strand is unknown.
RNA polymerase is a complex enzyme made up of several polypeptide
chains. At least one chain recognizes and binds with the promoter site on the
DNA sense, or coding, strand. Other sections of the enzyme move along the
length of the DNA coding strand binding ribonucleotides (A, C, G, or U) to the
lengthening RNA strand. When RNA polymerase reaches a specially coded
portion of the DNA molecule, it stops transcribing mRNA and leaves the DNA.
The RNA also detaches itself. (it is interesting to note the RNA
polymerase works only in the 3’ to 5’ direction [upstream] on its DNA template
and codes RNA [downstream] from the 5’ to 3’ direction.)
The messenger RNA, which now contains the complementary code of its
template DNA, moves out of the nucleus and becomes attached to a ribosome in
Protein synthesis begins when mRNA becomes attached to a small subunit of the
ribosome. The positioning of the mRNA on this ribosomal subunit is important. The first
codon (AUG) on the mRNA must be positioned correctly in order for protein synthesis to
begin. Meanwhile, individual amino acid molecules are activated by enzymes using
energy from ATP. An AMP –amino acid- enzyme complex is formed:
This complex is activated by transfer RNA (tRNA).
Transfer RNA consists of a short chain of about 75 nucleotides arranged in
a four-leaf clover configuration. Two segments of four-leaf clover are active. A
third segment is composed of three protein bases which form an anticodon. The
anticodon is attracted to its complementary codon on mRNA. The fourth segment
of tRNA recognizes the amino acid complex and binds with the amino acid
molecule. Now tRNA is ready to carry its specific amino acid molecule to the
activated ribosome. The anticodon of tRNA binds to the AUG codon on mRNA>
The amino acid carried by tRNA is methionine.
At this point, the small ribosomal unit is a complex consisting of mRNA
and its AUG codon, the anticodon of tRNA, and the amino acid, methionine.
Next, a large ribosomal unit becomes attached to the complex. This large
ribosomal unit has two binding sites fro tRNA known as peptidyl (P) site and an
aminoacyl (A) site. Now the ribosome is in a condition to affect the building of a
Now another tRNA with a complementary anticodon binds to the next
codon in mRNA. Through the action of the enzyme peptidyl transferase the first
amino acid and the second amino acid are joined by a peptide bond, leaving the
first tRNA empty. Both amino acids are held by the second tRNA. The empty
tRNA detaches from the ribosome and is now free to pick up more methionine.
Next the ribosome moves along the mRNA in a process known as
translocation. The second tRNA and mRNA are moved along the ribosome from
the A site to the P site. By means of translocation, the third codon in mRNA is
brought to the A site. The second codon is now at the P site. (The cycle of tRNA
anticodon binding to its complementary codon on mRNA starts over again.) The
complementary anticodon from an amino acid-bearing tRNA binds to the third
mRNA codon. Peptidyl transferase caused peptide bond formation between the
second and third amino acid. The second tRNA is now free, detaching from the
ribosome. This process repeats until the ribosome reaches a stop codon. A special
protein molecule known as Releasing Factor binds to the stop codon. This results
in the detaching of mRNA from the ribosome. Then the ribosomal subunits
separate. Now, the ribosome releases the newly formed polypeptide.
Figure 5.4 four leaf clover configuration Figure 5.5