are characterized by the sequence of amino acids in their structure. This
sequence is controlled and dictated by DNA. Proteins have many
important functions in the body, and the type of proteins an organism
synthesizes characterizes that organism. Organisms are the result of their
protein. The type of protein made by the cell is controlled by the DNA
through the genetic code. A segment of DNA carries an order of nitrogen
bases. This segment of nitrogen bases is a gene. Each gene codes for a
specific sequence of amino acids, which in turn, codes for a protein.
Protein synthesis occurs in two main stages: transcription and translation.
2.6.2 1 State one function of messenger RNA and one function of transfer
RNA.
mRNA - the sequence of bases on mRNA determines the sequence of a
polypeptide. It is copied code from DNA, or the DNA template.
tRNA – carries codons to the mRNA.
rRNA – major component of ribosomes.
2.6.3 2 Outline DNA transcription in terms of the formation of a RNA strand
complementary to the DNA strands by RNA polymerase.
The process of protein synthesis can be divided into two parts:
Transcription: the process by which RNA is produced from a DNA
template. This occurs in the nucleus of a cell. Translation: the assembly
of a polypeptide in a sequence specified by the order of the base sequence
in the mRNA. Transcription is similar to DNA replication in that it takes
place in the nucleus and involves sections of DNA which need to unzip.
Then, only one of the two strands of DNA is transcribed and a RNA
complimentary strand to this strand is made. This strand is called mRNA.
After transcription, the mRNA leaves the nucleus through nuclear pores
and goes into the cytoplasm. The steps of transcription are: 1. The two
strands of DNA unwind and the hydrogen bonds between them break. 2.
Free nucleotides in the nuclear fluid start to pair up with the nucleotides of
one of the DNA strands. This DNA strand is called the template, the other
strand is termed as inactive in the process of transcription. 3. The three
nucleotides that bind to the DNA form a mRNA strand by the action of the
enzyme nRNA polymerase. 4. The RNA formed is now called messenger
RNA. This is because it carries the message from the DNA in the form of
the sequence is complimentary to the sequence of the DNA strand.
2.6.4 2 Describe the genetic code in terms of codons composed of triplets of
bases.
The genetic code is based on sets of 3 nucleotides call codons. Since 20
amino acids are commonly found in cells we need to use a set of 3
nucleotides, which will create 64 possibilities. This is more than is
required for the 20 different amino acids. As a result, the code is said to
be degenerate meaning that more than one codon can code fore a single
amino acid. The sequence of the codons in the mRNA determines the
sequence of the amino acids in the polypeptide.
2.6.5 3 Describe translation including the roles of mRNA codons, tRNA anti-
codons, and ribosomes leading to peptide linkage formation.
After the mRNA has reached the cytoplasm, translation takes place. In the
cytoplasm, ribosomes attach to the mRNA. The ribosome covers an area
of two codons on the mRNA. TRNA carrying an amino acid will come in
and the anti-codon exposed on the tRNA will have a complementary
binding with the codon of the mRNA. The second tRNA with its own
anti-codon, and consequently carrying a specific amino acid, will
complementary bind to the second codon on the mRNA, filling the other
site in the ribosome. The second amino acid will attach to the first and the
first amino acid will be released from the tRNA. The ribosome and
mRNA will move relative to each other and the tRNA will separate from
the mRNA. The ribosome now covers the second and third codon. A new
tRNA with an amino acid comes in, the second tRNA is released after its
amino acids has attached to the third one. This process continues until a
stop codon is reached. The stop codon does not code for an amino acid
but terminated translation. The steps for translation are below:
- The transcribed mRNA leaves the nucleus through the nuclear pores in
the nuclear membrane. 2. It reaches the cytoplasm and attaches to a ribosome. 3. The triplets of nucleotides in mRNA are called codons. 4. In the cytoplasm, there is another kind of RNA, tRNA. It is folded with hydrogen bonds between nucleotides. This folding results in a cloverleaf shape. 5. tRNA is attached to a certain amino acid at one end and at another end it has a specific triplet of nucleotides, called the anticodon. There are forty-five different kinds of tRNA, each with a different anticodon. 6. tRNA carrying a specific amino acid attaches itself to mRNA by means of hydrogen bonds between the complementary nitrogen bases. So each codon on mRNA can bind with one tRNA, which is the one that has the anticodon that is complementary to it. 7. Another tRNA carrying a specific amino acid attaches to the next codon through its anticodon by hydrogen bonds. 8. The two amino acids now make a peptide bond with each other. 9. The first tRNA breaks off and another tRNA attaches to the next codon bringing with it another amino acid according to that specific codon. The amino acid makes a peptide bond with the previous one. 10. This process continues, and the polypeptide grows and elongates, and process that is called elongation. 11. At a certain stage a codon is met on mRNA that does not code for an amino acid. This codon is called a stop codon. 12. This terminates the process of translation, and the polypeptide is released. MRNA fragments and returns to the nucleus, where its nucleotides are recycled and used again for protein synthesis. TRNA also return to their free state, each carrying a certain amino acid and ready for the process of translation again when needed. The process is called translation because mRNA is translated into a polypeptide.
2.6.6 1 Define the terms degeneracy and universal as they relate to the
genetic code.
The genetic code is said to be degenerate since more than one codon cedes
for an amino acid. There are twenty different types of amino acids and
sixty-four different types of triplet codons. This results in the fact that
each amino acid can be coded for by more than one codon. Degeneracy is
important in decreasing the effects of mutation. If CCU mutates into
CCC, CCA, or CCG, the amino acid will be proline, and no effect will be
expresses by the mutation. This is one of the reasons that evolution is a
long and slow process. The DNA is also universal, meaning that a certain
codon codes for the same amino acid in all organisms from bacteria to
man. This is very important in genetic engineering and gene
manipulation.
2.6.7 3 Explain the relationship between one gene and one polypeptide and its
significance.
The genetic code is universal and a specific codon will code for a specific
amino acid in all organisms. So, if a certain human gene is transferred to a
bacterium, the bacterium will make the human protein coded for by that
gene. Hence “one gene, on protein.” However, since all reactions in
biological systems require enzymes to proceed at a reasonable speed,
control the enzymes essentially means controlling the rate of reaction. It
is clear that every polypeptide chain in an enzyme is coded for by a
specific gene. By transcribing and translating the genes, we can control
the rate of enzyme production and the rate of any reaction. Therefore, the
saying one gene, one protein, should be state one gene, one polypeptide.
