From DNA to Proteins
DNA carries out the assembly and minute to minute operation of a cell.
In transcription, molecules of RNA are produced on the DNA templates in the nucleus. In
translation, RNA molecules shipped from the nucleus to the cytoplasm are used as templates
for polypeptide assembly. The resulting protein is structural or functional, such as an
enzyme to perform work.
DNA is life.
- The Three Classes of RNA
- Messenger RNA (mRNA) carries the
"blueprint" for protein assembly to the ribosome.
- Ribosomal RNA (rRNA) combines with
proteins to form ribosomes upon which polypeptides are assembled.
- Transfer RNA (tRNA) brings the correct
amino acid to the ribosome and pairs up with an mRNA code for that amino acid.
- How RNA Is Assembled
- RNA differs from DNA in many ways:
- RNA uses ribose sugar, not deoxyribose.
- RNA uses URACIL (U) in place of Thymine (T).
- RNA leaves the nucleus, DNA is confined
- Transcription differs from replication in three ways:
- Only one region of one DNA strand is used as a template.
- RNA polymerase is used instead of DNA polymerase.
- (m)RNA is single stranded; DNA is double.
- Transcription begins when RNA polymerase binds to a promoter region (a base sequence at
the start of a gene) and then moves along to the end of a gene; an RNA transcript is the result.
- Finishing Touches on the mRNA Transcripts
- Newly formed mRNA is an unfinished molecule, not yet ready for use.
- mRNA transcripts are modified before leaving the nucleus.
- The 5 end is capped with a special
nucleotide that may serve as a "start" signal for translation.
- A "poly-A tail" of about 100200 molecules
of adenylic acid is added to the 3 end.
- Noncoding portions (introns) are snipped out, and
actual coding regions (exons) are spliced together to
produce the mature transcript. These introns can have regulatory purposes.
- Deciphering the mRNA Transcripts
- The Genetic Code
- Both DNA and its RNA transcript are linear sequences of nucleotides carrying the
- Every three bases (a triplet) specify an amino acid to be included into a growing
polypeptide chain; the complete set of triplets of is called the genetic
- Each base triplet in RNA is called a codon.
- The genetic code consists of sixty-one triplets that specify amino acids and three that
serve to stop protein synthesis. (43 = 64)
- AUG (specifies methionine) is the "start" codon.
- With few exceptions, the genetic code is universal for all life.
- Roles of tRNA and rRNA
- Each kind of tRNA has an anticodon that is
complementary to a mRNA codon; each tRNA also carries its own specific amino acid.
- After the mRNA arrives in the cytoplasm, an anticodon on tRNA bonds to the codon on the
mRNA, and thus a correct amino acid is brought into place (molecular
- The first two bases of the anticodon must pair up with the codon by the usual rules of
base pairing (A with U ; G with C), but there is some latitude in the pairing of the third
base (called the wobble effect).
- A ribosome has two subunits (each composed of rRNA and proteins) that perform together
only during translation.
- Stages of Translation
- In initiation, a complex forms in this sequence:
initiator tRNA + small ribosomal subunit + mRNA + large ribosomal subunit.
- In elongation, a start codon on mRNA defines the
reading frame; a series of tRNA's deliver amino acids in sequence by codon-anticodon
matching; a peptide bond joins each amino acid to the next in sequence.
- In termination, a stop codon is reached and the
polypeptide chain is released into the cytoplasm or enters the cytomembrane system for
- The three steps just outlined can be repeated many times on the same mRNA because
several ribosomes may be moving along the mRNA at the same time.
- A gene mutation is a change in one to several bases
in the nucleotide sequence of DNA, which can result in a change in the protein
- Causes of Gene Mutations
- Mutations are rare, chance events but each gene has a characteristic mutation rate. We will discuss mutation rates more during
- Mutations can be caused by mutagens such as
ultraviolet radiation, ionizing radiation (gamma and X-rays) and chemicals such as
alkylating agents, which act as carcinogens.
- The Proof is in the Protein
- If a mutation arises in a somatic cell, it will affect only the owner of that cell and
will not be passed on to offspring.
- If however, the mutation arises in a gamete, it may be passed on and thus enter the
- Either kind of mutation may prove to be harmful (most likely), beneficial, or neutral in
- Types of Mutations (see Sources of Genetic Change