RNA: 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.
- Deciphering the mRNA Transcripts
- The Genetic Code
- 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 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 synthesized.
- Causes of Gene Mutations
- Mutations can be caused by mutagens
such as ultraviolet radiation, ionizing radiation (gamma and X-rays) and
chemicals such as carcinogens.
Genetic Regulation : Controls Over Genes
How do these simple things called genes
control all of the functions of life from birth to death?...
- Overview of Gene Controls
- Because all cells in your body have a complete genome (well almost) only about 90 % are
used at a time.
- Which genes are expressed depends on the type of cell, its responses to chemical
signals, and built-in control systems.
- Regulatory proteins interact with DNA, RNA, or actual gene products.
- Two kinds of control systems are used by cells:
- In negative control systems, a regulatory protein
binds to the DNA to block transcription; it can be removed by an inducer.
- Controls in Bacterial Cells
- Negative Control of Transcription
- Escherichia coli bacteria (common in the human digestive tract) can
metabolize lactose because of a series of genes that code for lactose-digesting enzymes.
- A promoter and operator precede the three genes and together they are called an operon.
- A regulator gene nearby codes for a repressor protein that binds to the operator
when lactose concentrations are low and effectively blocks RNA polymerases access to
- When milk is consumed, the lactose binds to the repressor changing its shape and
effectively removing its blockage of the promoter; thus RNA polymerase can now initiate
transcription of the genes.
- Controls in Eukaryotic Cells
- Much less is known about gene controls in multicelled eukaryotes because patterns of
gene expression vary within and between body tissues.
- Examples of Signaling Mechanisms
- Hormone Signals
- Hormones are major signaling molecules that can stimulate or inhibit gene
activity in target cells.