Molecular Biology Processes: Cancer, Protein Synthesis, and DNA

Development of Cancer

• Oncogenes are transformed by carcinogens.
• Cancerous cells do not respond to signals from other cells, so they continue to divide (rapid mitosis).
• Cancerous cells are not removed by the immune system.
• The tumor gets bigger.
• Cells change their characteristics and look different under the microscope.
• The tumor is supplied with blood and lymph vessels.
• Tumor cells spread in blood and lymph to other parts of the body (Metastasis).
• Secondary cancers form throughout the body as tumor cells invade other tissues.

Protein Synthesis

Transcription

The double helix of the DNA is unwound by RNA Polymerase, which docks on the DNA at a special sequence of bases. RNA Polymerase moves downstream, unwinding the DNA. As the double helix unwinds, ribonucleotide bases (A, C, G, and U) attach themselves to the DNA template strand by complementary base pairing.

• RNA Polymerase catalyzes the formation of covalent bonds between the nucleotides.
• In the wake of transcription, DNA strands recoil into the double helix.
• The RNA transcript is released from the DNA, along with the RNA polymerase.

Translation

1. A ribosome attaches to the mRNA molecule at the start codon (always AUG), signaling the start of the gene to be transcribed.
2. The ribosome can enclose two codons at a time.
3. tRNAs (transfer RNAs) act as couriers; each tRNA is bonded to a specific amino acid.
4. As AUG is the start codon, the first amino acid to be 'couriered' is always Methionine.
5. Amino acids are added stepwise to the growing polypeptide chain. The next amino acid tRNA attaches to the adjacent mRNA codon.
6. The bond holding the tRNA and amino acid together is broken, and a peptide bond is formed between the adjacent amino acids.
7. The ribosome shuffles down to cover a new codon, releasing the first tRNA, which is now free to collect another amino acid.
8. The polypeptide chain continues to grow until a 'stop' codon is exposed on the ribosome (UAA, UAC, or UGA).

DNA Replication

• The DNA double helix unwinds and 'unzips' as the hydrogen bonds between the bases break.
• In the nucleus, there are nucleotides to which two extra phosphates have been added. These extra phosphates activate the nucleotides, enabling them to take part in the following reactions.
• Each of the bases of the activated nucleotides pairs up with its complementary base on each of the old DNA strands.
• An enzyme, DNA polymerase, links the sugar and innermost phosphate groups of next-door nucleotides together.
• The two extra phosphates are broken off and released into the nucleus.

Stages of Mitosis

Prophase – Metaphase – Anaphase – Telophase – Cytokinesis

DNA replication takes place during interphase to produce 2 identical copies of each DNA molecule in the nucleus. Each chromosome is made up of one DNA molecule, so after replication is complete, each chromosome is made up of 2 identical DNA molecules. During mitosis, the 2 chromatids split apart and are moved to opposite ends of the cell. A new nuclear envelope forms around each group. These 2 nuclei each contain a complete set of DNA identical to those in the parent cell. After mitosis is complete, the cell usually divides into 2, with one of the nuclei in each one of the 2 cells.

Immobilizing Enzymes

One way enzymes can be immobilized is by trapping them in jelly (alginate) beads. This is commercially useful because the enzyme can be re-used, and the product is separate from (uncontaminated by) the enzyme.

Immobilized enzymes are more tolerant of temperature changes and pH changes than enzymes in solution. This is because their molecules are held firmly in shape by the alginate in which they are embedded, and so do not denature as easily.

Sickle Anemia

The DNA sequences for the $\text{Hb}^{\text{A}}$ (normal) and $\text{Hb}^{\text{S}}$ (sickle cell) alleles of the gene for the $\beta$-globin polypeptide differ by only one base. The triplet CTT in $\text{Hb}^{\text{A}}$ is replaced by CAT in $\text{Hb}^{\text{S}}$, swapping the amino acid from glutamic acid to valine. This single difference in the polypeptide results in sickle cell anemia in individuals with two $\text{Hb}^{\text{S}}$ alleles.

DNA and RNA Structure

DNA and RNA are polynucleotides. They are made of long chains of nucleotides linked together. They are made up of:

• A 5-carbon sugar (deoxyribose in DNA, ribose in RNA)
• A phosphate group
• A nitrogen-containing base (A, G, C, T in DNA; A, G, C, U in RNA)