Molecular Genetics: Unit Review

They labelled DNA with ³²P and protein with ³⁵S in bacteriophages. Radioactive phosphorus (DNA) entered the bacteria, so DNA is the genetic material.

Chargaff's rules (A = T, G = C) and Rosalind Franklin's X-ray diffraction image (Photo 51) showing a helix of constant width.

C = 22%, so A + T = 56%: A = 28%, T = 28%.

Sugar: deoxyribose vs ribose. Bases: thymine in DNA, uracil in RNA. Strands: DNA is usually double-stranded, RNA usually single-stranded.

3′-TACGGCAA-5′ (written 5′ to 3′: 5′-AACGGCAT-3′).

G-C pairs form three hydrogen bonds; A-T pairs form two.

It breaks down the cell and nuclear membranes (lipids), releasing the DNA.

DNA is insoluble in alcohol, so it precipitates where the layers meet. Alcohol is flammable, so it is kept away from heat and flames, and an adult handles it.

Meat tenderizer contains a protease, which digests histones and other proteins bound to DNA, so more DNA is released and can clump together.

Helicase; primase; DNA polymerase III; DNA ligase.

DNA polymerase can only add nucleotides to the 3′ end, building 5′ to 3′. The lagging strand runs the wrong way relative to fork movement, so it is made in short pieces away from the fork.

Each new DNA molecule has one parent strand and one new strand.

The lagging strand cannot be finished at the very end of a linear chromosome, so chromosomes shorten each round; telomeres are non-coding repeats that protect the genes.

If it adds a mismatched nucleotide, it uses its 3′ to 5′ exonuclease activity to remove it, then adds the correct one.

Thymine dimers; fixed by nucleotide excision repair.

After replication, it finds wrongly paired bases that proofreading missed and replaces a section of the new strand.

The template (antisense) strand, read 3′ to 5′; RNA is built 5′ to 3′.

5′-AUGCCUAAG-3′.

A DNA sequence (such as the TATA box in eukaryotes) where RNA polymerase and transcription factors bind to start transcription.

Adding a 5′ cap, adding a 3′ poly-A tail, and splicing out introns.

Joining different combinations of exons from one gene, so one gene can code for several proteins.

Bacterial genes usually lack introns, and transcription and translation happen together in the cytoplasm.

With 4 bases, 2-base codons give only 16 combinations, fewer than the 20 amino acids; 3-base codons give 64.

Met–Phe–Gly–Stop.

Most amino acids are coded by more than one codon.

Codon 5′-AUG-3′; methionine.

A site (arriving), P site (holds the growing chain), E site (exit).

A release factor binds, the polypeptide is released, and the ribosome comes apart.

Inducible. Allolactose binds the repressor, which leaves the operator, so the genes are transcribed.

Repressible. Tryptophan acts as a corepressor, activating the repressor, which binds the operator and blocks transcription.

Lactose removes the repressor, and low glucose raises cAMP, so CAP binds and boosts transcription.

It generally silences it, by keeping chromatin tightly packed and blocking transcription.

Activator proteins bind the enhancer, the DNA loops, and they contact the proteins at the promoter, helping RNA polymerase start.

It base-pairs with a matching mRNA and blocks its translation or causes it to be broken down.

A nonsense mutation: TGG (Trp) becomes TGA (stop), so the protein is cut short.

Deleting one causes a frameshift, changing every codon downstream; deleting three removes one amino acid but keeps the frame.

Physical: UV light, X-rays or other ionizing radiation. Chemical: e.g., benzo[a]pyrene in tobacco smoke, or base analogues.

They have short single-stranded overhangs (AATT) that can base-pair with any other end cut by the same enzyme.

Bacteria cannot splice out introns. cDNA, made from mature mRNA by reverse transcriptase, has no introns.

Only bacteria that took up the plasmid survive on antibiotic plates, so transformed cells can be selected.

Denaturation (about 95 °C), annealing of primers (about 50 to 65 °C), extension by Taq polymerase (about 72 °C).

2²⁰ ≈ 1 million (1 048 576).

DNA is negatively charged (phosphate groups), so it moves to the positive end. Smaller fragments move farthest.

It transfers part of its Ti plasmid into plant DNA; scientists replace its disease genes with the desired gene.

A matching guide RNA and an adjacent PAM (NGG). Without a template, end joining often adds or deletes bases, knocking out the gene.

The donor of the body-cell nucleus (except for mitochondrial DNA from the egg).

Creating a human clone and altering the genome of a human cell or embryo in a way that could be inherited.

Harvard mouse (2002): higher life forms cannot be patented; or Monsanto v. Schmeiser (2004): growing patented GM canola without a licence infringed the patent on the gene and cells.

Any two: genetic counsellor, medical laboratory technologist, forensic DNA analyst, bioinformatician, plant breeder, research scientist, patent agent, bioethicist.