Key Points

A nucleotide consists of a nitrogenous base, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group. Nucleotides are linked by $3'-5'$ phosphodiester bonds to form a polynucleotide chain.

DNA consists of two antiparallel polynucleotide chains, with one running in the $5' \rightarrow 3'$ direction and the other in the $3' \rightarrow 5'$ direction. The backbone is sugar-phosphate, and the bases project inwards.

In a DNA double helix, Adenine (A) always pairs with Thymine (T) through two hydrogen bonds, and Guanine (G) always pairs with Cytosine (C) through three hydrogen bonds. This makes the two strands complementary.

Negatively charged DNA is wrapped around a positively charged histone octamer to form a structure called a nucleosome. Repeating nucleosomes form chromatin, which is further condensed to form chromosomes.

This experiment provided unequivocal proof that DNA is the genetic material. They used bacteriophages with radioactive phosphorus ($^{32}P$ in DNA) and radioactive sulfur ($^{35}S$ in protein) to show that only DNA entered the host bacteria.

Watson and Crick proposed that during replication, the two DNA strands separate, and each acts as a template for a new complementary strand. The resulting DNA molecules each have one parental and one new strand. This was confirmed by the Meselson and Stahl experiment.

DNA-dependent DNA polymerase is the main enzyme that catalyzes polymerization in the $5' \rightarrow 3'$ direction. DNA ligase joins the discontinuously synthesized fragments (Okazaki fragments) on the lagging strand.

Proposed by Francis Crick, it describes the flow of genetic information: $\text{DNA} \xrightarrow{\text{Transcription}} \text{RNA} \xrightarrow{\text{Translation}} \text{Protein}$. In some viruses, reverse transcription (RNA to DNA) can occur.

A transcription unit in DNA consists of three regions: a promoter (binding site for RNA polymerase), the structural gene, and a terminator. The template strand ($3' \rightarrow 5'$) is transcribed into RNA.

The primary transcript (hnRNA) is processed before translation. This involves splicing (removal of introns and joining of exons), capping (addition of methyl guanosine triphosphate at the $5'$-end), and tailing (addition of poly-A tail at the $3'$-end).

The code is a triplet, with 61 codons for 20 amino acids and 3 stop codons (UAA, UAG, UGA). It is degenerate (multiple codons for one amino acid), non-overlapping, and nearly universal. AUG acts as the initiator codon.

Transfer RNA (tRNA) acts as an adapter molecule. It has an anticodon loop that reads the codons on mRNA and an amino acid acceptor end that binds to a specific amino acid.

Translation is the synthesis of a polypeptide from an mRNA template on a ribosome. The ribosome moves along the mRNA, and tRNAs bring the corresponding amino acids, which are joined by peptide bonds.

The lac operon is an inducible system for gene regulation in E. coli. In the presence of lactose (the inducer), the repressor protein is inactivated, allowing RNA polymerase to transcribe the structural genes needed for lactose metabolism.

HGP was a mega-project to sequence the entire human genome. Key findings include a genome size of $3.16 \times 10^9$ bp, an estimated 20,000-25,000 genes, and that less than 2% of the genome codes for proteins.

This technique identifies individuals by analyzing variations in repetitive DNA sequences called Variable Number of Tandem Repeats (VNTRs). It is widely used in forensic science and paternity testing.