Genetics and DNA structure

• Genetics is the science of heredity.
• Molecular Biology deals with DNA and protein synthesis.
• The genome is the total DNA contained in the cell.
• The genome consists of the chromosome(s) and any plasmids.
• Chromosomes contain genes.
• Genes are sections of DNA that code for a functional product.
• DNA is a macromolecule made of nucleotides.

Nucleotide Components:

• Nitrogenous base (A, T, G, C)
• Sugar (deoxyribose) numbered 1' to 5'
• A phosphate group

DNA Structure

• DNA forms a double helix.
• Two strands are held together by hydrogen bonds between bases.
• The base pairing rule dictates that A always pairs with T (A-T) and G always pairs with C (G-C).
• Strands of DNA are complementary, meaning the sequence of one strand determines the sequence of the other.
• Nucleotides are linked together by covalent phosphodiester bonds.
• The 5' carbon of one nucleotide is joined to the 3' carbon of the next nucleotide, with a phosphate in between.
• DNA is usually considered in the 5' to 3' direction.
• Two strands of DNA run antiparallel.

Flow of Genetic Information:

• Replication is the process where DNA is copied before cell division.
• Gene expression is the process where DNA is used to make proteins.
• Recombination is the process where DNA can flow between two different bacterial cells.

DNA Replication

• One parental double stranded DNA molecule makes 2 identical double stranded DNA molecules.
• Because the strands are complementary, one strand serves as a template for synthesis of the other strand.
• DNA polymerase reads the order of nucleotides in the template strand to make a complementary new strand.
• dsDNA unwinds, and the strands are separated, forming the replication fork.
• Each separated strand serves as a template for synthesis of a complementary strand.
• A Short RNA primer is produced by the enzyme Primase.
• The primer serves as the starting site for nucleotides to form a new strand of DNA.
• Template must be read in the 3' 5' direction to that the leading DNA strand is synthesised in the 5' -> 3' direction
• Synthesis of the leading strand is continuous in the 5' to 3' direction.
• DNA polymerase synthesizes Okazaki fragments in the 5' to 3' direction for the lagging strand.
• Afterwards, the RNA primers are removed and the fragments are joined together by enzyme DNA ligase.

Gene Expression

• Transcription is when information stored in DNA is copied into RNA.
• Translation is when information in RNA is decoded to make protein.

Transcription

• RNA is synthesized from a DNA template.
• The RNA sequence is complementary to a gene
• Except: it contains U instead of T
• Ex. If Gene is 3'-ATGCAT-5', then mRNA will be 5'-UACGUA-3'

Three types of RNA

• Messenger RNA (mRNA) carries information for making specific protein.
• Ribosomal RNA (rRNA) forms part of the ribosome.
• Transfer RNA (tRNA) transports specific amino acids for protein synthesis.

Steps in Transcription:

• RNA polymerase binds to the gene at a specific site called the promoter; this is initiation.
• The two strands separate.
• Only one DNA strand is copied, known as the template.
• The template is read in the 3' 5' direction so that RNA can be made in the 5' 3' direction.
• RNA polymerase moves along the template, synthesizing new RNA - Elongation step,
• Termination is when RNA polymerase encounters the terminator (end of the gene), it falls off the template and releases the newly synthesized RNA.

Genetic Code:

• Information in mRNA must be translated to make proteins.
• Organized into sets of 3 nucleotides, called codons.
• Each codon specifies an amino acid to be added during protein synthesis.
• Ex. GGC specifies the amino acid glycine
• The sequence of codons in an mRNA determines the sequence of amino acids in the protein.
• Three codons specify no amino acid.
• UAA, UAG, UGA - "Stop codons" signal the end of protein synthesis.

Translation

• A ribosome assembles on the mRNA.
• A tRNA carrying the amino acid formyl-methionine enters the P site, which is intiation
• A tRNA carrying a second amino acid enters the ribosome.
• The codon in the A site specifies the tRNA.
• The ribosome joins the amino acids together by a peptide bond.
• The ribosome moves a distance of one codon down the mRNA during elongation.
• The next codon is now in place in the A site.
• The correct tRNA enters the A site, bringing with it the next amino acid to be added.
• The amino acid is joined to the chain, forming a polypeptide.
• Elongation continues until a "stop codon" is reached.
• When a stop codon enters the A site, the ribosome disassembles and releases the polypeptide which is termination
• The polypeptide is folded into the correct shape and becomes a protein.
• The ribosome can initiate translation of another mRNA.