Transcription and Translation

• The nucleic acids DNA and RNA are polymers of nucleotides.
• DNA differs from RNA in the number of strands normally present, the base composition and the type of pentose.
• DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.
• Nucleotides consist of three parts: a sugar, which has five carbon atoms, so is a pentose sugar; a phosphate group, which is the acidic, negatively-charged part of nucleic acids; and a base that contains nitrogen and has either one or two rings of atoms in its structure.
• The full names of DNA and RNA are based on the type of sugar in them – deoxyribonucleic acid and ribonucleic acid.
• There are usually two polymers of nucleotides in DNA but only one in RNA. The polymers are often referred to as strands, so DNA is double-stranded and RNA is single-stranded
• Each strand of DNA consists of a chain of nucleotides linked by covalent bonds
• The strands are held together by hydrogen bonds between the nitrogenous bases. Adenine (A) is always paired with thymine (T) and guanine (G) with cytosine (C). This is referred to as complementary base pairing.
• The replication of DNA is semi-conservative and depends on complementary base pairing.
• Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds.
• DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template.
• Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase.
• Translation is synthesis of polypeptides on ribosomes.
• The amino acid sequence of polypeptides is determined by mRNA according to the genetic code.
• Codons of three bases on mRNA correspond to one amino acid in a polypeptide.
• Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA.
• Before DNA replication can occur, the two strands of the molecule must separate so that they can each act as a template for the formation of a new strand. The separation is carried out by helicases, a group of enzymes that use energy from ATP.
• The enzyme RNA polymerase binds to a site on the DNA at the start of a gene.
• RNA polymerase forms covalent bonds between the RNA nucleotides.
• The product of transcription is a molecule of RNA with a base sequence

  that is complementary to the template strand of DNA.
• The DNA strand with the same base sequence as the RNA

  is called the sense strand.
• The other strand that acts as the template and has a complementary base sequence to both the RNA and the sense strand is called the antisense strand.
• To begin transcribing a gene, RNA polymerase binds to the DNA of the gene at a region called the promoter. 
• A promoter contains DNA sequences that let RNA polymerase or its helper proteins attach to the DNA.
• The Initiation process includes RNA polymerase II unwinding/unzipping DNA strands.
• RNA polymerase adds a matching (complementary) RNA nucleotide to the 3' end of the RNA strand.
• The RNA transcript is nearly identical to the non-template, or coding, strand of DNA. However, RNA strands have the base uracil (U) in place of thymine (T).
• During elongation, RNA polymerase moves along the DNA molecule, adding more nucleotides to the growing RNA chain.
• mRNA leaves the nucleus through nuclear pores and travels to ribosomes in the cytoplasm where translation takes place.
• A sense (coding) strand is the segment within double-stranded DNA that carries the translatable code in the 5′ to 3′ direction, and which is complementary to the antisense (template) strand of DNA.
• Translation takes place on cell structures in the cytoplasm known as ribosomes.
• Amino acids are the building blocks of proteins.
• Three main types of RNA are involved in protein synthesis: mRNA, rRNA and tRNA.
• The Central Dogma: DNA -> RNA -> Protein
• Messenger RNA carries the genetic information from the DNA in the nucleus of the cell to the ribosomes in the cytoplasm.
• Transfer RNA is responsible for bringing amino acids to the ribosomes during protein synthesis.
• Ribosomal RNA makes up part of the structure of the ribosome where translation occurs.
• Ribosomal RNA helps to catalyse the formation of peptide bonds between amino acids, facilitating the assembly of the amino acids into a polypeptide chain during translation.
• There are 64 different codons: 61 specify amino acids and 3 are used as stop signals.
• Methionine is specified by the codon AUG, which is also known as the start codon. Consequently, methionine is the first amino acid to dock in the ribosome during the synthesis of proteins.