2.3

• Both DNA and RNA carry information, for instance DNA holds genetic information whereas RNA then transfers this genetic information from DNA to ribosomes made of RNA and proteins.
• Both deoxyribonucleic and ribonucleic acid are polymers of nucleotides.
• Nucleotides consist of pentose which is a 5 carbon sugar, a nitrogen containing organic base and a phosphate group.
• The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine.
• tRNA is a single stranded molecule with a binding site at one end that can only carry one type of amino acid, and a triplet of bases at the other.
• tRNA attaches itself to mRNA by complementary base pairing, with two molecules attaching to mRNA at a time.
• The amino acids attached to two tRNA molecules join by a peptide bond and then tRNA molecules detach themselves from the amino acids, leaving them behind.
• This process is repeated, leading to the formation of a polypeptide chain until a stop codon is reached on mRNA and ends the process of protein synthesis.
• The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil.
• Nucleotides join together by phosphodiester bonds formed in condensation reactions.
• A DNA molecule is a double helix composed of two polynucleotides joined together by hydrogen bonds between complementary bases whereas RNA is a relatively short single polynucleotide chain.
• ATP Adenosine triphosphate is a nucleotide derivative and consists of ribose, adenine and three phosphate groups.
• Energy is released when ATP is hydrolysed to form ADP and a phosphate molecule.
• The process of hydrolysing ATP to form ADP and a phosphate molecule is catalysed by ATP hydrolase.
• The inorganic phosphate can be used to phosphorylate other compounds, as a result making them more reactive.
• Condensation of ADP and inorganic phosphate catalysed by ATP synthase produces ATP during photosynthesis and respiration.
• The semi-conservative replication of DNA ensures genetic continuity between generations of cells meaning that genetic information is passed on from one generation from the next.
• The steps of semi conservative replication of DNA are as following: The double helix unwinds and the hydrogen bonds between the complementary bases break using DNA helicase thus separating the two strands of DNA.
• Both strands are used as templates and complementary base pairing occurs between the template strands and free nucleotides.
• Not all the genome codes for proteins are written in the form of genes, the non-coding sections of DNA are called introns and the coding regions are called exons.
• The genetic code is non-overlapping, meaning that each triplet is only read once and triplets don’t share any bases.
• The genetic code is also degenerate, meaning that more than one triplet codes for the same amino acids, this reduces the phenotypic effect of mutations.
• A change in the base sequence of DNA may alter the amino acid sequence and the protein, therefore it can have various effects.
• Some mutations are harmful such as the mutation which leads to production of sticky mucus and causes cystic fibrosis or sickle cell anaemia in which a mutated form of haemoglobin distorts the shape of red blood cells.
• The genetic code contains start and stop codons which either start or stop protein synthesis.
• There are two stages of protein synthesis: Transcription which occurs in the nucleus and involves DNA and mRNA, and Translation which involves mRNA, tRNA and ribosomes.
• During transcription, DNA strand is transcribed into mRNA and translation is the process during which the amino acids are assembled together to form a polypeptide chain/protein.
• During transcription, a molecule of mRNA is made in the nucleus: The hydrogen bonds between the complementary bases break and the DNA uncoils thus separating the two strands.
• One of the DNA strands is used as a template by RNA polymerase to make the mRNA molecule, this DNA template is called the antisense strand.
• Free nucleotides line up by complementary base pairing and adjacent nucleotides are joined by phosphodiester bonds made by RNA polymerase thus forming a single stranded molecule of mRNA.
• mRNA then moves out of the nucleus through a pore and attaches to a ribosome in the cytoplasm which is the site of next stage of protein synthesis called translation.
• During translation amino acids join together to form a polypeptide chain.
• mRNA attaches to a ribosome and transfer RNA collects amino acids from the cytoplasm and carries them to the ribosome.
• Adjacent nucleotides are joined by phosphodiester bonds formed in condensation reactions using DNA polymerase.
• The order of bases on DNA is called the genetic code which consists of triplets of bases, each triplet of bases codes for a particular amino acids and is known as a codon.
• The amino acids are then joined together by peptide bonds and form a polypeptide chain.
• A gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.