Nucleotides

• DNA has a double helix structure, containing two polynucleotide chains bonded together with hydrogen bonds between the nitrogenous bases. The chains are anti-parallel to each other.
• RNA is a single stranded molecules of one polynucleotide chain of a single helix.
• The bases of DNA are Adenine, Thymine, Cytosine, Guanine.
• The bases of RNA are adenine, guanine, uracil and cytosine.
• The structure of nucleotides include pentose sugar, phosphate group, and nitrogenous base.
• RNA has a ribose pentose sugar whilst DNA has a deoxyribose pentose sugar.
• The bonds involved in a nucleotide involve an N-glycosidic bond between the sugar and the nitrogenous base. Between the phosphate group and the sugar, there is a phosphoester bond. Between the phosphate of one group and the sugar of another, there is a phosphodiester bond.
• Cytosine and guanine are bonded by 3 hydrogen bonds. Adenine and thymine/uracil are bonded by 2.
• Purines have a larger two ring structure, guanine and adenine.
• Pyrimidines have a smaller 1 ring structure, cytosine, thymine, uracil.
• ATP and ADP are made up of nucleotides, and consist of adenine, a pentose ribose sugar, and either 1,2 or 3 phosphate groups. When the bond joining the phosphate to the sugar is broken, lots of energy is released.
• Replicating DNA occurs in interphase of mitosis.
• DNA Replication, 1st stage: DNA uncoils from it's double helix, DNA helicase breaks the hydrogen bonds.
• DNA Replication 2nd stage: Each strand acts as a template, whilst DNA polymerase adds complimentary nucleotides as the DNA unzips. There is one lagging strand (joins backwards along strand) and one leading strand as the nucleotides must be added in the 5' to 3' direction.
• DNA Replication 3rd stage: DNA ligase facilitates the joining of DNA fragments which are created at the lagging strand. They are called Okazaki fragments. The enzyme catalyses the making of phosphodiester bonds, creating the new backbone nucleic acid.
• Conservative hypothesis is the complete parent DNA molecule acts as a template for the new daughter, which is assembled from new nucleotides. The parent molecule is unchanged.
• Semi-conservative hypothesis is when the parent DNA separates into its two component strands, each of which act as a template strand and forms two new daughter molecules, with half the parent DNA and half the new DNA.
• The experiment proving the semi-conservative: 15N strand is heavy therefore floats lower in the centrifuge tube, 15N +14N hybrid is less heavy so floats slightly higher in the centrifuge, proving the DNA is made up of one strand of the parents DNA. 14N is light so floats at the top of the centrifuge.
• Genes code for specific proteins, they consist of codons that make amino acids by pairing 3 nitrogenous bases which determines the order of amino acids.
• Genes are universal and are found in all living organisms.
• Genes are degenerate so there can be a few different ways to code for the same amino acids with different triplet bases.
• Genes are non-overlapping so successive triplets are read in order, so each nucleotide is part of only one triplet codon.
• RNA is involved in the synthesis of proteins and is the primary messenger of the genetic code.
• DNA is transcribed into a single strand of mRNA so that it can leave the nucleus as DNA is too large to fit through nuclear pores.
• tRNA decodes the messenger RNA sequences into a polypeptide chain. At one end is an anti-codon region of complimentary base pairs to bind to potential triplet codes. At the other end is the specific amino acid the codon codes for. The shape of tRNA is held together by hydrogen bonds.
• Ribosomal RNA (rRNA) moves along the mRNA molecules, catalysing the assembly of amino acids into protein chains. They also bind tRNA and various accessory molecules necessary for protein synthesis. Ribosomes are also made up of this and proteins.
• Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA by the enzyme RNA polymerase. It starts at a start codon and stops at a stop codon.
• Translation is the second step of gene expression when mRNA must transcribed into a protein.