HUMAN BIOCHEMISTRY  NUCLEIC ACIDS  THE GENETIC CODE

STRUCTURE OF NUCLEOTIDES AND NUCLEIC ACIDS  Nucleic acids are very long thread-like macromolecules with altering sugar and phosphate molecules forming the ’backbone’. There are two types of nucleic acid found in living cells: deoxyribonucleic acid (DNA), and ribonucleic acid (RNA). Nucleic acids are polymers made up of nucleotides.  Nucleotides join together to form nucleic acids. A nucleodite consists of three substances combined together. These are:  A nitrogenous base  the four bases are cytosine (C), guanine (G), adenine (A), thymine (T).  A pentose sugar  deoxyribose occurs in DNA and ribose in RNA.  A phosphoric acid.  Nucleic acids are joined by covalent bonds between the phosphate of one nucleotide and the sugar of the next, resulting in a backbone with a repeating pattern of sugar–phosphate–sugar–phosphate. Nitrogenous bases are attached to the sugar of the backbone.

A NUCLEODITE, A NUCLEIC ACID AND THE FOUR NITROGEN- CONTAINING BASES

THE DNA AND RNA MOLECULES  The DNA molecule consists of two polynucleotide strands paired together and held by hydrogen bonds. The two strands take the shape of a double helix. The pairing of bases is between adenine (A) and thymine (T), and between cytosine (C) and guanine (G). Adenine and thymine form 2 hydrogen bonds when they combine and cytosine and guanine form 3 hydrogen bonds.  The RNA molecule is a single strand of polynucleotide in which the sugar is the ribose the bases found in RNA are cytosine (C), guanine (G), adenine (A) and uracil (which replaces thymine of DNA). There are three types of RNA, known as messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA.

DIFFERENCES BETWEEN THE STRUCTURES OF DNA AND RNA FEATUREDNARNA NUMBER OF STRANDS IN THE MOLECULE Two strands forming a double helix One strand only TYPE OF SUGAR IN EACH NUCLEOTIDE Deoxyribose (Deoxyribose lacks an oxygen atom on C2) Ribose TYPES OF BASES CONTAINED A, C, G and TA, C, G and Uracil replaces thymine

DNA AND RNA REPRESANTATION

DNA REPLICATION  The cell produces many free nucleotides for DNA replication.  Helicase uncoils the DNA double helix and splits it into two template strands.  RNA primase adds a short length of RNA attached by base pairing to the template strand of DNA. This acts as a primer, allowing DNA polymerase to bind and begin replication.  DNA polymerase III adds nucleotides in a direction 5’-3’ next to the RNA primer.  Short lengths of DNA are formed between RNA primers, called Okazaki fragments.  DNA polymerase I removes the RNA primer and replaces it with DNA.  A nick is left where two nucleodites are still unconnected. This nick is sealed up by DNA ligase.

DNA REPLICATION REPRESENTATION

DNA TRANSCRIPTION  The one strand that forms the template and is transcribed is called antisense strand and the other one is called sense strand.  DNA is unwound by the front of RNA polymerase.  Free nucleoside triphosphates are used by RNA polymerase to extent the growing mRNA molecule.  Two phosphates are removed as they are linked on, converting them into RNA molecules.  The 5’ end of the nucleotide is added to the 3’ end of the growing chain-transcription thus moves in a 5’  3’ direction.  DNA is rewound into a double helix by the rear of RNA polymerase

REPRESENTATION OF DNA TRANSCRIPTION

DNA TRANSLATION  Messenger RNA carries the information needed for making polypeptides out from the nucleus, to the cytoplasm of eukaryotic cells.  The information is in a coded form which is decoded during translation.  The base sequence of mRNA is translated into the amino acid sequence of a polypeptide.  Three bases (codon) code for one amino acid and this is why the genetic code is called a triplet code.  It is possible for two or three codons to code for the same amino acid and this is why the genetic code is called degenerated.

DNA TRANSLATION REPRESENTATION

TRANSLATING THE GENETIC CODE

DNA PROFILING  Humans and other organisms have short sequences of bases that are repeated many times called satellite DNA. This satellite DNA varies greatly between different individuals in the number of repeats.  If it is coped using a methods which is called PCR and then cut up into small fragments using restriction enzymes, the length of the fragments vary greatly between individuals.  Gel electrophoresis can be used to separate fragmented pieces of DNA according to their charge and size.  The pattern of bands on the gel is very unlikely to be the same for two individuals.  This technique has many applications, including forensic investigations and investigating paternity.

REPRESENTATION OF DNA PROFILING