Nucleic Acids Second Lecture

NUCLEIC ACIDS NUCLEIC ACIDS Nucleic acids are organic molecules that act as the gene/c material of a cell. Nucleic acids store the coded instruc/ons for the expression of proteins, which determine the characteris/cs of a cell. These instruc/ons are passed between genera/ons – making the nucleic acids responsible for inheritance within a species. NUCLEOTIDES All nucleic acids are composed of monomeric subunits known as nucleo< des. Each nucleo/de is comprised of three components: • A pentose sugar (organised into a 5-carbon cyclic structure) • A nitrogenous base (aXached to the sugar at the 1’-carbon) • A phosphate group (aXached to the sugar at the 5’-carbon) Nucleo/des differ according to the nitrogenous base they have PHOSPHATE 5 4 1 BASE SUGAR 3 2 SUGAR-PHOSPHATE BACKBONE Nucleo/des are joined together by condensa< on reac< ons to form long polynucleo/de chains. Nucleo/des are connected between the 5’-phosphate and the 3’-posi/on of the sugar. The sugars and phosphates form a backbone, while the bases extend outwards. The order of the base sequence forms the coded instruc/on. BASE ORDER NUCLEOTIDE BACKBONE BASE SEQUENCE The base sequence of nucleic acids func/ons as a four-leXer code that determines the characteris/cs of the cell. The nitrogenous bases can be organised into two groups: double-ringed purine bases or single-ringed pyrimidine bases. Adenine and guanine are purines, while cytosine and thymine (or uracil) are pyrimidines. PURINES (double-ringed) N H O H N H2N N N Guanine PYRIMIDINES (single-ring) N H2N N H N H N Adenine NH3 H N H N O Cytosine H3C N H H H O N O Thymine (DNA) H O N H N H O Uracil (RNA) DNA VERSUS RNA There are two main types of nucleic acids. DNA func/ons as the master template of the gene/c instruc/ons and remains in the nucleus of eukaryo/c cells (or in the nucleoid region of prokaryo/c cells). RNA func/ons as a transient copy of the template instruc/ons and is transported to the ribosomes to synthesise proteins. There are a number of key structural differences between the structures and func/ons of DNA and RNA: • In DNA nucleo/des the pentose sugar is deoxyribose, whereas RNA nucleo/des use the sugar ribose • In RNA, the nitrogenous base thymine is replaced by uracil (i.e. DNA uses T, whereas RNA uses U) • Whereas DNA is a double-stranded molecule, RNA is always a single-stranded molecule DNA STRUCTURE DNA is composed of double-stranded polynucleo/de chains. The two DNA chains are held together by hydrogen bonds that form between the two sets of bases. When they pair the two polynucleo/de strands will form a structure like a ladder – the backbones are struts and the bases are rungs. In order for the complementary bases to pair, the two DNA strands must be running in opposite direc/ons, and hence the two strands are an< parallel. Double-stranded DNA will arrange into the most stable configura/on: a double helix. NUCLEOTIDE A T G C BASE PAIRING LADDER A T G C A T T A LADDER RNA STRUCTURE RNA is a single-stranded molecule that uses the base uracil (U) instead of thymine. It is synthesised from the DNA template and is involved in protein assembly. There are three main types of RNA made by cells: • • • Messenger RNA (mRNA): A transcript of a DNA sequence that is translated by ribosomes into protein Transfer RNA (tRNA): Brings amino acids to the ribosome according to the specific mRNA sequence Ribosomal RNA (rRNA): A component of the ribosome that enables it to interact with mRNA and tRNA All three types of RNA are required for protein synthesis to occur at the ribosomes. When the protein has been synthesised, the mRNA transcripts are broken down and the nucleo/des can be recycled by the cell. COMPLEMENTARY BASE PAIRING In order for DNA to form a double stranded structure, be replicated or interact with RNA, the bases must be able to connect in a consistent paXern so as to maintain the DNA sequence. This involves what is known as complementary base pairing. A purine must always pair with a pyrimidine in order to maintain an op/mal distance between the two molecules that are interac/ng. The complementary base pairing that occurs is: • The purine adenine (A) always pairs with the pyrimidine thymine (T) by forming two hydrogen bonds • The purine guanine (G) always pairs with the pyrimidine cytosine (C) by forming three hydrogen bonds • In an RNA sequence, the base thymine is replaced by uracil (U), and so this pairs with adenine instead. N H O G N N H–N H–N C N O N H H N–H H N H N –– N H A N N NN H3C OO H N –– N H H O T H N H GENETIC CODE A sequence of DNA that codes for a par/cular protein is called a gene. Because a gene sequence can be of any length and involve any possible combina/on of bases, the capacity for DNA to store informa/on has no limits. A gene sequence will give rise to a polypep/de sequence because specific base combina/ons code for par/cular amino acids. The set of instruc/ons that determines which amino acid is produced by a given combina/on of bases is called the gene< c code. The gene/c code is conserved across all forms of life – this suggests that all living things share universal common ancestry (all exis/ng life came from a single source). CHROMOSOMES Within cells, molecules of DNA are organised into discrete structures called chromosomes. The way DNA is packaged and stored within the cell will differ between prokaryo/c and eukaryo/c cells. In prokaryo/c cells (bacteria), the DNA is unpackaged (naked) and forms a single circular chromosome known as a genophore. In eukaryotes, DNA is packaged with histone proteins to form nucleosomes, which allow the chromosomes to adopt a more compact structure known as chroma/n. This chroma/n will then condense (supercoil) to form linear chromosomes. Addi/onally, eukaryo/c cells are more complex and will consequently possess a larger number of chromosomes (i.e. more than one) than are typically present within most bacterial cells. PROKARYOTE EUKARYOTE DNA is naked DNA is bound to histones Chromosome is circular Chromosomes are linear Sequence is more compact (there are usually no introns) Sequence is less compact (genes may contain introns) Only one main chromosome plus addi/onal plasmids Mul/ple chromosomes (usually organised in pairs) DNA stored in the nucleoid DNA is stored in the nucleus