Structure and functions of RNA. RNA is single stranded, contains uracil instead of thymine and ribose instead of deoxyribose sugar. mRNA carries a copy of the DNA code from the nucleus to the ribosome. Ribosomal RNA (rRNA) and proteins form the ribosome. Each transfer RNA (tRNA) carries a specific amino acid.

RNA Like DNA, RNA is a nucleic acid. Differences between DNA and RNA DNARNA A DNA molecule is double stranded An RNA molecule is single stranded DNA contains the sugar deoxyribose RNA contains the sugar ribose DNA contains the base thymine RNA contains uracil instead of thymine Types of RNA and their functions Type of RNAFunction Messenger RNA (mRNA) Carries a copy of the genetic code on DNA to the ribosomes Transfer RNA (tRNA)Binds to a specific amino acid and transports it to the ribosomes Ribosomal RNA (rRNA) Component of the ribosomes

Transcription Transcription of DNA into primary and mature RNA transcripts to include the role of RNA polymerase and complementary base pairing. The introns of the primary transcript of mRNA are non-coding and are removed in RNA splicing. The exons are coding regions and are joined together to form mature transcript. This process is called RNA splicing.

Protein synthesis 2 stages: 1.Transcription Production of mRNA with a base sequence that depends on the base sequence of DNA. Transcription occurs in the nucleus. 2. Translation A protein is produced with an amino acid sequence that depends on the mRNA base sequence. Translation occurs on the ribosomes.

G C U A G C U A C RNA nucleotides Bonds form joining nucleotides to make a mRNA molecule - this needs ATP and an enzyme (RNA polymerase) Hydrogen bonds Part of DNA molecule uncoils this exposes the bases of the gene on one DNA strand RNA nucleotides form hydrogen bonds with complementary bases of the DNA strand Transcription summary A T G C C G A T G DNA strand

Transcription Transcription is the synthesis of mRNA from DNA. The base sequence of the mRNA is determined by the base sequence of the DNA and so is complementary to the DNA base sequence. RNA polymerase catalyses the synthesis of mRNA. The RNA formed is called the primary transcript, it consists of regions called exons which actually code for the protein and other regions called introns that are non-coding regions. RNA splicing The introns are cut from the primary transcript and the exons are spliced together to form the mature transcript. This mRNA then passes from the nucleus to the ribosomes where it is translated into a protein. Primary transcript of mRNA exons introns Introns cut out Exons spliced to form mature transcript of mRNA

Transcription 1.The DNA double helix uncoils at the site of the gene 2.Hydrogen bonds between the bases break and the 2 DNA strands separate to expose the bases of the gene on one of the strands 3.mRNA nucleotide bases form hydrogen bonds with the gene DNA bases adenine pairs with thymine on DNA uracil pairs with adenine on DNA guanine pairs with cytosine and vise versa 4.While the mRNA nucleotides are held in position, sugar-phosphate bonds are formed to join them into a mRNA molecule. This requires an enzyme and ATP. 5.Hydrogen bonds between the mRNA and DNA bases break to release the mRNA molecule from the DNA 6.The mRNA formed is the primary transcript – introns are cut out and the exons spliced to make the mature transcript 7.The mature transcript mRNA molecule leaves the nucleus through pores in the nuclear membrane

tRNA folds due to base pairing to form a triplet anticodon site and an attachment site for a specific amino acid. Transfer RNA (tRNA) The function of tRNA is to bind to a particular amino acid and carry it to the ribosomes. The diagram represents a tRNA molecule RNA is a single strand but part of the tRNA molecule is folded due to hydrogen bonds between it’s bases. The molecule has three unpaired bases called an anticodon that is able to form hydrogen bonds with a complementary codon (consisting of three bases) on mRNA. Another part of the tRNA molecule has an attachment site for a specific amino acid.

Translation of mRNA into a polypeptide by tRNA at the ribosome. Triplet codons on mRNA and anticodons translate the genetic code into a sequence of amino acids. Start and stop codons exist. Codon recognition of incoming tRNA, peptide bond formation and exit of tRNA from the ribosome as polypeptide is formed. RIBOSOMES contain enzymes essential for protein synthesis, and have 3 tRNA binding sites; E, P and A. -Site P holds the tRNA carrying the growing polypeptide chain. -Site A holds the tRNA carrying the next amino acid to be joined to the growing chain by a peptide bond. -Site E discharges a tRNA from the ribosome once its amino acid has become part of the polypeptide chain. E site P site A site Translation starts when the START CODON on an mRNA molecule binds to the P site on a ribosome. Eventually, once peptide bonds have formed between amino acids (the order of which is decided by the base sequence on mRNA) a STOP CODON is reached and the ribosome releases the last tRNA and the now complete polypeptide chain. The process of polypeptide formation from translation of mRNA at a ribosome is shown below:

3 bases on mRNA is a CODON Translation summary tRNA molecules transport amino acids to the ribosome Hydrogen bonds form between anticodons and codons holding amino acids in the correct sequence While amino acids are held in the correct sequence, peptide bonds form between them to join them into a polypeptide chain This requires an enzyme and ATP

Polyribosome Several ribosomes can become attached to an mRNA molecule at the same time – a string of ribosomes on the same mRNA is called a polyribosome. Polyribosomes allow many copies of the polypeptide to be made from a single mRNA molecule. Production of different mRNA molecules from the same primary transcript The same primary transcript of mRNA can be used to produce different mature RNA molecules depending on which sections are treated as exons and introns and therefore cut out or spliced. So mRNA with different base sequences can be produced from the same gene (DNA base sequence). These different RNA molecules produce different proteins after translation. Therefore a single gene can give rise to several different proteins.

Sulphur bridges Polypeptide chain folds and sulphur bridges form Post translation changes to protein structure The polypeptide chain formed by translation may be modified by: Cutting the chain using enzymes and combining sections, e.g. insulin is formed from a polypeptide that has its central section cut out leaving the two remaining sections joined by sulphur bridges The protein can be modified by adding other molecules, e.g. addition of a carbohydrate makes a glycoprotein, e.g. mucus or phosphate may be added to activate a regulatory protein. Enzymes cut out middle section Insulin consisting of 2 polypeptide chains joined by sulphur bridges Polypeptide chain