RNA, transcription & translation Unit 1 – Human Cells

Re-Cap / Introduction  Genotype:  – a cell’s genetic constitution  - determined by the DNA base sequence  Phenotype:  - a cell’s physical/chemical state  - determined by which genes are expressed  An individuals characteristics are the result of many biochemical reactions  - these are controlled by enzymes  - enzymes are made of protein  - built up of amino acids into a polypeptide chain  - a protein’s structure and shape depends on the order of amino acids  - this is determined by the order of bases in the organisms DNA  - this is the genetic code

Structure of RNA  Ribonucleic Acid  Composed of ribose sugar, a base and a phosphate  The base Uracil replaces Thymine  Is single stranded  Proteins are synthesised at structures called ribosomes  - these are found outwith the nucleus  RNA allows for information from the DNA to be carried to the ribosomes

Transcription  The synthesis of messenger RNA (mRNA) from a section of DNA  mRNA then leaves the nucleus and is taken to a ribosome  Process starts at a promoter  - a region of DNA where transcription is initiated  RNA polymerase moves along the gene, unwinding the DNA strand  - RNA nucleotides pair up with complementary DNA nucleotides  - it then binds the RNA nucleotides to form a strand of mRNA  - nucleotides are added to the 3’ end of the mRNA  - this continues until a region called a terminator is reached on the DNA  - the mRNA strand then separates from the DNA

Modification of primary mRNA transcript  On average, mRNA is initially around 8000 nucleotides in length  - only about 1200 is needed to code for a polypeptide chain  Long sections are not involved in coding for the polypeptide  - these non-coding sections are INTRONS  Before the mRNA leaves the nucleus, the introns must be cut out  - this ensures only coding sequences are left  - these are called EXONS  These are joined together by the process of SPLICING

Translation  Process where mRNA directs the synthesis of a polypeptide chain (protein)  mRNA carries the genetic message as triplets of bases  - these are CODONS  mRNA attaches itself to a ribosome  Once attached, each CODON of the mRNA is exposed  These are paired up with triplets on a molecule called transfer RNA (tRNA)  - found in the cytoplasm  Each molecule of tRNA carries a specific amino acid  - each tRNA triplet is called an ANTICODON  Ribosomes bring tRNA into contact with mRNA

Translation  Ribosomes have three binding sites  Site P holds the tRNA carrying the growing polypeptide chain  Site A holds the tRNA carrying the next amino acid to be added  Site E (exit) discharges the tRNA after it’s amino acid has become part of the chain  mRNA initially binds to site A at a START codon  - at the 5’ end  tRNA with a corresponding anticodon will bring in an amino acid  - from then on amino acids will be joined by peptide bonds  - to form a polypeptide chain  - process continues until a STOP codon on the mRNA is reached  - site A is occupied by a release factor  - polypeptide is now freed from the ribosome  Often, a strand of mRNA is translated at several ribosomes simultaneously  - known as a polyribosome

Translation

One gene, many proteins  Alternative RNA splicing:  - different segments of mRNA can be treated as exons and introns  - so, several mRNA transcripts can come from primary transcripts  - different triplets = different polypeptide  E.g. free or membrane bound antibodies  Post-Translational modification:  CLEAVAGE:  - polypeptide chain can be cut to be activated  E.g. insulin cut by protease enzymes  MOLECULAR ADDITION:  - proteins with molecules added for activation .e.g  - mucus with carbohydrate  - regulatory proteins (p53) with phosphate