Transcription and Translation Topic 3.5 Transcription and Translation

Central Dogma of Biology How does the information flow in biological systems? DNA molecule Gene 1 Gene 2 Gene 3 DNA strand TRANSCRIPTION RNA Polypeptide TRANSLATION Codon Amino acid The information encoded in the DNA is transferred to RNA and then decoded to produce proteins. Special cases: Retroviruses (genetic material is RNA), Ex.: HIV or Flu virus

Transcription and Translation Two Main Process: Transcription and Translation DNA copy itself: Replication DNA synthesize RNA Transcription RNA synthesize protein Translation DNA replication: to transmit the genetic information between parents and progeny, the DNA must be replicated faithfully. Replication is carried out by a complex group of proteins that unwind the superhelix, unwind the double-stranded DNA helix, and, using DNA polymerase and its associated proteins, copy or replicate the master template itself. DNA Transcription: Transcription is the process by which the information contained in a section of DNA is transferred to a newly assembled piece of messenger RNA (mRNA). It is facilitated by RNA polymerase and transcription factors. DNA Translation: Eventually, this mature mRNA finds its way to a ribosome, where it is translated. In prokaryotic cells, which have no nuclear compartment, the process of transcription and translation may be linked together. In eukaryotic cells, the site of transcription (the cell nucleus) is usually separated from the site of translation (the cytoplasm), so the mRNA must be transported out of the nucleus into the cytoplasm, where it can be bound by ribosomes.

3.5.1 Compare the structure of RNA and DNA (deoxyribonucleic acid) RNA (Ribonucleic acid) Two strands forming a double helix One strand only Deoxyribose Ribose A, C, G and T A, C, G and U H

3.5.2 DNA Transcription DNA helix unzips One of the 2 DNA strands acts as a template RNA nucleotides line up along one strand of the DNA following the base-pairing rules. No T, Instead U

3.5.2 DNA Transcription RNA polymerase forms covalent bonds between nucleotides. Transcription continues until one entire gene has converted to RNA. The single-stranded messenger RNA separate and the DNA strands rejoin.

3.5.3The Genetic Code How do nucleotides specify 20 amino acids? 4 different nucleotides (A, G, C, T) Possible codes: 1 letter code  4 AAs <20 2 letter code  4 x 4 = 16 AAs <20 3 letter code  4 x 4 x 4 = 64 AAs >>20 Three letter code with 64 possibilities for 20 amino acids suggests that the genetic code is degenerate (i.e., more than one codon specifies the same amino acid).

3.5.3The Genetic Code Triplete Code Degenerated Universal

Nucleic Acids: RNA types There are 3 different types of RNA: Messenger RNA: Transcription Single strand Transport nucleus – Cytoplasm Ribosomal RNA Structural & Functional component of Ribosomes Transfer RNA: Translation Bring aa to Ribosomes for protein assembly Types of RNA • mRNA – messenger RNA – Carries info from DNA in nucleus out into the cytoplasm and into the ribosome • rRNA – ribosomal RNA –Produce enzymes to bond amino acids during protein synthesis • tRNA – transfer RNA – Transfers the amino acids to the ribosomes for protein assembly

3.5.4 DNA Translation: DNA T A C G A U G G C U U A C C G G mRNA The base sequence of mRNA is used as a guide for assembling the sequence of amino acids that will be a polypeptide. 2. This process of protein production using a mRNA as a guide is called TRANSLATION. DNA T A C G A U G G C U U A C C G G mRNA Polypeptide: Metionine Alanine Tyrosine Arginine

3.5.4 DNA Translation: Translation take place on cell structures called Ribosomes, using mRNA & tRNA. Ribosomes are in the cytoplasm, outside the cell nucleus. Transfer RNA: 1. Amino acids are carried to the ribosomes by smaller RNA molecules called transfer RNA (tRNA). 2. Single strands folded back on themselves: “Clover Leaf structures” 3. Role = Carry aa to the ribosomes -aminoacid attachment site -anticodon –codon attachment site

3.5.4 DNA Translation: Process mRNA binds the small subunit of the ribosome tRNA molecules are present, each one carrying the specific aa & anticodon tRNA binds to the ribosome at the site where the anticodon matches the codon on the mRNA Two tRNAs binds at once and the first one in transfers the growing polypeptide chain to the second one in. The ribosome moves along the mRNA and the process continues until a stop codon is reached when the polypeptide is realeased.

3.5.4 DNA Translation: Process

3.5.4 DNA Translation: Overview Usually a number of ribosomes can be found on a single mRNA, each reading off the coded information at the same time. Such structures are known as Polyribosomes or Polysomes

3.5.5 One Gene – One Polypeptide Theory One gene is transcribed and translated to produce one polypeptide. Some protein are composed of a number of polypeptides and in this theory each polypeptide has its own gene. e.g. haemoglobin is composed of 4 polypeptides (2 of each type) and there is a gene for each type of polypeptide. This theory, like so many in biology has exceptions. 1) Some genes code for types of RNA which do not produce polypeptides. 2) Some genes control the expression of other genes.