Transcription and Translation

Objectives 3.5.1 - Compare the structure of RNA and DNA. 3.5.2 – Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase. 3.5.3 – Describe the genetic code in terms of codons composed of triplets of bases. 3.5.4 – Explain the process of translation leading to polypeptide formation. 3.5.5 – Discuss the relationship between one gene and one protein.

RNA vs. DNA structure Compare strands bases sugars DNA 2 RNA 1 RNA DNA UACG DNA TACG sugars

RNA vs. DNA structure Compare strands bases sugars Sugar: Deoxyribose Ribose

RNA vs. DNA structure Three types of RNA Messenger (mRNA): codes for protein Ribosomal (rRNA): makes ribosomes Transfer (tRNA): carries amino acids Protein synthesis

Protein synthesis: overview Transcription – one DNA strand (the template) is used for synthesis of a complementary messenger RNA (mRNA) molecule. Step 1

Protein synthesis: overview Translation - information contained in the order of nucleotides in mRNA is used to determine the amino acid sequence of a polypeptide. Step 2

Transcription Similar in prokaryotes & eukaryotes. Bacteria: Cytoplasmic process Eukaryotes Transcription occurs in the nucleus, and translation occurs at ribosomes in the cytoplasm. mRNA is modified before it is ex- ported to the cytoplasm.

Transcription Transcription: DNA-directed synthesis of RNA 1) Initiation: RNA polymerase attaches at a specific DNA sequence - the promotor.

Transcription 1) Initiation: Then RNA polymerase separates the DNA strands and bonds RNA nucleotides as they base-pair along the DNA template. C to G, G to C, A to T, but U to A. RNA polymerases can add nucleotides only to the 3’ end of the growing polymer.

Transcription 2) Elongation: Genes are read 3'⇒5', creating a 5'⇒3' mRNA. The mRNA is complementary to the template DNA strand.

Transcription Close-up of elongation. Complementary: A:T, U:A C:G, G:C

Transcription 3) Termination: A specific terminator sequence of nucleotides signals the end of transcription.

The genetic code In the genetic code, nucleotide triplets specify amino acids. Three consecutive base combinations specify 64 possible code words (4 x 4 x 4 = 64). But there are only 20 amino acids, so many combinations are repeats.

The genetic code Nucleotide triplets specify amino acids. Blocks of three nucleotides are codons. It would take at least 300 nucleotides to code for a polypeptide that is 100 amino acids long.

The genetic code To crack the code, scientists made artificial mRNA, such as “poly(U)” then added a mix- ture of amino acids, ribosomes, & other components for protein synthesis ⇒ phenyalanine. Try it: What is GAC? What is CGG? What is the code for Cystine?

Translation - overview Translation - RNA-directed synthesis of a polypeptide ribosomes interpret a series of codons along mRNA. transfer RNAs (tRNA) move amino acids from a cytoplasm pool to the ribosome. Note the codon & anticodon match. ribosomes add each amino acid to a growing polypeptide.

Translation - overview Each tRNA has a specific amino acid at one end and a specific nucleotide triplet (anticodon) at the other. The anticodon base-pairs with a complement- ary codon on mRNA. If the codon is UUU, a tRNA with an AAA anticodon and carrying phenyl- alanine will bind to it.

Translation - overview Ribosomes aide the specific coupling of tRNA anticodons with mRNA codons. Each ribosome has a large & small subunit. Ribosomes are composed of proteins and ribosomal RNA (rRNA). rRNA genes are transcribed in the nucleus, then proteins form the sub- units in the nucleolus.

Translation -overview Each ribosome has a binding site for mRNA and three binding sites for tRNA molecules. The P site holds the tRNA carrying the growing polypeptide chain. The A site carries the tRNA with the next amino acid. Discharged tRNAs leave the ribosome at the E site.

Translation Translation can be divided into four stages: 1) Initiation – a small ribosomal subunit, the mRNA, and an initiator tRNA are brought together by protein initiation factors, then the large ribosomal unit attaches ⇒ initiation complex. anticodon codon

Translation 2) Elongation - the rRNA catalyzes a peptide bond between the polypeptide in the P site with the new amino acid in the A site.

Translation 3) During translocation, the ribosome moves the tRNA with the attached polypeptide from the A site to the P site; this requires GTP energy.

Translocation 4) Termination occurs when a stop codon reaches the A site. A release factor binds to the stop codon and hydrolyzes the bond between the polypeptide and its tRNA in the P site. The parts fall away to be re-used later.

Translation Animation

Translation Typically a single mRNA is used to make many copies of a polypeptide simultaneously. Multiple ribosomes, polyribosomes, may trail along the same mRNA. During & after synthesis, a polypeptide coils and folds to its 3D shape spontaneously.

Translation Free ribosomes synthesize proteins for use within the cell; ER-bound ribosomes synthe-size proteins for secretion.

One gene: one polypeptide Proteins are the links between the genotype and the phenotype (physical appearance): One gene - one polypeptide hypothesis ~40,000 genes in the human genome

One gene: one polypeptide Many exceptions have been found! Proteins from some genes are spliced to make new proteins. Proteins can be altered after translation to produce more varieties. This theory may need to be modified or abandoned. Paradigm shift – when a theory is abandoned and replaced by something totally new. Ex: origin of life, extinction of dinosaurs.

Gene mutations A rare change in the DNA of a gene, ultimately creating genetic diversity.

Gene mutation A base substitution: one nucleotide base is changed (perhaps by UV light). Ex: sickle cell anemia - The DNA that codes for the hemoglobin molecule that carries oxygen in the blood is mutated. A change in one nucleotide changes one amino acid only.

Gene mutations A base deletion: ALL amino acids after the deletion are changed – more serious.

Chromosomal mutations A major disruption – many genes are affected. This occurs during meiosis. May lead to big evolutionary changes.