Transcriptionand 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 structureCompare

strandsDNA 2RNA 1

basesRNA

UACGDNA

TACG

sugars

RNA vs. DNA structureCompare

strands basessugars

Sugar: Deoxyribose Ribose

RNA vs. DNA structureThree types of RNA

Messenger (mRNA): codes for proteinRibosomal (rRNA): makes ribosomesTransfer (tRNA): carries amino acids

Protein synthesis

Protein synthesis: overviewTranscription – one DNA strand (the

template) is used for synthesis of a complementary messenger RNA (mRNA) molecule.

Step 1

Protein synthesis: overviewTranslation - information contained in the

order of nucleotides in mRNA is used to determine the amino acid sequence of a polypeptide.

Step 2

TranscriptionSimilar 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.

TranscriptionTranscription: DNA-directed synthesis of

RNA1) Initiation: RNA polymerase attaches at

a specific DNA sequence - the promotor.

Transcription1) 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.

Transcription2) Elongation: Genes are read 3'⇒5',

creating a 5'⇒3' mRNA. The mRNA is complementary to the template DNA strand.

Transcription

Close-up ofelongation.

Complementary: A:T, U:A C:G, G:C

Transcription3) Termination: A specific terminator

sequence of nucleotides signals the end of transcription.

The genetic codeIn 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 codeNucleotide 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 codeTo 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 - overviewTranslation - RNA-directed synthesis of a

polypeptideribosomes 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 - overviewEach 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 - overviewRibosomes 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 -overviewEach 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.

TranslationTranslation 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

Translation2) Elongation - the rRNA catalyzes a peptide

bond between the polypeptide in the P site with the new amino acid in the A site.

Translation3) During translocation, the ribosome moves

the tRNA with the attached polypeptide from the A site to the P site; this requires GTP energy.

Translocation4) 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.

TranslationAnimation

TranslationTypically 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.

TranslationFree ribosomes synthesize proteins for

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

One gene: one polypeptideProteins are the links between the

genotype and the phenotype (physical appearance):

One gene - one polypeptide hypothesis

~40,000 genesin the humangenome

One gene: one polypeptideMany 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 mutationsA rare change in the DNA of a gene, ultimately creating genetic diversity.

Gene mutationA 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 mutationsA base deletion: ALL amino acids after the deletion are changed – more serious.

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