Unit 6 (Transcription and Translation)

Proteins

Polypeptides made up of amino acids linked by peptide bonds

Gene expression

The process by which DNA directs the synthesis of proteins

• Transcription and translation

• Occurs in all organisms

• DNA → RNA (transcription)

• RNA → Protein (translation)

Transcription

The synthesis of DNA using information from DNA

• Allows for the “message” of the DNA to be transcribed

• Occurs in the nucleus

Translation

The synthesis of a polypeptide using information from RNA

• Occurs at the ribosome

• A nucleotide sequence becomes an amino acid sequence

Messanger RNA

• Synthesized during transcription using a DNA template

• Carries information from the DNA (at the nucleus) to the ribosomes in the cytoplasm

Transfer RNA

• Important in translation

• Each tRNA carry a specific amino acid

• Can attach to mRNA via their anticodon (complementary codon to mRNA)

• Allow information to be translated into a peptide sequence

Ribosomal RNA

• Helps form ribosomes

• Helps link amino acids together

The Genetic Code

• DNA contains the sequence of nucleotides that codes for proteins

• Read in groups of three (triplet coded)

• During transcription, only one DNA strand is being transcribed (template strand, noncoding strand, minus strand, or antisense strand)

• mRNA molecules formed are antiparallel and complementary to the DNA nucleotides (A=U and C=G)

• The mRNA nucleotide triplets are codons (code for amino acids)

• 64 different codon combinations

• 61 codes for amino acids (3 are stop codons)

• Reclunclancy: more than one codon code for each amino acid

Leading frame

The codons of the mRNA must be read in the correct grouping during translation to synthesize the correct proteins

• A shift in even one letter will produce a completely different outcome

Steps of Transcription

1. Initiation

2. Elongation

3. Termination

Transcription Step 1: Initiation

• Begins when RNA polymerase molecules attach to a promoter region of DNA

• Does not need a primer to attach

• Promoter regions are upstream (placed before) of the desired gene to be transcribed

• Eukaryotes: The promoter region is called the TATA box

  • Transcription factors help RNA polymerase bind

• Prokaryotes: RNA polymerase can bind directly to promoter

Transcription Step 2: Elongation

• RNA polymerase opens the DNA and reads the triplet code of the template strand

  • Moves in the 3’ to 5’ direction

  • The mRNA transcript elongates 5’ to 3’

• RNA polymerase moves downstream

  • Only opens small sections of DNA at a time

  • Pairs complementary RNA nucleotides

  • The growing mRNA strand peels away from the DNA template strand

  • The DNA double helix then reforms

• A single gene can be transcribed simultaneously by several RNA polymerase molecules

  • Helps increase the amount of mRNA synthesized and protein production

Transcription Step 3: Termination

Prokaryotes

• Proceeds through a termination sequence

  • Causes a termination signal

  • RNA polymerase detaches

  • mRNA transcript is released and proceeds to translation

  • mRNA does not need modifications

Eukaryotes

• RNA polymerase transcribes a sequence of DNA called the polyadenylation signal sequence

  • Codes for a polyadenylation signal (AAUAAA)

  • Released the pre-mRNA from the DNA

  • Must undergo modifications before translation

Pre-mRNA modifications

1. 5’ Cap (GTP): The 5’ end of the pre-mRNA receives a modified guanine nucleotide “cap.”

2. Poly-A tail: The 3’ end of the pre-mRNA receives 50-250 adenine nucleotides

   • Both help the mature mRNA leave the nucleus

   • Help protect the mRNA from degradation

   • Help ribosomes attach to the 5’ end of the mRNA when it reaches the cytoplasm

3. RNA splicing: sections of the pre-mRNA, called introns, are removed, and then exons are joined together

   • Introns: intervening sequence, do not code for amino acids

   • Exons: expressed sections, code for amino acids

   • Allows a single gene to code for more than one kind of polypeptide (alternative splicing)

• Once all modifications are done, pre-mRNA becomes mature RNA

Translation

The synthesis of a polypeptide using information form mRNA

• at the ribosome

• A nucleotide sequence becomes an amino acid sequence

Transfer RNA

• Has an anticodon region which is complementary and antiparallel to mRNA

• Carries the amino acid that the mRNA codon codes for

  • ACU codes for Thr

• The enzyme aminoacyl-tRNA synthetase is responsible for attaching amino acids to tRNA

  • At tRNA carrying on amino acid “charged"

Ribosomes

• Ribosomes have two subunits (small and large)

• Prokaryotic and eukaryotic ribosomal subunits differ in size

  • Prokaryotes: small subunits (30s) large subunit (40s)

  • Eukaryotes: small subunits (40s) large subunit (60s)

• A large subunit has three sites

  • A site: amino acid site → holds the next tRNA carrying an amino acid

  • P site: polypeptide site → holds the tRNA carrying the growing peptide chain

  • E site: exist site

Translation Step 1: Initiation

• Begins when the small ribosomal subunit binds to the mRNA, and a charged tRNA binds to the start codon, AUG, on thte mRNA

• The tRNA carries methionine

Translation Step 2: Elongation

• Starts when the next tRNA comes into the A state

  • mRNA is moved through the ribosome, and its codons are read

  • Each mRNA codon codes for a specific amino acid

  • Codon charts are used to determine the amino acid

1. Codon recognition: The appropriate anticodon of the next tRNA goes to the A-site

2. Peptide bond formation: Peptide bonds are formed that transfer the polypeptide to the A-site tRNA

3. Translocation: The tRNA in the A-site moves to the P-site, and the tRNA in the P-site goes to the E-site. The A site is open for the next tRNA

Translation Step 3: Termination

• Occurs when a stop codon in the mRNA reaches the A site of the ribosome

• Stop codons do not code for amino acids

• The stop codons for a release factor

  • Hydrolyzes the bond that holds the polypeptide to the P site

  • Polypeptide releases

  • All translational units disassemble