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Proteins

  • Proteins are polypeptides made up of amino acids
    • Amino acids are linked by peptide bonds
  • Gene expression: the process by which DNA directs the synthesis of proteins
    • Includes two stages: transcription and translation
      • Occur in all organisms

DNA

RNA

Protein

Transcription

Translation

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Transcription and Translation

  • Transcription: the synthesis of RNA 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

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Types of RNA

As we go through transcription and translation there will be three key RNA molecules

  1. Messenger RNA (mRNA)
  2. Ribosomal RNA (rRNA)
  3. Transfer RNA (tRNA)

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Messenger RNA

  • Messenger RNA is synthesized during transcription using a DNA template
    • mRNA carries information from the DNA (at the nucleus) to the ribosomes in the cytoplasm

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Transfer RNA

  • Transfer RNA molecules are important in the process of translation
    • Each tRNA can carry a specific amino acid
    • Can attach to mRNA via their anticodon
      • A complementary codon to mRNA
    • Allow information to be translated into a peptide sequence

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Ribosomal RNA

  • rRNA helps form ribosomes
  • Helps link amino acids together

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The Genetic Code

  • DNA contains the sequence of nucleotides that codes for proteins
    • The sequence is read in groups of three called the triplet code
  • During transcription, only one DNA strand is being transcribed
    • Known as the template strand (also known as the noncoding strand, minus strand, or antisense strand)

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The Genetic Code

  • mRNA molecules formed are antiparallel and complementary to the DNA nucleotides
    • Base pairing: A=U and C=G
    • The mRNA nucleotide triplets are called codons
      • Codons code for amino acids

DNA Template Strand

mRNA

3’

3’

5’

5’

codon

Tyr

Ala

Leu

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The Genetic Code

  • 64 different codon combinations
    • 61 code for amino acids
    • 3 are stop codons
    • Universal to all life
  • Redundancy: more than one codon code for each amino acid

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The Genetic Code

Example: the fat cat ate the rat

  • If the reading frame shifts even by one letter, it will produce a completely different outcome
    • Hef atc ata tet her at

Reading frame: the codons on the mRNA must be read in the correct groupings during translation to synthesize the correct proteins

​

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Quick Check

  1. If the DNA template strand reads: 3’-ACGAGA- 5’ what will the mRNA transcript read?
    1. Answer: 5’- UGCUCU-3’
  2. Using the mRNA transcript from #1, what amino acids will be produced during translation? (refer to a codon chart)
    • Answer: cysteine (cys) and serine (ser)

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Steps of Transcription

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Transcription

There are three steps in transcription

  1. Initiation
  2. Elongation
  3. Termination

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Step 1: Initiation

  • Transcription begins when RNA polymerase molecules attach to a promoter region of DNA
    • Do not need a primer to attach
    • Promoter regions are upstream of the desired gene to transcribe

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Step 1: Initiation

  • Eukaryotes:
    • Promoter region is called TATA box
    • Transcription factors help RNA polymerase bind
  • Prokaryotes:
    • RNA polymerase can bind directly to promoter

Notice that the promoter is always upstream of the gene of interest to be transcribed

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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’

Moves 3’ to 5’

Elongates 5’ to 3’

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Step 2: Elongation

  • 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
        • DNA double helix then reforms
  • A single gene can be transcribed simultaneously by several RNA polymerase molecules
    • Helps increase the amount of mRNA synthesized
      • Increases protein production

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Step 3: Termination

Prokaryotes

  • Transcription 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

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Step 3: Termination

Eukaryotes

  • RNA polymerase transcribes a sequence of DNA called the polyadenylation signal sequence
    • Codes for a polyadenylation signal (AAUAAA)
      • Releases the pre-mRNA from the DNA
        • Must undergo modifications before translation

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Pre-mRNA modifications, aka RNA Processing

​

There are three modifications that must occur to eukaryotic pre-mRNA before it is ready for translation

  1. 5’ cap
  2. Poly-A tail
  3. RNA splicing

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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 the 5’ cap and the poly-A tail function to:
    • 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

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Pre-mRNA modifications

  1. RNA Splicing: sections of the pre-mRNA, called introns, are removed and then exons are joined together
    1. Introns: intervening sequence, do not code for amino acids
    2. Exons: expressed sections, code for amino acids

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Pre-mRNA modifications

How is mRNA splicing accomplished?

​

​

Small nuclear�ribonucleoproteins

form a spliceosome�that cuts the

pre-mRNA

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Pre-mRNA modifications

Why does splicing occur?

  • A single gene can code for more than one kind of polypeptide
    • Known as alternative splicing

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Pre-mRNA modifications

Once all modifications have occurred, the pre-mRNA is now considered mature mRNA and can leave the nucleus and proceed to the cytoplasm for translation at the ribosomes

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Quick Check

  1. Discuss the overall “big picture” of transcription.
    1. Answer: transcription writes down the information from the DNA and allows it to leave the nucleus to be translated into a protein
  2. How does initiation differ between eukaryotes and prokaryotes?
    • Answer: eukaryotes require transcription factors to help RNA polymerase bind to the DNA, whereas prokaryotic RNA polymerase can bind directly to the DNA

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