Unit 9: Protein Synthesis

Central Dogma

Central Dogma

DNA→RNA→Protein

Base pairing rules of DNA

A→T

C→G

Base pairing rules of RNA

A→U

C→G

Structure and Function of mRNA

Structure:

• single stranded

• linear/one straight line

Function:

• copy DNA code during transcription

• carry DNA code to the ribosome

Structure and Function of tRNA

Structure:

• single stranded

• clover leaf shape

Function:

• carry the correct amino acids to the ribosome during translation

Structure and Function of rRNA

Structure:

• single stranded

• globular

Function:

• combine with proteins to make up ribosomes

Why do we need Central Dogma?

chromosomes have info to determine what proteins are made

• mostly enzymes

Why do we need transcription?

the DNA can’t leave the nucleus and ribosomes are in the cytoplasm so DNA has to turn into mRNA to leave the nucleus

What is the product of transcription?

a single stranded mRNA

Where does transcription happen?

in the nucleus

Steps of Transcription

1. Initiation

2. Elongation

3. Termination

transcription factors

a protein that controls the rate of transcription

TATA box

• adenine and thymine

• promoter region

• site where transcription starts/the start of a gene

Transcription Initiation

1. helicase, single stranded binding proteins, and topoisomerase do their thing and keep the DNA strand open (essentially makes a template)

2. transcription factors find the promoter region (TATA box)

3. transcription factors tell RNA polymerase where to attach

Transcription Elongation

1. RNA polymerase adds the base pairs complementary to the template strand of DNA

   • RNA polymerase works 3’→5’

   • mRNA = a leading strand

Transcription Termination

1. termination sequence of nucleotides signal the end of transcription

• yellow light for transcription

2. 10-35 base pairs later RNA polymerase stops

termination sequence

AAUAAA

Where does translation happen?

generally in cytoplasm but specifically in ribosomes

What is the product of translation?

proteins !!

codon?

• set of three nitrogenous bases

• ex. AUU , CGC, AGU

• every codon codes for an amino acid

  • 1 codon = 1 amino acid

tRNA

• brings correct amino acids to the ribosomes

  • has complementary anticodons that attach to the

anticodon

• a trinucleotide

• at the end of a tRNA

• corresponds with a codon in the mRNA

peptide bonds

• bonds that attach amino acids to the polypeptide chain

ribosomes

• small subunit = ground

• large subunit = wall

  • func = making proteins

5’ GTP cap

• guanine triphosphate (guanine nucleotide w/ 2 additional phosphates attached

  • where the mRNA molecule attaches to the ribosome

poly-A-Tail

• 140-200 adenine nucleotides on the 3’ end

  • provides stability

introns

non-coding sequences

stay in the nucleus

extrons

coding sequences

exit the nucleus

heterogenous nuclear mRNA

uncleaved mRNA

what cleaves introns

• small nuclear ribonucleoproteins (snRNPs) = “biological scissors”

• found in large spliceosomes

  • do the cutting and putting back together of actual mRNA

Translation Initiation

1. 5’ GTP cap attaches to the small ribosomal subunit

2. tRNA w/ the start anticodon (UAC) attaches to the mRNA start codon (AUG) at the P site

3. the large ribosomal subunit attaches to the complex which makes a complete ribosome

Translation Elongation

1. the next tRNA arrives at the A site

2. Methionine is removed from the tRNA at the P site

3. Methionine attaches to the new amino acid in the A site

4. everything shifts over - the mRNA moves over which switches everything over

5. the tRNA exits from the E site

6. the next tRNA goes in the A site and the cycle repeats

APE sites

A = attachment site

• where the next tRNA attaches to the mRNA

P site:

• holds the tRNA with the amino acid chain/ growing polypeptide

E = exit site

• where the empty tRNA can exit and attach to a new but same type of amino acid

methionine

• essential in humans

• we don’t naturally make it

  • have to consume through meat, fish, dairy, etc.

Translation Termination

1. one of the stop codons (UAA, UGA, UAG) comes up in the mRNA sequence

2. a release factor comes instead of an amino acid

3. the 2 subunits of the ribosome, the last tRNA, and the completed polypeptide are released from the mRNA

4. the above components can be recycled and complete translation somewhere else in the cell

   1. the protein can now fold into it’s secondary, terriatry, and quaternary structure

stop codons

• UAA

• UGA

• UAG

point mutations

change one base pair

substitution mutations

• silent

• missense

• nonsense

silent mutation

• no effect on the amino acid being created

• inserted at the “wobble” position

  • there is >1 codon for each amino acid

    • when you change the third base pair it doesn’t always change the amino acid

      • ex. CCG and CCA both code for glycine

missense mutation

• changes the amino acid being created

  • can change the shape/function of entire polypeptide

nonsense mutation

• creates a stop codon

• ultimately creates a nonfunctional polypeptide

  • stop making protein

• amino acids get recycled

frameshift mutations

• changes reading frame of code

• Insertion

• Deletion

Insertion mutation

A nucleotide is added to a gene

Changes all of the amino acids”downstream” of the mutation

Shifts the reading frame to the right→

Results in a non functional protein

Deletion mutation

A nucleotide is removed from the gene

All of the amino acids downstream of the change are affected

Shifts the reading frame to the left <—