gen bio - lecture 15: gene expression

central dogma of molecular bio (crick!)

unidirectional flow of genetic info!
DNA → transcription → RNA → translation → protein

transcription (def)

information in DNA is used to synthesize mRNA (messenger RNA)

translation (def)

ribosome uses mRNA to synthesize a polypeptide

RNA composition - each nucleotide contains…

1. phosphate

2. sugar (ribose)

3. nitrogenous base (A, C, G, U)

single stranded!

why is RNA less stable than DNA??

RNA has sugar ribose! (vs DNA which has deoxyribose)

• ribose has an extra oxygen!! = less stable bc oxygen is highly electronegative and highly reactive 

uracil

• replaces thymine

  • complementary to adenine

• pyrimidine base (one ring)

• forms 2 hydrogen bonds

mRNA (purpose)

messenger RNA

• encodes amino acid sequence; creates “language”

rRNA

ribosomal RNA

• structural part of ribosome

tRNA

transfer RNA

• brings amino acids to ribosome during translation

codons

• each triplet of mRNA sequence = codon =  codes for one amino acid

• 64 different combos possible

• must be grouped and read correctly!!w

codons are…

• unambiguous: each codon codes for just one amino acid

• redundant: most amino acids can be coded by more than one codon

wobble hypothesis

3rd bases on the codons can vary WITHOUT changing the amino acid

• mutation tolerance!!

transcription OVERVIEW (4 parts)

DNA to RNA

1. transcription initiation

2. transcription elongation

3. transcription termination

4. mRNA modification (technically not part of transcription — more of an in between step between transcription and translation)

transcription initiation

DNA is read in 3’ to 5’ so RNA is synthesized in 5’ to 3’ !!

• single DNA strand = template strand

1. promoter: starter site for transcription

• specific DNA sequence on transcribed strand designates start site (the specific DNA sequence is NOT TRANSCRIBED)

2. RNA polymerase: 

• does not need primer!

• binds to promoter, unwinds helix + begins transcription

• synthesizes RNA complementary to DNA in 5’ to 3’ direction!

transcription elongation

• RNA synthesis = anti parallel to template strand

• adds RNA nucleoside triphosphate (has 2 extra phosphates compared to a nucleotide)

  • energy comes from hydrolysis of the extra 2 phosphates

example:

nontranscribed strand: 5’ ATGACT 3’

transcribed strand: 3’ TACTGA 5’

RNA: 5’ AUGACU 3’

transcription termination

• specific DNA sequence (NOT A STOP CODON) causes RNA polymerase to release from DNA

  • RNA transcript dissociates from DNA template

mRNA modification

ONLY IN EUKARYOTES!!

• transcription creates pre-mRNA (not useful for protein synthesis!)

1. add 5’ cap and 3’ poly-A tail (helps stabilize molecule, export from nucleus, help w protection — no 5’ no translation)

2. in nucleus w/help of spliceosome, RNA splicing occurs

• remove introns (non coding regions)

• connect externs (coding regions)

translation OVER (3 parts)

mRNA → polypeptide

occurs in ribosomes!! (cytoplasm or rough ER)

1. translation initiation

2. translation elongation

3. translation termination

components of translation (2)

1. tRNA

• transcribed from DNA

• 3D structure (base pairing within molecule; unpaired nucleotides create loops)

• anticodon

  • complementary and anti-parallel to mRNA codon

  • 3 specific bases on loop of tRNA = 1 specific amino acid (one amino acid per tRNA)

**aminoacyl tRNA: tRNA molecule linked to an amino acid

• high energy

• delivers amino acid to ribosomes

2. ribosomes: structure that carries out translation

• comprised of rRNA and proteins

• small + large subunits

• 3 binding sites: A (tRNA binds), P, E (exit site)

translation initiation

• small ribosomal unit binds mRNA + first tRNA (complementary to start codon)

• large subunit joins and completes the initiation complex (tRNA @ P site)

translation elongation

• series of repeated cycles

  • each tRNA adds a single amino acid to growing polypeptide chain

  • ribozyme catalyzes peptide bond formation

translocation: tRNA reaches E site and the ribosome subunits separate

translation termination

• stop codon recognized by release factors (proteins)

• release factors promotes hydrolysis + ribosomal subunits and other components dissociate