3. nucleic acids, chromatin, gene expression, and mutations 3

two major steps of gene expression

• transcription: DNA first makes mRNA

• translation: mRNA makes proteins in the cytoplasm

what does translation require

rRNA and tRNA

what are protein-coding DNA

DNA sequence that can make protein

• 2% of human genome

3 steps of transcription

• initiation: RNA polymerase (pol) is positioned on the DNA of the gene that needs to be transcribe

• elongation: RNA pol transcribes the DNA sequence of the gene into an RNA molecule

• termination: RNA pol is released from the DNA

step 1 transcription initiation

• each gene contains a promoter region

• the general transcription factors bind to the promoter

• RNA pol binds to the general transcription factors to form a transcription initiation complex

promoter contains _____

consensus sequences

• CAAT box (GGNCAATCT)

• GC box (GC-rich region)

• TATA box (TATAA)

step 2 transcription elongation

• RNA pol “walks” along one strand of DNA, called template/non-coding strand

• the strand of DNA not used as a template for transcription is called the non-template/coding strand

• coding strand and newly synthesized mRNA have the same nucleotide sequence

what direction does the RNA strand grow

5’ → 3’

• transcription initiation complex moves along the non-coding strand in a 3’ to 5’ direction and makes a new strand of mRNA with complementary base pairs in 5’ to 3’ direction

transcription termination

• termination in eukaryotes is not well understood

• RNA pol transcribe a consensus AAUAAA sequence, called poly (A) signal

• after synthesizing poly (A) signal, the mRNA is cleaved and then about 200 adenine residues are added to form a poly-A tail

in eukaryotes first a _____ is formed

precursor-mRNA

• when an RNA transcript is first made in a eukaryotic cell, it is considered a pre-mRNA

• pre-mRNA must be processed into a messenger RNA before leaving the nucleus

processing of eukaryotic pre-mRNA: cap and poly-A tail

• a cap is added to the 5’ end

• a poly-A tail is added to the 3’ end

what is RNA splicing

to splice out introns (noncoding segments) and combine exons (coding segments)

processing of eukaryotic pre-mRNA: RNA splicing

done by the spliceosome that contains small nuclear RNAs and proteins

what is a codon

3 consecutive bases in mRNA that specify a particular amino acid

mRNA and codons

• mRNA codons are read from 5’ to 3’

• codons specify the order of amino acids in a protein from N-terminus to C-terminus

in human, multiple codons can _____

code for the same amino acid

human genetic code

• 20 amino acids commonly found in proteins

• 64 codons

• each codon specify an amino acid

• three “stop” codons mark the end of a protein

• one “start” codon, AUG, marks the beginning of a protein

RNA polyermase is a _____

therapeuticc target

• promising target for the discovery of new antimicrobial agents

• CX-5461: selective RNA pol 1 inhibitor used to treat hematological cancers

mRNA can make _____ by _____

DNA by reverse transcription

• some RNA virus (retrovirus) makes a complementary DNA (cDNA), which then integrates into the host’s genome

• reverse transcriptase makes DNA, called complementary DNA (cDNA) from RNA

_____ is a therapeutic target for treating AIDS

reverse transcriptase

• reverse transcriptase inhibitors (RTIs) are a class of antiretroviral drugs used to treat HIV infection or AIDS, and in some cases hepatitis B

_____ are the sites in a cell in which translation takes place

ribosomes

ribosome make up

• a large and a small subunit

• during translation, the two subunits come together around a mRNA molecule, forming a complete ribosome

• 1/3 protein and 2/3 rRNA

what are the 3 sites for tRNA binding on ribosome

• P site: peptidyl or donor site

• A site: aminoacyl or acceptor site

• E site: exit site

3 steps of translation

initiation, elongation, termination

translation initiation

• the initiator tRNA carrying methionine attaches to the small ribosomal subunit

• the complex “walk” along the mRNA in the 3’ direction to find the start codon (AUG)

• the large ribosomal subunit joins to form initiation complex at the site of AUG on mRNA

translation elongation

• methionin-carrying tRNA occupies the P site

• next aminoacyl tRNA with the complementary anticodon sequence then binds to the A site

• peptide bond connects one amino acid to another

• once the peptide bond is formed, the mRNA is pulled onward through the ribosome by exactly one codon

translation termination

• happens when a stop codon in the mRNA (UAA, UAG, or UGA) enters the A site

• stop codon are recognized by proteins called release factors, which fit neatly into the A site of ribosome

• upon termination, the ribosome is disassembled and the completed polypeptide is released

antibiotics kill bacteria by _____

inhibiting key translational molecules

• streptomycin, tetracycline, and chloramphenicol “stall” bacterial protein synthesis

_____ may be affected due to change in DNA sequence

• a different protein may form

• protein may not form at all

• less amount of protein may form

• protein production may increase

• may not affect protein production