Gene Expression at the Molecular Level I: Production of mRNA and Proteins

Beadle and Tatum’s One-Gene-One-Enzyme Hypothesis

Proposed that onen gene codes for one enzyme, States the following

1. All proteins come from genes but not all proteins function as enzymes

2. Some proteins are composed of more than one polypeptide

3. Some genes can code for more than one protein if the mRNA is spliced in multiple ways during RNA modification

4. Some genes encode for non-coding RNAs which do not generate a protein

DNA —> RNA —> Protein

Central dogma of molecular biology

Transcription

An RNA copy of a gene is made from DNA

mRNA (messenger RNA)

The coding RNA

tRNA (transfer RNA), rRNA (ribosomal RNA), other RNAs not used for translation

Non-coding RNAs (3)

Translation

A polypeptide is made from an RNA transcript of a gene

Genes

Organized units of nucleotide sequences that enables a segment of DNA to be transcribed into RNA and ultimately results in the formation of a functional product

Promoter

Region of DNA that designates where transcription will begin for each gene

Terminator

Region of the DNA that designates where transcription will end for each gene

Regulatory Sequence

Region ahead (upstream) of the promoter that regulates the rate of transcription when bound to regulatory proteins

Initiation

Stage of transcription that consists of the recognition of the promoter region by RNA polymerase and separation of DNA strands

Elongation

Stage of transcription where one strand of DNA is read and a polymer of RNA is synthesized as RNA polymerase moves down the template strand in the 3’ to 5’ direction, synthesizing RNA ins a 5’ to 3’ direction

Template Strand

The strand of DNA that is being read to form the RNA transcript

Coding Strand

The strand of DNA that holds the same sequence as the new mRNA transcript

Termination

Stage of transcription where RNA polymerase reaches the terminator, and the DNA-RNA-Protein complex dissociates

• Occurs in cytoplasm

• Sigma factor recognizes promoter region

• One RNA polymerase transcribes all genes

• No modification afterward

Traits of prokaryotic transcription (4)

• Occurs in nucleus

• Five transcription factors bound to RNA polymerase (preinitiation complex) recognize promoter region

• Three RNA polymerases transcribe different genes

• Modification occurs in between transcription and translation

Traits of eukaryotic transcription (4)

RNA Polymerase II

RNA polymerase that transcribes mRNA in eukaryotes

RNA Polymerase I and III

RNA polymerases that transcribe other non-coding RNAs such as tRNA and rRNA

pre-mRNA

A long RNA that contains extra nucleotide sequences that will not be coded into the final polypeptide, Made directly after transcription in eukaryotes

Mature mRNA

A modified mRNA sequence that is released from the nucleus into the cytosol to be translated into protein

5’ capping

3’ Poly-A tailing

Splicing

Three types of modifications

5’ Capping

Modification that adds a modified guanine (7-methylguanosine) to the 5’ end, Aids in nucleus ext, Prevents degradation in the cytosol, Helps ribosome recognitions

3’ Poly-A Tailing

Modification that adds 100-200 adenine nucleotides to the 3’ end, Aids in nucleus exit and prevents degradation in the cytosol

Splicing

Modification that removes the mRNA regions that do not code for the final polypeptide

Introns

mRNA regions that get spliced out and remain in the interior of the nucleus

Exons

The coding portion of the mRNA that exits the nucleus to be translated to protein

Spliceosome

Cuts out introns and connects exons during RNA splicing

snRNPs

Small nuclear RNAs and proteins that make up spliceosomes

3’ binding site

5’ binding site

branch site

Three binding sites during RNA splicing

Genetic code

Required for the translation from the language of nucleotides in the mRNA to the language of amino acids in protein

Codon

The amino acid coding unit in mRNA, Every three nucleotides on an mRNA translates to one amino acid in the polypeptide

Degenerate

What the genetic code is considered as when different codons can code for the same amino acid

Start Codon

The first codon that initiates translation

Stop Codon

The final codon that terminates translation

Insertions

Deletions

Substitutions

Frameshift mutations (3)

Insertions

Frameshift mutation that involves the addition of a nucleotide

Deletions

Frameshift mutation that involves the removal of a nucleotide

Substitution

Frameshift mutation that involves the replacement of one nucleotide for another (does not shift the frame)

Missense Mutation

Mutation that changes one amino acid to another

Nonsense Mutation

Mutation that changes to a stop codon

Silent Mutation

Mutation that does not cause change in an amino acid sequence

Anticodon

An amino acid

Two crucial factors that tRNA holds that allows for translation

Anticodon

A factor of tRNA that consists of the complementary bases to the codons on the mRNA

An Amino Acid

A factor of tRNA that determines what the codon codes for

mRNA - Holds the code

tRNA - Bridges the gap between nucleotides and amino acids

rRNA - The molecular machinery that puts everything together via ribosomal activity

What mRNA, tRNA, and rRNA do during translation

1. mRNA binds to the small ribosomal subunit

2. tRNA charged with methionine recognizes the start codon and initiates translation

3. The large ribosomal subunit meets with the small ribosomal subunit

What happens during initiation during translation (3)

1. The next correctly coded charges tRNA enters the ribosome at the A site

2. The amino acid that is attached to the tRNA in the P site will be transferred to the amino acid on the tRNA in the A site, creating a peptide bond and extending the polypeptide chain

3. The ribosome shifts down the mRNA by one codon and the tRNA at each site moves to the adjacent site

What happens during elongation during translation (3)

1. A stop codon enters the A site

2. The bond between the polypeptide and the tRNA is cleaved and the polypeptide and tRNA are released from the ribosome

3. The mRNA, ribosomal subunits, and release factor all dissociate

What happens during termination during translation (3)