U8 Biology honors

MB Questions

Activators bind to DNA enhancer regions.

• Other choices: RNA polymerase, promoters, introns, exons

• Activators are a type of transcription factor that bind to enhancer regions.

The operon model of the regulation of gene expression in bacteria was proposed by Jacob and Monod.

• Other choices: Watson and Crick, Franklin, Darwin, Mendel

• In 1961, Jacob and Monod proposed the operon model of gene expression.

Regulatory proteins bind to the operator.

• Other choices: The lactose-utilization genes, the regulatory gene, RNA polymerase, transcription factors

• Transcription is inhibited when a regulatory protein binds to the lac operon operator.

Translation & Transcription Notes

Types of RNA:

1. Messenger RNA (mRNA) → Carries DNA message from the nucleus to the ribosome.

2. Transfer RNA (tRNA) → Transfers amino acids to the ribosomes.

3. Ribosomal RNA (rRNA) → Along with proteins, makes up ribosomes.

• Also 6 others, not necessary for this unit.

Transcription:

• RNA is synthesized complementary to DNA

• RNA has uracil base instead of thymine

• RNA is made by RNA polymerase in the 5’ to 3’ direction

• RNA is complementary to only one DNA strand → A gene is encoded on only one strand of DNA

Transcription Initiation:

• RNA polymerase binds DNA at promoter

• RNA polymerase unwinds DNA and begins making a complementary strand on one strand of the DNA

• The promoter signifies which strand is the template (coding) strand

• Transcription factors can make a promoter more or less active, regulating gene expression

RNA Modification:

• Messenger RNA (mRNA) is used to make proteins

• In eukaryotes, mRNA is modified in 3 ways:

1. 5’ cap modified G to tell where to start translation

2. Poly A tail to 3’ end: 150-200 A to inhibit degradation

• These structures aid in RNA stability and translation 

3. Splicing 

• Cutting out the introns and putting together the exons 

Splicing:

• Introns = Non-coding DNA

• Exons = Coding DNA

• Splicing = Putting together the exons

• Occurs in the nucleus before exiting to the cytoplasm

If genes make proteins but there are only 19,000 different genes in the human genome, how do we make the almost 100,000 necessary proteins in humans? → Alternative Splicing

• Putting together the exons in different orders

The Genetic Code:

• Each three base pairs of mRNA encodes an amino acid, a codon

• This code is universal (virtually all organisms use the same code)

• The code is redundant (The same amino acid may be encoded by multiple combinations of letters 

Translation:

• Once an mRNA is made, it is exported from the nucleus to the cytoplasm

• Then, the making of a protein based on the information in the mRNA can begin

Ribosomes:

• Large and small subunits, made of rRNA and protein

• Small subunits bind to mRNA first

• 3 sites within the subunit:

1. A-Site → Acceptor side where tRNA enters

2. P-Site → Peptide bond forms between amino acids

3. E-Site → Exit of tRNA (gets released)

Transfer RNA:

• tRNA contains an anticodon, a sequence of RNA bases that complements the codon sequence in the mRNA

• Enzyme recognizes the anticodon of the tRNA and links the appropriate amino acid

Initiation of Translation:

• Once ribosome has bound to mRNA, it slides along mRNA molecule until it reaches an AUG codon, which is the initiator codon

• All proteins begin translation with a methionine amino acid

Translation Elongation:

• Initiator codon occupies P-Site with tRNA bound on anticodon

• The next codon occupies A-Site, tRNA with correct anticodon binds to codon in A-Site

• Peptide bond formed between amino acids

• Ribosome moves so tRNA from A-Site is now at P-Site

• A new tRNA with anticodon is now bound at A-Site, next peptide bond formed

• The initiator codon establishes the reading frame for the mRNA, codons do not overlap, once AUG is found it determines how a ribosome will translate that mRNA

Translation Termination:

• Stop codon in A-Site causes release factor to release tRNA at the P-Site

• Ribosome disassembles, polypeptide and mRNA released

Polyribosomes:

• Many ribosomes may be translating a given mRNA

Modification:

• There may be posttranslational modification of a polypeptide:

1. Addition of carbohydrate in Endoplasmic Reticulum/Golgi Apparatus

2. Addition of lipid

3. Cutting of polypeptide into functional units

Mutation:

• A change in the genetic material, results in a change in protein

• Point mutations → Change in a single base pair

1. Silent → Redundancy in code inserts the same amino acid at that position

2. Missense → Inserts another amino acid

3. Nonsense → Creates a termination codon, premature termination

Frame-Shift Mutations:

• Causes a change in the reading frame of a message, rather than changing only one amino acid. This can drastically change the protein.

• Insertion → Insertion of one or more bases

• Deletion → Deletion of one or more base pairs