Biology - Chapter 14: Transcription & Translation

Why is Wahoo (NE) famous?

Wahoo, NE is the birthplace of George Beadle, who hypothesized that one gene entails the creation of one, specific enzyme. This hypothesis led to a Nobel Prize.

How did the Beadle and Tatum experiments on bread mold mutations lead to the "one-gene, one-enzyme" hypothesis?

Beadle and Tatum grew Neurospora, a type of mold, in a minimal medium, which could catalyze all necessary metabolic reactions for growth. Using X-Rays, mutations were induced, and Neurospora lost the ability to grow. For each mutant strain, a single, additional supplement allowed growth again. This experiment led to the "one-gene, one-enzyme" hypothesis.

Why is the "one-gene, one-enzyme" hypothesis now called the "one-gene, one-polypeptide hypothesis"?

Many proteins contain several polypeptide chain subunits, composing a protein's quaternary structure. A gene specifies a specific polypeptide subunit; thus, it is more accurate to consider a gene a building block for a polypeptide.

Do all genes code for polypeptides?

No, some genes are transcribed into RNA and have other functions. Examples include mRNA, rRNA and tRNA.

Define Transcription...

The information in a DNA sequence is copied into a complementary RNA strand.

Define Translation...

An RNA sequence is used to create the amino acid sequence of a polypeptide.

Define DNA Replication...

Information in a DNA molecule is copied to daughter DNA molecules.

What is the Central Dogma of Molecular Biology?

The Central Dogma describes the ways in which information flows in the cell: from DNA to DNA, from DNA to RNA, and from RNA to polypeptides.

How does RNA differ from DNA?

RNA is:

a.) Single Stranded

b.) Contains Uracil

c.) Contains a Ribose Sugar

What are the three kinds of RNA that are involved in protein synthesis and briefly describe them?

1.) Messenger RNA (mRNA): The message which produces polypeptides.

2.) Ribosomal RNA (rRNA): Forms ribosomes with proteins.

3.) Transfer RNA (tRNA): Transfers amino acids to the ribosome, contains an anticodon.

Why are retroviruses exceptions to the "central dogma"? Give an example of a retrovirus.

a.) Retroviruses synthesize DNA from RNA in a process called Reverse Transcription, an exception to the "central dogma."

b.) An example of a retrovirus is HIV.

What three basic groups of macromolecules are required for transcription to occur?

1.) DNA Template Strand

2.) Ribonucleotides (rNTPs)

3.) RNA Polymerase

In what 4 ways does RNA polymerase differ from DNA polymerase?

1.) RNA polymerase does not require a primer.

2.) RNA polymerase catalyzes the addition of ribonucleotides, synthesizing RNA.

3.) RNA polymerases are processive, one enzyme-template binding results in many copies of mRNA being produced.

4.) RNA polymerase has no proofreading mechanism, but since many copies of mRNA are produced, it is less of an issue compared to DNA.

Where in the cell does transcription occur?

Nucleus

What are the three stages of transcription?

Initiation, Elongation, and Termination

What region of DNA is needed to start transcription?

Promoter Sequence

What occurs on the promoter sequence of DNA during transcription?

RNA polymerase binds to this sequence on the DNA template, initiating transcription.

In what direction is the newly formed RNA transcript produced?

Newly formed RNA is produced in the 5' to 3' direction.

Why does transcription terminate at a specific point in the DNA?

Transcription terminates at a specific point due to a specific DNA sequence indicating completion. There is an END to Transcription.

How is nucleic acid hybridization used in research and what information has been found?

Nucleic acid hybridization is used to study transcription, highlighting key DNA and RNA sequences by the creation of a hybrid. The results show that NOT all DNA is transcribed into mRNA.

What are introns?

Noncoding intervening regions of DNA that are spliced out of the pre-mRNA in the nucleus. An intron resides within a gene but does not remain in the final mature mRNA.

What are exons?

The coding sequences that make up the mature mRNA that travel to the ribosome.

Do introns appear in the final mRNA transcript?

No

How is mRNA processed?

Introns are spliced THEN...

A.) 5' Cap is added to the 5' end, which facilitates binding to the ribosome for translation.

B.) A poly A (Adenine) tail is added to the 3' End, which assists in export from the nucleus and stability.

Present the Central Dogma of Molecular Biology...

DNA Replication: DNA ---> DNA

Transcription: DNA ---> RNA

Translation: RNA ---> Polypeptide

What is the genetic code?

The genetic code specifies which amino acids will be used to build a specific protein.

What is a codon?

A sequence of three mRNA ribonucleotide bases, which corresponds to a particular amino acid.

Why is a codon three (ribo)nucleotides long?

The codon is specifically three ribonucleotides long as to produce all of the 20 different amino acids. A three nucleotide base pair can produce a total of 64 different mRNA base combinations. Some codons are redundant, having different combinations produce the same amino acid. If a codon was only two nucleotides long, it could only produce 16 different base sequences, and 4 ribonucleotides would produce 256 different base sequences.

Why is the genetic code redundant, but not ambiguous?

The genetic code is redundant as there is more than one codon for almost all amino acids, but not ambiguous as a codon can only specify for one amino acid.

Does the genetic code vary among organisms? What is the evolutionary significance of this?

Genetic code is essentially identical across all the species on our planet, indicating that it must have been intact throughout evolution. This evolutionary significant support that all life comes from pre-existing life.

Where in the cell does translation occur?

Ribosomes in the Cytoplasm

The code for a human gene is the same as that for a __________________ gene.

Bacterial

What is tRNA and what is its role in translation?

Transfer RNA links mRNA codons with specific amino acids. Additionally, tRNA transfers amino acids to the ribosomes, where protein synthesis, occurs, and it contain an anticodon which connects to the codon on the mRNA.

Compare and contrast the anticodon and the codon.

The codon is a sequence of three bases that specify an amino acid on an mRNA strand while an anticodon is the complementary sequence to an codon on a tRNA strand.

What is "wobble" and what is its relation to the genetic code?

Wobble is the base pairing that occurs at the third position of the codon-anticodon interaction, the 3' end of the codon. The codons that produce the same amino acid usually have the same first two bases but differ in the final base. This interaction (and inherently a pattern) allows cells to produce fewer tRNA types.

What two macromolecules make up ribosomes?

rRNA and Proteins

What is the basic structure of a ribosome?

A ribosome consists of two subunits, a large one and a small one, each containing rRNA and proteins.

Are ribosomes specific for specific proteins produced?

No

How are ribosomal subunits attached to each other?

Noncovalently

What are the three stages of translation? List components...

1.) Initiation: Charged tRNA + mRNA + Ribosome

2.) Elongation of Polypeptide

3.) Termination

Which mRNA codon starts translation?

AUG, Methionine

What occurs in the 'A' site, the 'P' site, and the 'E' site of the ribosome during polypeptide elongation.

A site: Amino acid on tRNA binds

P site: Polypeptide grows with the addition of amino acids

E site: Exit of tRNA

How and why does translation end?

Translation ends when a stop codon enters the A site. A protein release factor hydrolyzes the bond between the polypeptide and the terminal tRNA in the P site, separating the polypeptide from the final ribosome.

What is a polyribosome?

A strand of mRNA with several associated ribosomes which can make multiple copies of the same polypeptide.

Is a protein complete immediately following translation?

Immediately following translation, a signal sequence may be added to the polypeptide to direct it to a place in the cell and other post-translational changes may occur to the protein to make it functional.

How does a newly synthesized protein move into the nucleus or become exported from the cell?

The newly synthesized protein moves via the signal sequence that directs the synthesized protein to specific organelles.

What are three common post-translational protein modifications?

Proteolysis: Polypeptide is cut

Glycosylation: Addition of sugars

Phosphorylation: Addition of phosphate groups

Why are the mRNA ends processed?

The ends are processed to distinguish between the two. Additionally, the 3' cap allows the mRNA to bind to the ribosome, the location of protein synthesis, while the poly A tail assists the mRNA in exportation from the nucleus.

What is a 'signal sequence'?

A signal sequence is a short stretch of amino acids attached to the polypeptide that will tell the polypeptide where in the cell in belongs. The sequence on the polypeptide binds to a specific receptor protein at the surface of the organelle. Once it has bound, the targeted protein moves into the organelle.

Can the N terminus be removed after translation?

Yes, The N terminus, AUG, methionine, may be removed during post-translational changes.