Lecture 20: Central Dogma

Question 1

Beadle and Tatum’s Experiments

Answer 1

Beadle and Tatum grew nutritional mutants (auxotrophs) of Neurospora on a minimal medium supplemented with a single amino acid such as arginine (arg)

They hypothesized that each auxotrophic strain had a defect in a gene that codes for an enzyme needed to synthesize a particular amino acid (one gene–one enzyme hypothesis)
•Their hypothesis was later updated to the one gene–one polypeptide hypothesis

•Their hypothesis was later updated to the one gene–one polypeptide hypothesis

Question 2

How to grow a mould

Answer 2

Provide all the required nutrients: complete medium

Provide only the simplest most basic nutrients needed for growth: minimal medium

Beadle & Tatum’s Question:
•If some mutant moulds cannot grow on minimal medium then do their mutant genes fail to specify the enzymes needed to produce the nutrient?

Question 3

DNA to RNA to Protein

Answer 3

Beadle and Tatum proposed the one-gene-one-enzyme hypothesis

• One- gene-one –polypeptide hypothesis: The majority of genes code for cellular proteins. Each gene encodes one polypeptide (the simplest building block of a protein).
• Updated because one enzyme (or protein) can have more than one polypeptide component each encoded by a different gene

Question 4

Pathway from Gene to Polypeptide

Answer 4

In 1956, Francis Crick gave the name central dogma to the flow of information from DNA → RNA → protein

• Transcription is the mechanism by which information encoded in DNA is made into a complementary RNA copy
• Translation uses the information encoded in the RNA copy to assemble amino acids into a polypeptide

Question 5

Transcription and Translation

Answer 5

In transcription, RNA polymerase copies the DNA sequence of a gene into an RNA sequence
•A protein-coding gene is transcribed into messenger RNA (mRNA)
•In translation, an mRNA associates with a ribosome, on which amino acids specified by the mRNA are joined one by one to form the polypeptide
•Some genes do not encode a polypeptide – they encode various molecules that function in transcription, translation, and other processes

Question 6

Reading the DNA sequence

Answer 6

Reading the DNA sequence:
•The DNA alphabet: A, T, C and G.
•The RNA alphabet: A, U, C and G.
•Text p. 206
•The genetic code:
•The four mRNA bases in combinations of 3 code for 20 amino acid “words”

Question 7

Codons

Answer 7

Start codon = AUG

Stop Codon = UAA, UAG, UGA

Question 8

RNA polymerase

Answer 8

Prokaryotes
Binds the promoter sequence of the gene directly

Eukaryotes
Transcription factors facilitate the binding of RNA polymerase

Question 9

Gene expression regulation

Answer 9

PRO - At transcription
EUK - Transcriptional and
Translational regulation

Question 10

Transcription termination

Answer 10

PROK - Specific terminator sequences are involved

EUK - There is a polyadenylation signal at the 3’ end and poly (A) polymerase adds a poly (A) tail

Question 11

Translation

Answer 11

PROK - Occurs throughout cell and may begin while mRNA is still being made

EUK - Occurs in the cytoplasm and begins only after processing the mRNA

Question 12

Features of the Genetic Code

Answer 12

Sense codons
61 codons specify amino acids
•Most amino acids specified by several codons (degeneracy or redundancy)
•Ex: CCU, CCC, CCA, CCG all specify proline

Start codon or initiator codon
•First amino acid recognized during translation
•AUG; specifies amino acid methionine

Stop codons or termination codons
•End of a polypeptide-encoding mRNA sequence
•UAA, UAG, UGA

Question 13

Wobble hypothesis

Answer 13

The pairing of the anticodon with the first two nucleotides is specific but the third nucleotide of the codon may be less precise

Question 14

Know this…

Answer 14

Every protein is assembled on ribosomes according to instructions which are specified by genes and coded in DNA.

Question 15

Polysomes

Answer 15

Multiple ribosomes can simultaneously translate a single mRNA

Question 16

Simultaneous Transcription and Translation

Answer 16

Can occur in prokaryotes (no nuclear envelope)

Fig.

Question 17

Genetic Changes That Affect Protein Structure and Function

Answer 17

Two types of genetic change can alter protein structure and produce an altered phenotype (function):
•Mutation of a base pair in the DNA (a change from one base pair to another)
•Movement of transposable elements (TEs) from one location to another in the genome

Four types of base-pair-substitution mutations affect protein-coding genes: missense mutation, nonsense mutation, silent mutation, and frameshift mutation

Question 18

Missense Mutation

Answer 18

A sense codon is changed to a different sense codon that specifies a different amino acid
•Whether the function of a polypeptide is altered significantly depends on the amino acid change that occurs

Genetic diseases caused by missense mutations include sickle-cell disease, albinism, hemophilia, and achondroplasia

Question 19

Nonsense Mutation

Answer 19

A sense codon is changed to a nonsense (stop) codon
•Translation of an mRNA containing a nonsense mutation results in a shorter than normal polypeptide – in many cases, this polypeptide will be only partially functional

i.e. Cystic fibrosis

Question 20

Silent Mutation

Answer 20

A sense codon is changed to a different sense codon but that codon specifies the same amino acid as in the normal polypeptide

The function of the polypeptide is unchanged

Question 21

Frameshift Mutation

Answer 21

A single base pair deletion or insertion in the coding region of a gene alters the reading frame of the resulting mRNA
•After the point of mutation, the ribosome reads codons that are not the same as for the normal mRNA, producing a different amino acid sequence in the polypeptide

•Resulting polypeptide typically is nonfunctional because of the significantly altered amino acid sequence