cHAPTER 15

Question 1

The Nature of Genes

Answer 1

-Early ideas to explain how genes work came from studying human diseases
-Archibald Garrod – 1902
-Recognized that alkaptonuria is inherited via a recessive allele
-Proposed that patients with the disease lacked a particular enzyme
These ideas connected genes to enzymes

Question 2

Beadle and Tatum – 1941

Answer 2

• Used X-rays to damage DNA—to create mutations in chromosomes and verify that they behaved in a -Mendelian fashion in crosses
• Studied Neurospora crassa—mold
• One gene/one enzyme hypothesis
• Today—modified—one gene/one polypeptide hypothesis

Question 3

Central Dogma

Answer 3

First described by Francis Crick
-How do we get from: 
genotype (DNA)phenotype (protein)?
-Information only flows from
-DNA → RNA → protein
-Transcription = DNA → RNA 
-Translation = RNA → protein
-Retroviruses violate this order using reverse transcriptase to convert their RNA genome into DNA

Question 4

Transcription

Answer 4

-DNA-directed synthesis of RNA
-Only template strand of DNA used
-T (thymine) in DNA replaced by U (uracil) in RNA
mRNA used to direct synthesis of polypeptides

Question 5

Translation

Answer 5

-RNAs are used to synthesize polypeptides
-Takes place at ribosome
Requires several kinds of RNA

Question 6

RNA

Answer 6

All synthesized from DNA template by transcription

• Messenger RNA (mRNA)
• Ribosomal RNA (rRNA)
• Transfer RNA (tRNA)
• Small nuclear RNA (snRNA)
• Signal recognition particle RNA
• Micro-RNA (miRNA)

Question 7

Genetic Code

Answer 7

• Francis Crick and Sydney Brenner determined how the order of nucleotides in DNA encoded amino acid order
• Codon – triplet of mRNA nucleotides corresponding to an amino acid
• Marshall Nirenberg identified the codons that specify each amino acid

Question 8

Code practically universal

Answer 8

• Strongest evidence that all living things share common ancestry
• Advances in genetic engineering
• Mitochondria and chloroplasts have some differences in “stop” signals

Question 9

Prokaryotic transcription

Answer 9

Single RNA polymerase
Initiation of mRNA synthesis does not require a primer
Requires Recognition and binding site on DNA
transcription begins
Promoter
Start site Transcription unit
Termination site

Question 10

Elongation

Answer 10

-RNA nucleotides are added in the 5′-to-3′ direction
Transcription bubble – contains RNA polymerase, DNA template, and growing RNA transcript
-After the transcription bubble passes, the now-transcribed DNA is rewound as it leaves the bubble

Question 11

Termination

Answer 11

-Marked by sequence that signals “stop” to polymerase
-Hairpin forms
RNA–DNA hybrid within the transcription bubble dissociates
RNA polymerase releases the DNA
DNA rewinds

Question 12

Prokaryotic transcription is coupled to translation

Answer 12

-mRNA begins to be translated before transcription is finished
Prokaryotic cells also have operons.
–Operon
-Grouping of functionally related genes on mRNA that code for multiple enzymes for a pathway
-Can be regulated together
Eukaryotes only have one gene on a mRNA

Question 13

Eukaryotic Transcription

Answer 13

• -3 different RNA polymerases
• RNA polymerase I transcribes rRNA
• RNA polymerase II transcribes mRNA and some snRNA
• RNA polymerase III transcribes tRNA and some other small RNAs
• -Each RNA polymerase recognizes its own promoter

Question 14

Initiation of transcription

Answer 14

-Requires a series of transcription factors (proteins)
-Necessary to get the RNA polymerase II enzyme to a promoter and to initiate gene expression
-Interact with RNA polymerase to form initiation complex at promoter
-Termination
Termination sites not as well defined

Question 15

mRNA modifications

Answer 15

-In eukaryotes, the primary transcript (pre-mRNA) must be modified to become mature mRNA
-Addition of a 5′ cap (methyl-guanylate cap)
-Protects from degradation; involved in translation initiation (helps line up mRNA on ribosome)
-Addition of a 3′ poly-A tail (100-200 Adenines)
Created by poly-A polymerase; protection from degradation
-Removal of non-coding sequences (introns)
-Pre-mRNA splicing done by spliceosome

Question 16

Addition of a 5′ cap (methyl-guanylate cap)

Answer 16

Protects from degradation; involved in translation initiation (helps line up mRNA on ribosome)

Question 17

Removal of non-coding sequences (introns)

Pre-mRNA splicing done by

Answer 17

spliceosome

Question 18

Eukaryotic pre-mRNA splicing

Answer 18

Introns have to be cut out and the exons have to be spliced back together

Question 19

-snRNA—small nuclear RNA—recognize exon-intron boundaries
-snRNP—small nuclear
ribonucleoprotein particles (snurps)—cuts introns out

Answer 19

Spliceosome—splicing

organelle

Question 20

Alternative splicing

Answer 20

• Single primary transcript can be spliced into different mRNAs by the inclusion of different sets of exons
• 15% of known human genetic disorders are due to altered splicing
• 35 to 59% of human genes exhibit some form of alternative splicing
• Explains how 25,000 genes of the human genome can encode the more than 80,000 different mRNAs

Question 21

tRNA and Ribosomes

Answer 21

tRNA molecules carry amino acids to the ribosome. Amino acids can then bond
by peptide bondspolypeptide

Question 22

tRNA has to have an amino acid loaded onto the acceptor end of the molecule.

Answer 22

Aminoacyl-tRNA synthetases add amino acids to the acceptor stem of tRNA

Question 23

tRNA charging reaction

Answer 23

-Each aminoacyl-tRNA synthetase recognizes only 1 amino acid but several tRNAs
-Charged tRNA – has an amino acid added using the energy from ATP
-Can undergo peptide bond formation without additional energy
-Ribosomes do not verify amino acid attached to tRNA—only verify anticodon-
codon matching

Question 24

Charged tRNA

Answer 24

has an amino acid added using the energy from ATP

Question 25

Ribosome | The ribosome has multiple tRNA binding sites

Answer 25

P site – binds the tRNA attached to the growing peptide chain A site – binds the tRNA carrying the next amino acid E site – binds the tRNA that carried the last amino acid (exit site)

Question 26

The ribosome has two primary functions

Answer 26

Decode the mRNA | Form peptide bonds

Question 27

Peptidyl transferase

Answer 27

Enzymatic component of the large subunit of the ribosome | Forms peptide bonds between amino acids

Question 28

Translation

Answer 28

-Initiation complex formed: -Initiator tRNA charged with N-formylmethionine (Pro.); methionine (Eu.) mRNA strand attaches to small subunit of ribosome -Large subunit now added Initiator tRNA bound to P site with A site empty

Question 29

-There are fewer tRNAs than codons so a tRNA anticodon can bond with several codons -Wobble pairing allows less stringent pairing between the 3′ base of the codon and the 5′ base of the anticodon -This allows fewer tRNAs to accommodate all codons

Answer 29

B

Question 30

Termination

Answer 30

- Elongation continues until the ribosome encounters a stop codon - Stop codons are recognized by release factors which release the polypeptide from the ribosome

Question 31

Protein targeting

Answer 31

- In eukaryotes, translation may occur in the cytoplasm or the rough endoplasmic reticulum (RER) - Signal sequences at the beginning of the polypeptide sequence bind to the signal recognition particle (SRP) - The signal sequence and SRP are recognized by RER receptor proteins - Docking holds ribosome to RER - Beginning of the protein-trafficking pathway

Question 32

Mutation: Altered Genes

Answer 32

-Point mutations alter a single base -Base substitution – substitute one base for another -Silent mutation – same amino acid inserted -Missense mutation – changes amino acid inserted -Nonsense mutations – changed to stop codon -Frameshift mutation—addition or deletion of a single base—much more profound consequences alter reading frame downstream from mutation

Question 33

Triplet repeat expansion mutation

Answer 33

Huntington disease | Repeat unit is expanded in the disease allele relative to the normal

Question 34

Chromosomal mutations

Answer 34

- -Change the structure of a chromosome - Deletions – part of chromosome is lost - Duplication – part of chromosome is copied - Inversion – part of chromosome in reverse order - Translocation – part of chromosome is moved to a new location

Question 35

- Mutations are the starting point for evolution - Too much change, however, is harmful to the individual with a greatly altered genome - Balance must exist between amount of new variation and health of species

Answer 35

O