Chapter 3: Cells, Membranes, and Homeostasis

Question 1

Describe Griffith’s and follow-up experiments that lead to the discovery of DNA as the hereditary material

Answer 1

Griffith: demonstrated that one strain of bacteria (nonvirulent) can be transformed into another (virulent) strain killing mice
-> TRANSFER OF GENETIC INFO!

Avery, Macloeod, McCartney: showed that the molecule responsible for transforming nonvirulent strains to a virulent strain was DNA using the process of elimination
-> DNA IS RESPONSIBLE FOR TRANSFER OF GENES!

Hershey & Chase: once again proved DNA carried the genetic material using viruses
-> MORE EVIDENCE!

Watson & Crick: discovered DNA structure
-> STRUCTURE INDICATES FUNCTION!

Question 2

What is the Central Dogma of biology?

Answer 2

DNA
-> Transcription
RNA
-> Translation
PROTEIN

Question 3

What are two vital elements of the creation and structure of DNA? How many H bonds form between A-T and G-C?

Answer 3

• 2 Strands must run “antiparallel” because hydrogen bonding can only occur if polarity of the 2 strands runs in opposite directions
• Hydrogen bonding between bases forms double helix (DOUBLE BOND between A-T, TRIPLE BOND between G-C)

Question 4

Describe the process of DNA Replication. What is the purpose of:
- Helicase
- Single Strand Binding Proteins
- Leading & Lagging Strand
- Primase & RNA Primer
- DNA Ligase

Answer 4

1) Double-helix is unwound by helicase
2) Single strand binding proteins stabilize the unwound parental DNA
3) The leading strand is synthesized continuously in the 5’ –> 3’ direction
4) The lagging strand is synthesized discontinuously: Primase synthesizes an RNA Primer so that DNA Polymerase can create Okazaki Fragments
5) After the RNA Primer is replaced by DNA, DNA Ligase joins the Okazaki Fragments to the growing Lagging Strand

Question 5

Why are the 5’ and 3’ so important in DNA replication?

Answer 5

• A polymerase can only add to the 3’ OH end of a DNA strand!

Question 6

What are the major differences between RNA and DNA?

Answer 6

RNA
- single stranded
- sugar is ribose (OH)
- A, U, C, G

DNA
- double stranded
- sugar is deoxyribose (H)
- A, T, C, G

Question 7

What are the differences between the different types of RNA? (mRNA, tRNA, rRNA)

Answer 7

mRNA: messenger RNA (carries code)
tRNA: transfer RNA (carries amino acids)
rRNA: ribosomal RNA (helps translate info from mRNA into protein)

Question 8

What is the difference between the Template and Non-Template DNA strands during Transcription?

Answer 8

Only one strand serves as a template for transcription at any given time. This TEMPLATE strand is called the NONCODING strand. The NONTEMPLATE strand is referred to as the CODING strand because its sequence will be the same as that of the new RNA molecule (U replacing T).

Question 9

Describe the process of transcription including proteins involved, products, initiation and termination IN EUKARYOTES

Answer 9

DNA TO RNA

INITIATION
- In eukaryotes, the process involves the assembly of transcription factors and RNA polymerase at the promoter

ELONGATION
- As RNA polymerase moves along the DNA template strand, it adds complementary RNA nucleotides to the growing RNA strand (U replaces T)

TERMINATION
- In eukaryotes, termination is more complex and involves the cleavage and polyadenylation of the RNA transcript. After RNA polymerase transcribes a specific sequence called the polyadenylation signal, the pre-mRNA is cleaved, and a poly-A tail is added to the 3’ end.

Question 10

Describe the process of transcription including proteins involved, products, initiation and termination IN PROKARYOTES

Answer 10

DNA TO RNA

INITIATION
- In prokaryotes, RNA polymerase binds directly to the promoter region

ELONGATION
- As RNA polymerase moves along the DNA template strand, it adds complementary RNA nucleotides to the growing RNA strand (U replaces T)

TERMINATION
- In prokaryotes, termination can be rho-dependent or rho-independent. Rho-dependent termination involves the rho protein binding to the RNA and causing the RNA polymerase to dissociate. Rho-independent termination involves the formation of a hairpin loop in the RNA that causes RNA polymerase to detach.

Question 11

What is the principle behind Chargaff’s ratios?

Answer 11

There is a 1:1 correspondence between purine and pyrimidine bases

Question 12

What is the function of the sigma factor in prokaryotic transcription?

Answer 12

• In PROKARYOTES, the sigma factor is a crucial component in the initiation phase of transcription.
• The sigma factor recognizes and binds to specific DNA sequences called promoter regions.
• When the sigma factor associates with RNA polymerase, the resulting complex is called the holoenzyme (σ-RNA polymerase holoenzyme). The holoenzyme is capable of recognizing and binding to promoter sequences with high specificity.
• Once a short RNA transcript is synthesized, the sigma factor may be released.

Question 13

What is the difference between prokaryotic and eukaryotic transcription?

Answer 13

PROKARYOTES:
- Polycistronic (mRNA codes for multiple proteins)
- Transcription & Translation occur in cytoplasm

EUKARYOTES:
- Monocistronic (mRNA codes for one protein)
- Transcription occurs in nucleus & Translation occurs in cytoplasm
* 5’ Cap and Poly A Tail
* Introns & Extrons (SPLICING!)