Cell Reproduction and Protein Synthesis

Question 1

Classify and describe the phases of the cell cycle. Draw a diagram

Answer 1

INTERPHASE
Go
- Resting phase
- Completely differentiated cells or cells with DNA viability/sustainability problems
- Non-availability of mitogen and energy rich compounds
- Remain metabolically active, grow and differentiate

G1

• Growth Phase 1
• Longest phase
• Rapid growth and routine functions
• Multiplication organelles

S

• Synthesis Phase
• DNA replication and repair

G2 (Preparation for Mitosis)

• Growth phase 2
• DNA synthesis stops (RNA/proteins continues)
• Multiplication of cell organelles
• Lasts 2 - 5 hours

MITOTIC PHASE
Prophase (chromatin --> chromosomes)
Metaphase (Chromosomes line up)
Anaphase (Sister chromatids pulled opposite pokes)
Telophase (chromosomes --> chromatin)
Cytokinesis - cytoplasmic division

Question 2

What are the 3 options for a cell in G1 phase

Answer 2

1. To enter G0 resting phase
2. Leave cycle - To enter synthesis phase (DNA replication)
3. Leave cycle - and undergo cell differentiation

Question 3

What are the checkpoints within the cell cycle

Answer 3

Regulatory proteins assess viability for the cell to continue into the next phase of the cell cycle at various checkpoints in the cell cycle

1. End G1 –> ? big enough ? made enough proteins
2. S Phase –> Enough + correct DNA replication
3. Mitosis –> ? mitosis complete

Question 4

List and describe the 4 factors that control cell reproduction

Answer 4

1. Minimum growth
   - Some amount of minimum growth is required before the cell obtains the ability to undergo division
2. Surface - Volume ratio
   - Increase size –> decrease surface:volume –> reduced efficacy of surface exchange required to maintain optimum metabolism
3. Nucleocytoplasmic ratio
   - As this decreases, the cell is stimulated to divide
4. Mitogens
   - Substances which bring about cell division (E.g. EGF and PDGF)

Question 5

Name and describe the functions of the various types of DNA

Answer 5

mRNA - messenger RNA: carries information from gene in the DNA to the cytoplasm RER. Determines the sequence of amino acids for a protein

tRNA - Brings the individual amino acid to the Ribosome and mRNA in translation

rRNA - Found on ribosomes and is used to “connect” the tRNA to the mRNA

snRNA- Spliceosomes –> used to remove introns

Question 6

Define and differentiate exons and introns

Answer 6

Introns are noncoding regions of mRNA transcript that are spliced out of mRNA before translation.

the sections of RNA and DNA that code for proteins are called exons

Question 7

What is a codon and what is an anticodon

Answer 7

A codon is a triplet of DNA/mRNA nucleotides which is specific for a particular amino acid

More than one codon codes for the same amino acid

Anticodon is the triplet of nucleotides that are complementary to its corresponding codon - i.e. those found on tRNA.

Question 8

How many different codons are there? How many of these code for amino acids?

Answer 8

64 codons
61 code for amino acids
3 code for stop codons
1 code for start (also for methionine)

Question 9

Describe and define the two major steps in protein synthesis

Answer 9

1. Transcription - the genetic information from a strand of DNA is copied into a strand of mRNA
2. Translation - the mRNA, with the help of ribosome, rRNA and tRNA, forms a polypeptide chain of amino acids, eventually forming a protein

Question 10

Name and describe the stages of transcription

Answer 10

Initiation

• RNA polymerase attaches to ‘promoter region’ - the TATA box
• Transcription factors assist with this
• RNA polymerase unwinds the DNA at the start of the gene

Elongation

• RNA polymerase unwinds DNA and base pairs RNA nucleotides to the DNA gene.
• RNA is made in a 5’ to 3’ direction (DNA gene 3” to 5’)
• Adenine - Uracil
• Guanine - Cytosine

Termination
- RNA polymerase proceeds until stop codon

Processing

• 5’ Modified guanine nucleotide is added
• 3’ Poly -A-tail (30 - 200 adenine nucleotides)

Question 11

What is the purpose of the Poly-A- tail

Answer 11

1. Prevent degradation of mRNA
2. Signal ribosomes where to attach
3. Determines how many time mRNA can be translated before it is destroyed

Question 12

What is the average length of an immature mRNA vs a mature mRNA. why is this different

Answer 12

Immature: 8000 nucleotides long

Mature: 1200 nucleotides long

The noncoding regions (introns) on the immature mRNA are spliced/removed by spliceosomes (snRNA)

Question 13

What are the functions of the large and small subunits of the ribosomes

Answer 13

Large - two binding sites for tRNA (that bring aa’s)

Small - contains the binding site for mRNA

Question 14

On the Ribosome (large subunit), what is the E site, P site and A site

Answer 14

E site - exit site for the TRNA leaving

P site - Peptidyl site (containing peptide chain)

A site - aminoacyl site (for arriving amino acids)

Question 15

Name and describe the stages of translation

Answer 15

Initiation

• small ribosomal subunit attaches to mRNA
• Initiator tRNA attaches to the mRNA in the P-site
• Large ribosomal subunit binds to the complex
• New tRNA then joins onto the A site
• Peptide bond forms connecting the aa’s

elongation
- tRNA repeatedly transferred from A to P site with lengthening chain

termination

• termination codon reached
• polypeptide released
• ribosomal complex splits