Chapter 4: Cell Processes and Applications (Protein Synthesis)

Question 1

explain transcription

Answer 1

Transcription begins when enzyme RNA polymerase binds tightly to a promoter, which is a region of DNA that contains a special sequence of nucleotides. This enzyme opens up the DNA helix just before the promoter so complementary base pairing occurs (like DNA replication). RNA polymerase inserts RNA nucleotides resulting in an mRNA molecule. Messenger RNA (mRNA) carries genetic information from DNA to ribosomes for protein synthesis. mRNA has a sequence of bases complementary to DNA: A, T, C, G in DNA translate to U, A, G, C, in mRNA. mRNA must be processed before entering cytoplasm. Newly synthesized primary mRNA molecules become mature mRNA after processing and enter the cytoplasm.

Question 2

explain translation’s first step: initiation

Answer 2

brings all translation components together and initiation factor proteins assemble small ribosomal unit, mRNA, initiator tRNA and large ribosomal unit for start of protein synthesis. In prokaryotes, small ribosomal unit attaches to mRNA in vicinity of start codon, initiator tRNA pairs with this codon in the P site, then large ribosomal unit joins the small ribosomal unit. A ribosome is a small structural body found in cytoplasm and endoplasmic reticulum. Ribosomes have one binding site for mRNA and three for tRNA. Transfer RNA (tRNA) molecules bring amino acids to ribosomes, the site of protein synthesis. The single-stranded polynucleotide doubles back on itself so complementary base pairing creates a boot-like shape, on one end is an amino acid and on the other is an anticodon.

Question 3

explain translation’s second step: elongation

Answer 3

protein synthesis where polypeptide increases in length one amino acid at a time, requires tRNA molecules and elongation factors that facilitate binding of tRNA anticodons to mRNA codons at ribosome. tRNA with attached peptide is at the P site and tRNA carrying next amino acid in chain is arriving at the A site. Once in place at the A site, the peptide chain will be transferred to the A site tRNA. Energy and the ribosomal subunit is needed to bring about transfer: energy contributes to peptide bond formation making resulting polypeptide chain one amino acid longer. Once translocation occurs, mRNA moves forward one codon length and peptide-bearing tRNA is now at ribosome P site. The “spent” tRNA exits.

Question 4

explain translation’s third step: termination

Answer 4

the final step of protein synthesis. The polypeptide and the assembled components separate. This occurs at the stop codon and requires a protein called a release factor. This cleaves polypeptide from last tRNA and the polypeptide is set free. It begins to take its own 3D shape. The ribosome will dissociate into 2 subunits.

Question 5

translate the following DNA sequence into
-an mRNA codon
-an anti codon
-a protein (using an mRNA chart)
TAC GTA CTA AAT ATC

Answer 5

Codon (mRNA)
AUG CAU GAU UUA UAG

Anticodon (tRNA)
UAC GUA CUA AAU AUC

Amino Acid (protein)
methionine (start) histidine 
aspartic acid
leucine
stop

Question 6

give two examples of environmental mutagens that can cause mutations in humans

Answer 6

radiation:
radioactive elements, X-rays, UV radiation
certain organic chemicals:
chemicals in cigarette smoke, certain pesticides

Question 7

using examples, explain how mutations (3) in DNA affect protein synthesis and may lead to genetic disorders

Answer 7

(1) Abnormal Protein - a point mutation changes one base in a codon which could then code for a completely different amino acid, causing an abnormal protein that can have a different function (point mutation). An abnormal functioning or non-functioning amino acid can wreak havoc on our bodies. This is what produces sickle cell disease. This can cause a genetic disorder.
(2) New Sequence of Codons/Non-Functional Protein - addition or deletion of a nucleotide can cause entire frame to switch (THE CAT → HEC AT) and every codon to code for a different amino acid (frameshift mutation). If this creates a non-functioning protein, it can be very dangerous to the human body. This can cause a genetic disorder.
(3) Incomplete Protein - if a mutation codes for a stop codon early in the polypeptide chain, the chain will be cleaved incredibly early and result in a shorter protein that function differently or is non-functioning. This can cause a genetic disorder.