DNA, Genes and Protein Synthesis

Question 1

Similarities and Differences between the DNA in Eukaryotic and Prokaryotic cells.

Answer 1

Similarities - Both double stranded, both have the same nucleotide variation, and universal code.

Differences - Eukaryotic; DNA is associated with proteins (histones) which allow the DNA to be formed into chromosomes.
Prokaryotic; DNA isn’t associated with proteins, so doesn’t form into chromosomes, and instead circles. They are also shorter.

Question 2

Define: Allele

Answer 2

Allele - One of a number of alternative forms of a gene.

Question 3

Describe the structure of ribonucleic acid.

Answer 3

• A single-strand of mono-nucleotide sub-units, which are made up of;
  > Ribose (pentose sugar)
  > Organic base (Adenine, Guanine, Cytosine or Uracil)
  > Phosphate group

Question 4

Describe the two types of ribonucleic acid.

Answer 4

Messenger RNA (mRNA):
- Long, single helix strand of thousands of mononucleotides.

Transfer RNA (tRNA):
- Relatively small molecule of about 80 nucleotides. Single-stranded chain folded into a clover-leaf shape with one end extending beyond the other (the amino acids attachment site), with three anticodon sites at the other end of the molecule.

Question 5

Define and Describe the Transcription sequence of protein synthesis:

Answer 5

• It is where one gene on the DNA is copied using mRNA.

Method:
- A complementary mRNA copy of one gene on the chosen DNA strand is created by the DNA helix unwinding (catalysed by DNA helicase by breaking the H bonds between bases), to expose the bases as a template. (Only one chain acts as a template)
- Free nucleotides in the nucleus along opposite exposed complementary DNA bases on the template.
- RNA polymerase catalyses bonding together of the nucleotides to create a Pre-mRNA strand.
- Pre-mRNA is then modified by splicing the introns out of the base sequence to form mRNA.
- mRNA carries the genetic code from the nucleus to the ribosomes (through the nuclear pores) in the cytoplasm to enable the protein to be formed.

Question 6

Define and Describe the Translation stage of protein synthesis.

Answer 6

• When the polypeptide chain is formed using both the mRNA base sequence and the tRNA.

Method:
- mRNA from the nucleus attaches to the ribosomes in the cytoplasm at the START codon.
- The tRNA codon with the complementary anticodon to the START codon aligns opposite the mRNA, both being held in place by the ribosomes.
- The ribosome moves along the mRNA molecule allowing the next complementary tRNA anticodon molecule to attach to the next codon.
- The two amino acids that have been delivered by the tRNA molecules are joined by a peptide bond (catalysed by peptyl transferase and requires ATP).
- The ribosome moves along the mRNA strand again, and the first tRNA molecule can detach and leave as it is now empty.
- The tRNA molecules continue delivering amino acids which are joined by peptide bonds, forming the peptide chain.
- When the ribosome reaches the STOP codon at the end of the mRNA molecule, it detaches, and the translation ends.
- The polypeptide chain then goes to the Golgi apparatus for folding and modification.