9: Genes & Protein Synthesis

Question 1

Gene Definition:
Allele Definition:
What is the Locus?

Answer 1

• section of DNA that codes for 1 polypeptide
• a different variants of a gene
• the genes fixed position on a particular chromosome

Question 2

Why are DNA and proteins vital for cell function?

Answer 2

• DNA carries the genetic code to allow the cell to make proteins
• enzymes (proteins) are needed to control chemical processes going on inside cells

Question 3

How many different Amino Acids could be coded for?
How many different Amino Acids are actually coded for?

Answer 3

1. 64
2. 20

Question 4

Describe the 3 important features of Genetic Code:

Answer 4

1. the code is non-overlapping:
   
   • each base is only apart of one triplet code
2. the code is degenerate
   
   • most amino acids are coded for by more than one triplet, due to 64 different triplet codes but only 20 amino acids
3. the code is universal:
   - given triplet specifies the same amino acid in all organisms

Question 5

What are the 3 types of RNA?

Answer 5

• ribosomal RNA
• messenger RNA
• transfer RNA

Question 6

Describe the structure/role of rRNA:

Answer 6

• rRNA and proteins form ribosomes, this is the site of mRNA translation and protein synthesis
• rRNA is coded for by numerous genes in many different chromosomes

Question 7

Describe the structure/role of mRNA:

Answer 7

• mRNA consists of lots of nucleotides in a single linear strand
• mRNA is formed through the transcription of a gene in DNA (from nucleus), of which is complementary to the DNA in its base sequence
• amino acids are coded for by triplet of bases on mRNA called codons
• mRNA contains unpaired bases, so is easily broken down in cytoplasm (only needs to exist temporarily until protein is manufactured

Question 8

Describe the structure/role of tRNA:

Answer 8

• relatively small molecule (about 80 nucleotides), which is a single strand which folds back on itself
• the tRNA molecule forms hydrogen bonds within complementary sections of this molecule, to stabilise the molecule
• one end of chain attaches to an amino acid
• each tRNA molecule carries a specific amino acid
• at the base of tRNA molecule, there is a sequence of 3 bases called anticodon (each amino acid has a different anticodon)

Question 9

Where does Transcription take place?
Where does Translation take place?

Answer 9

• nucleus
• ribosome

Question 10

Describe Transcription:

Answer 10

1. DNA helix unwinds:
   - hydrogen bonds between DNA bases are broken, separating the two strands of DNA, by the enzyme DNA helicase
2. One strand of DNA acts as a template strand with exposed DNA bases
3. free RNA nucleotides, found in nucleoplasm, attracted to exposed DNA bases on the template strand, aligning by complementary base pairing
4. RNA polymerase joins adjacent nucleotides together to form pre mRNA, as well as forming phosphodiester bonds between adjacent nucleotides

Question 11

Describe Splicing: (eukaryotic)

Answer 11

• exons are sections of DNA that are expressed to produce proteins, and introns are intevening sequences of junk DNA
• in pre-mRNA (eukaryotic), introns are spliced by enzymes/other molecules and extrons are joined together to form mRNA which exits nucleus into cytoplasm via nuclear pores

Question 12

Describe protein synthesis: (prokaryotic)

Answer 12

• transcription takes place in cytoplasm of cell and involves of functional mRNA, that is complementary to bases of DNA (same as eukaryotes)
• no splicing/pre mRNA as prokaryotes do not contain non-coding sections
• translation (occurs on ribosomes), involving translation of mRNA into sequence of amino acids to form polypeptide

Question 13

Describe Translation:

Answer 13

1. each ribosome contains two subunits (1 large 1 small), which binds to start codon on mRNA
2. ribosome can hold 2 tRNA molecules
3. specific tRNA molecules with anticodon to the mRNA start codon, will base pair and the mRNA start codon carries the amino acid Methionine. this requires ATP to attach amino acid to tRNA
4. a different tRNA molecule with a specific anticodon will base pair to the complementary mRNA codon, carrying the second amino acid
5. this new amino acid forms a peptide bond with the start amino acid, this requires ATP hydrolysis to provide the required energy, and an enzyme
6. empty tRNA now moves away from ribosome back to cytoplasm to pick up another molecule of the same amino acid e.g methionine, ribosome now moves onto next codon
7. this process is repeated until the ribosome comes to a STOP codon, therefore no more tRNA molecules will attach to it, so the tRNA molecule and its polypeptide chain detaches from ribosome
8. polypeptide is now free in cytoplasm to fold into tertiary stucture