8: DNA, genes and protein synthesis

Question 1

Gene

Answer 1

section of DNA that contains the coded information to make polypeptides and functional RNA. locus, the location of a gene on a chromosome

Question 2

The genetic code

Answer 2

theres a minimum of 3 bases that could for an amino acid;
- only 20 different amino acids regularly occur in proteins
- only four bases in dna

Question 3

Features of a genetic code

Answer 3

• degenerate (most amino acids are coded for by more than one triplet)
• non overlapping (each base in the sequence is only read once)
• universal (each triplet codes for the same amino acid in all organisms)

Question 4

DNA in eukaryotic and prokaryotic cells

Answer 4

• both made of dna nucleotides containg a deoxyribose, phosphate group and nitrogenous base
• nucleotides are joined together by phosphodiester bonds to make a polymer chain
• eukaryotic is longer, pro is shorter
• eukaryotic is linear, pro is circular
• eukaryotic is associated with histones, prokaryotic is not associated with proteins

Question 5

Mitochondria and chloroplasts of eukaryotic cells contain DNA which is like prokaryotic:

Answer 5

• short
• circular
• not associated with proteins

Question 6

Introns and exons

Answer 6

• introns are sections of dna that dont code for polypeptides. only found in eukaryotic dna
• exons are sequences of dna that do code for amino acids

Question 7

Codons, start codons and stop codons

Answer 7

• codon: three bases on mRNA that code for a specific amino acid
• start code: three bases at the start of every gene that initiate translation
• stop codon; three bases at end of every gene that causes ribosomes to detach and therefore stop translation

Question 8

Genome

Answer 8

• organisms complete set of genes in a cell

Question 9

Proteome

Answer 9

• full range of proteins that a cell is able to produce

Question 10

messenger RNA

Answer 10

• short
• single stranded
• found in cytoplasm and nucleus
• its made during transcription, copied from dna and therefore complementary to dna sequence.
• groups of three adjacent bases are called codons

Question 11

transfer rna (tRNA)

Answer 11

• in cytoplasm, amino acids become attached to tRNA. each amino acid has a specific trna molecule
• each has a sequence of three bases called an anticodon, these are complementary to the codons on the mrna molecule
• involved in translation. carries amino acids used to make proteins to the ribosomes
• single polynucleotide strand that is folded into a clover shape
• h bonds between base pairs hold the shape

Question 12

Protein synthesis

Answer 12

• proteins created on ribosomes
• production of proteins from dna code occurs in two stages;
  1) transcription (one gene from dna is copied into mrna)
  2) translation (mrna joins with ribosome, and corresponding trna brings the specific amino acid the codon codes for)

Question 13

Transcription

Answer 13

• complementary mRNA copy of one gene on the DNA is created in the nucleus
• mrna is much shorter than dna so can carry the genetic code needed to the ribosome in the cytoplasm for the protein to be made

1) dna helix unwinds to expose bases to act as a template
2) only one chain acts as template
3) this is catalysed by DNA helicase which breaks down the hydrogen bonds between bases
4) free mRNA nucleotides in the nucleus align opposite the exposed complementary dna bases
5) the enzyme rna polymerase joins together the rna nucleotides to create a new rna polymer chain. one entire gene is copied.
6) the mrna is modified then leaves through nuclear pores
- modified by splicing, introns are spliced out by a splicesome

Question 14

Translation - polypeptide synthesis

Answer 14

polypeptide chain is created using both the mRNA base sequence and the tRNA
1) modified mrna leaves nucleus and attaches to ribosome in the cytoplasm
2) ribosome attaches at the start codon
3) trna molecule with the complementary anitcodon to start codon aligns opposite the mrna, held in place by a ribosome
4) the ribosome will move along one codon of the mrna to allow the next complementary trna to attach to the next codon on the mrna
5) the two amino acids that have been delivered by the trna are joined by a peptide bond. this is catalysed by an enzyme and requires atp.
6) this continues until the ribosome reaches the stop codon at end of mrna
7) ribosome detaches and translation ends
8) polypeptide is now created and enters golgi body for modification