3.4.1 and 3.2.2 DNA, GENES & CHROMOSOMES, PROTEIN SYNTHESIS

Question 1

Describe the DNA in prokaryotic cells: (3)

Answer 1

• Short
• Circular
• Not associated with proteins/histones

Question 2

Describe the DNA in eukaryotic cells: (4)

Answer 2

• long
• Linear
• Associated with proteins called histones
• DNA molecule and its associated histones form a chromosomes.

Question 3

Describe the DNA in mitochondria and chloroplast of eukaryotic cells: (3)

Answer 3

• Short
• Circular
• Not associated with proteins/histones

Question 4

What is a gene?

Answer 4

• base sequence of DNA that codes for the amino acid sequence of a polypeptide and a functional RNA (including mRNA and tRNA)

Question 5

What does the gene occupy?

Answer 5

Occupies a fixed position called a LOCUS on a particular DNA molecule.

Question 6

What is a triplet?

Answer 6

A sequence of three DNA bases which codes for a specific amino acids.

Question 7

The genetic code a sequence of base triplets in mRNA is universal, non overlapping, degenerate: what do these words mean?

Answer 7

1. UNIVERSAL = The same specific DNA base triplets code for the same amino acids in all living organisms
2. NON-OVERLAPPING = discrete, each base can only be used once and in only one triplet.
3. DEGENERATE = the same amino acid can be coded for by more than one base triplet.

Question 8

What are exons?

Answer 8

sequences of DNA bases which code for amino acids.

Question 9

What are introns?

Answer 9

non-coding sequences (separate exons by one)

Question 10

What’s a genome?

Answer 10

The complete set of genes in a call including those in organelles.

Question 11

What’s a proteome?

Answer 11

the full range of proteins that a cell/genome is able to produce.

Question 12

What are alleles?

Answer 12

Different versions of the same gene (different bases/triplets)

Question 13

What are homologous pair of chromosomes?

Answer 13

same shape+size chromosomes with the same genes on the same gene loci but different alleles.

Question 14

Describe the structure of messenger RNA (mRNA): (5)

Answer 14

1. made by transcription in the nucleus
2. Straight, single polynucleotide strand
3. Carries genetic code to the ribosomes for protein synthesis.
4. Sequence of bases (codon) determines the sequence of amino acids in polypeptide chain.
5. Chemically unstable - breaks down after a few days.

Question 15

Describe the structure of transfer RNA (tRNA): (4)

Answer 15

= transfers amino acids to the ribosome and mRNA in translation

1. Single polynucleotide strand folded into a clover leaf shape
2. Hydrogen bonds between C-G, A-U
3. Carries an amino acid binding site (attachment site)
4. 3 free bases = anti codon which is complementary to mRNA codon.

Question 16

What’s a similarity between mRNA and tRNA?

Answer 16

Both a single polynucleotide strand.

Question 17

What are the 3 differences between mRNA and tRNA?

Answer 17

• mRNA is a single helix/straight but tRNA is folded into a clover shape.
• mRNA is longer
• mRNA containers no paired bases or H bonds but tRNA does.

Question 18

What is a codon?

Answer 18

groups of three adjacent bases on mRNA

Question 19

What is an anti-codon?

Answer 19

specific sequence of 3 free bases on tRNA.

Question 20

What is transcription and where does it happen in eukaryotes and prokaryotes?

Answer 20

the production of mRNA from DNA in the nucleus for eukaryotes.
cytoplasm in prokaryotes.

Question 21

Describe the stages of transcription: (4)

Answer 21

1. DNA helicase unzips and separated the two strands - H bonds broken
2. free RNA nucleotides align next to their complementary bases on the template strand - U replaces T in RNA
3. RNA polymerase line up and joins adjacent nucleotides - condensation reaction, forming phosphodiester bonds and goes on to assemble the rest.
4. Hydrogen bonds reform.
5. mRNA leaves nucleus via nuclear pores to the ribosomes.

Question 22

What happens to the mRNA in eukaryotes?

Answer 22

Transcription results in the production of pre-mRNA which has to be spliced to remove introns and join exons to form mRNA.

Question 23

What happens to the mRNA in prokaryotes?

Answer 23

transcription results in the production of mRNA directly from the DNA so no splicing required.

Question 24

What is translation and where does it occur?

Answer 24

The production of polypeptides from the sequence of codons carried by mRNA.
At the ribosomes in the cytoplasm.

Question 25

What are the stages of translation: (5)

Answer 25

1. mRNA attaches itself to a ribosome at the start codon.
2. tRNA carries specific amino acids and an anti-codon that is complementary to the first codon on mRNA. A second tRNA does the same thing to the next codon.
3. Two amino acids joined by condensation reaction form a peptide bond using energy from ATP.
4. tRNA detaches (without its amino acid) and ribosome moves along mRNA to next codon.
5. process continues until stop signal in the mRNA molecule and polypeptide chain is released.

Question 26

What is the role of ribosomes? (2)

Answer 26

• attaches to mRNA and houses tRNA allowing codon-anti-codon complementary bases to pair
• allows peptide bonds to form

Question 27

What is the role of tRNA? (4)

Answer 27

• attaches to and transported specific amino acids in relation to its anti-codon
• its complementary to mRNA codon, forming H bonds.
• two tRNA’s bring amino acids together to form peptide bonds
• there are about 60 different types of tRNA’s to carry 20 different amino acids –> degenerate.

Question 28

What is the role of ATP? (3)

Answer 28

• Hydrolysis of ATP releasing energy
• for the bond between amino acid and its corresponding tRNA molecule for amino acid to attach.
• for peptide bond formation