Protein Synthesis

Question 1

what is the amino acid sequence of a protein coded by?

Answer 1

triplets (codons) in the sequence of bases along a strand of DNA.

Question 2

what is a gene?

Answer 2

a section of DNA that contains the complete sequence of codons to code for an entire protein.

Question 3

where is DNA contained within a eukaryotic cell?

Answer 3

in the nucelus, contained by the double membrane nuclear envelope - protecting the DNA from being damaged.

Question 4

where does protein synthesis occur?

Answer 4

in the cytoplasm at the ribosomes.

Question 5

what is transcription?

Answer 5

the base sequences of genes have to be copied and transported to the ribosome. and produces shorter molecules of RNA from DNA.

Question 6

does transcription result in a different polynucleotide?

Answer 6

yes.

Question 7

is the DNA unzipped during transcription?

Answer 7

yes - under the control of DNA helicase, beginning at a start codon; involves breaking of hydrogen bonds between the bases.

Question 8

what is the sense strand?

Answer 8

only one of the two strands of DNA contains the code for the protein to be synthesised - this is the sense strand.

Question 9

does the sense strand run from 5’ to 3’ ?

Answer 9

yes

Question 10

what is the antisense strand?

Answer 10

a complementary copy of the sense strand which does not code for a protein.

Question 11

does the antisense strand run from 3’ to 5’ ?

Answer 11

yes

Question 12

what is the role of the antiense strand?

Answer 12

acts as a template strand during transcription so complementary RNA strand formed carries the same base strand as the sense strand.

Question 13

what happens after the DNA helix is unzipped?

Answer 13

free RNA nucleotides will base pair with complementary bases exposed on the antisense strand when the DNA unzips. The thymine base in RNA is replaced with uracil so uracil binds with adenine on DNA template strand.

Question 14

what bonds are formed between RNA nucleotides?

Answer 14

phosphodiester bonds.

Question 15

which enyzme forms phosphodiester bonds between RNA nucleotides?

Answer 15

RNA polymerase.

Question 16

when does transcription stop?

Answer 16

at the end of a gene.

Question 17

what is mRNA?

Answer 17

the completed short strand of RNA produced at the end of transcription.

Question 18

what happens after mRNA is formed?

Answer 18

it detaches from the DNA template and leaves the nucleus through the nuclear pores. The DNA double helix reforms. the mRNA then travels to the ribosome in the cell cytoplasm for protein synthesis.

Question 19

how many subunits are ribosomes made up of?

Answer 19

2 - one small and one large.

Question 20

what are the subunits of ribosomes composed of?

Answer 20

• almost equal amounts of protein.
• form of RNA called ribosomal RNA.

Question 21

what is the function of ribosomal RNA?

Answer 21

it is important in maintaining the structural stability of the protein synthesis sequence and plays a biochemical role in catalysing the reaction.

Question 22

what is translation?

Answer 22

when mRNA binds to a specific site on the small subunit of a ribosome. the ribosome holds mRNA in position while it is translated into a sequence of amino acids. (translation)

Question 23

what is transfer RNA?

Answer 23

another form of RNA which is necessary for the translation of mRNA.

Question 24

what is transfer RNA composed of?

Answer 24

a strand of RNA folded in such a way that three bases, called the anticodon, are at one end of the molecule.

Question 25

what does the anticodon on tRNA do?

Answer 25

the anticodon will bind to a complemetary codon on mRNA following the normal base pairing rules. the tRNA carries an amino acid corresponding to that codon.

Question 26

what happens when tRNA anticodons bind to complementary codons along te mRNA?

Answer 26

the amino acids are brought together in the correct sequence to form the primary structure of the protein coded for by the mRNA.

Question 27

are amino acids added all at once or one at a time?

Answer 27

one at a time.

Question 28

what happens when amino acids are added one at a time?

Answer 28

the polypeptide chain (protein) grows

Question 29

what are the roles of the ribosomes?

Answer 29

act as a binding site of mRNA and tRNA and catalyse the assembly of the protein.

Question 30

what is the sequence of events during translation?

Answer 30

1. the mRNA binds to the small subunit of the ribosome at its start codon (AUG)
2. a tRNA with complementary anticodon (UAC) binds to the mRNA start codon. this tRNA carries the amino acid methionine.
3. another tRNA with the anticodon UGC and carrying the corresponding amino acid; threonine, then binds to the next codon on the mRNA (ACG). A maximum of two tRNAs can be bound at the same time.
4. the first amino acid, methionine, is trasferred to the amino acid (threonine) on the second tRNA by the formation of a peptide bond. This is catalysed by the enzyme peptidyl trasferase, which is an rRNA component of the ribosome.
5. the ribosome then moves along the mRNA, releasing the first tRNA. the second tRNA becomes the first.

• stages 3 to 5 are repeated, which an amino acid added to the chain each time.
• this process keeps repeating until the ribosome reaches the end of the mRNA at a stop codon and the polypeptide is released.

Question 31

what forms the primary structure of a protein?

Answer 31

amino acids joined together.

Question 32

what determines the folding of proteins into their secondary and teriary structures?

Answer 32

the sequence of amino acids in the primary structure.

Question 33

how can multiple identical polypeptides be synthesised simultaneously

Answer 33

many ribosomes can follow the mRNA behind the first so that multiple identical polypeptides can be synthesised simultaneously.