3.10 Protein synthesis

Question 1

Where in a eukaryotic cell is the DNA?

Answer 1

• contained within a double membrane called a nuclear envelope that enclose the nucleus
• this is used to protect the DNA from being damaged within the cytoplasm

Question 2

Where does protein synthesis occur within a cell?

Answer 2

• in the cytoplasm at the ribosomes

Question 3

What is an issue with chromosomal DNA?

Answer 3

• a chromosomal DNA molecule is too large to leave the nucleus to supply the coding information needed to determine the protein’s amino acid sequence

Question 4

What is transcription (simply)?

Answer 4

• the base sequence of genes have to be copied and transported to the site of protein synthesis, a ribosome

Question 5

What does transcription produce?

Answer 5

• shorter molecules of RNA

Question 6

What other process is transcription similar to?

Answer 6

• DNA replication

Question 7

What is the similarity of both transcription and DNA replications process?

Answer 7

• the section of DNA that contains the gene unwinds and unzips under the control of a DNA helicase, beginning at a start codon
• this involves the breaking of hydrogen bonds between the bases

Question 8

What is the sense strand?

Answer 8

• one of the two strands of DNA which contains the code for the protein to be synthesised
• it runs 5’ to 3’

Question 9

What is the antisense strand and what else can it be called?

Answer 9

• the template strand, used during transcription
• 3’ to 5’
• a complementary copy of the sense strand and does not code for a protein

Question 10

What is the template strand used for?

Answer 10

• used during transcription
• so that the complementary RNA strand formed carries the same base sequence as the sense strand

Question 11

What do the free RNA nucleotides join to?

Answer 11

• base pair with complementary bases exposed on an antisense strand when the DNA unzips

Question 12

What is the thymine base of RNA molecules replaced with?

Answer 12

• uracil
• so RNA uracil binds with adenine on the DNA template strand

Question 13

When does transcription stop?

Answer 13

• at the end of the gene

Question 14

What is mRNA?

Answer 14

• messenger RNA
  -the completed short strand of RNA
• it has the same base sequence as the sequence making up the gene on the DNA, except that it has uracil in place of thymine

Question 15

What is RNA polymerase?

Answer 15

• phosphodiester bonds formed between the RNA nucleotides by the enzyme RNA polymerase

Question 16

What happens to the mRNA?

Answer 16

• it detaches from the DNA template and leaves the nucleus through a nuclear pore
• this mRNA molecule then travels to a ribosome in the cell cytoplasm for the next step of protein synthesis

Question 17

What happens to the DNA once mRNA has left the nucleus?

Answer 17

• the DNA double helix reforms

Question 18

What is the structure of ribosomes in eukaryotic cells?

Answer 18

• they are split into two sub-units
• one large and one small

Question 19

What are the sub-units in ribosomes formed from?

Answer 19

• composed of almost equal amounts of protein and a form of RNA known as ribosomal RNA

Question 20

What is rRNA important for?

Answer 20

• maintaining the structural stability of the protein synthesis sequence
• plays a biochemical role in catalysing the reaction

Question 21

What happens to mRNA when it leaves the nucleus and what is this called?

Answer 21

• it binds to a specific sit on the small sub-unit of a ribosome
• the ribosome holds the mRNA in position when it is decoded, or translated, into a sequence of amino acids
• translation

Question 22

What forms an anticodon loop?

Answer 22

• three bases

Question 23

What is transfer (tRNA)?

Answer 23

• another form of RNA
• necessary for the translation of the mRNA
• it is composed of a strand of RNA folded in such a way that three bases called the anti-codon, are at one end of the molecule
• carry an amino acid corresponding to that codon

Question 24

What does an anti-codon do?

Answer 24

• bind to a complementary codon on mRNA following the normal base pairing rules

Question 25

What happens after the tRNA anti-codons bind to complementary codons along the mRNA?

Answer 25

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

Question 26

What is the four step process of translation?

Answer 26

1. The mRNA binds to the small sub-unit of the ribosome at its start codon (AUG)
2. A tRNA with the complementary anti-codon (UAC) binds to the mRNA start codon. this tRNA carries the amino acid methionine
3. Another tRNA with the anti-codon UGC and carrying the corresponding amino acid, threonine, then binds to the next codon on the mRNA (ACG). A maximum of two tRNA’s can be bound at the same time
4. The first amino acid, methionine, is transferred to the amino acid (threonine) on the second tRNA by the formation of a peptide bond. This is catalysed by the enzyme ‘peptidyl transferase’, which is an rRNA component of the ribosome
5. The ribosome then moves along the mRNA, releasing the first tRNA. The second tRNA comes before the first tRNA

Question 27

How long is the translation process continued for?

Answer 27

• the process keeps repeating until the ribosome reaches the end of the mRNA at a stop codon and a polypeptide is released

Question 28

What structures do the amino acids form with proteins?

Answer 28

• form the primary structure of a protein
• then they fold into secondary and tertiary structures

Question 29

What happens to the proteins once they complete protein synthesis?

Answer 29

• they may undergo more modifications at the Golgi apparatus before it is fully functional and ready to carry out specific role for which it has been synthesised

Question 30

Why do ribosomes follow on the mRNA behind the first?

Answer 30

• so that multiple identical polypeptides can be synthesised simultaneously