3.1.8 Protein Synthesis

Question 1

When is mRNA made?

Answer 1

During transcription

Question 2

Describe the structure of mRNA

Answer 2

It’s a single polynucleotide strand (& has groups of 3 adjacent bases)

Question 3

What are 3 adjacent bases called?

Answer 3

Codons (or sometimes called triplets or base triplets)

Question 4

What does mRNA do?

Answer 4

Carries genetic code from DNA to ribosomes & is then used to make a protein during translation

Question 5

What does tRNA do?

Answer 5

Carries amino acids that are used to make proteins to ribosomes

(involved in translation)

Question 6

Describe the structure of tRNA

Answer 6

It’s a single polynucleotide strand that’s folded into clover shape

Question 7

How does tRNA stay in a clover shape?

Answer 7

H-bonds between specific base pairs hold it in shape

Question 8

What does every tRNA molecule have? (2x)

Answer 8

• Anticodon at one end - specific sequence of 3 bases
• An amino acid binding site at other end

Question 9

What is the main thing that happens in transcription?

Answer 9

mRNA copy of gene is made from DNA

Question 10

In eukaryotic cells, where does transcription takes place?

Answer 10

Nucleus

Question 11

Describe the stages in transcription

Answer 11

1. DNA helicase (in eukaryotes) breaks H-bonds between 2 DNA strands (in beginning of a gene)
2. Only one DNA strand acts as a template (to make mRNA copy)
3. Free bases in RNA nucleotides are attracted to exposed bases
4. Attraction occurs according to complementary base pairing (∴ mRNA strand becomes a complementary copy of DNA template strand)
5. RNA nucleotides are joined together by RNA polymerase
6. (In eukaryotes) mRNA moves out through nuclear pore

Question 12

Where does transcription take place in prokaryotes?

Answer 12

Cytoplasm

Question 13

When does the RNA polymerase stop making mRNA and detachs from DNA in transcription?

Answer 13

When RNA polymerase reaches a particular sequence of DNA called stop signal

Question 14

Where does translation occur in both eukaryotes and prokaryotes?

Answer 14

At ribosomes in cytoplasm

Question 15

What is the main thing occuring during translation?

Answer 15

Amino acids are joined together to make polypeptide chain, following the sequence of codons (triplets) carried by mRNA

Question 16

Describe the stages in translation

Answer 16

1. mRNA moves into cytoplasm & attaches to ribosome
2. tRNA carry amino acids to it
3. Specific tRNA molecule for specific amino acid
4. Anitcodon of tRNA binds to complementary codon on mRNA (attaches by specific base pairing)
5. Peptide bond forms between amino acids
6. tRNA detaches and collects another amino acid
7. Ribosome moves along mRNA = forming a long polypeptide chain

Question 17

What determines how the folded protein structure of protein will be?

Answer 17

Position of R groups in polypeptide chain

Question 18

What issue can happen as polypeptide chains are formed on a ribosome?

Answer 18

• Regions where amino acids have hydrophobic R groups can to clump together or join with other hydrophobic molecules spontaneously
• Leads to forming a non-functional, ‘mis-folded’ protein & many of them = disease

Question 19

What do cells produce to ensure the polypeptide chains made on ribosomes are folded correctly?

Answer 19

Chaperone proteins

Question 20

Describe the role of chaperone proteins (3)

Answer 20

• Chaperone proteins bind to hydrophobic groups (on the secretory proteins)
• Prevent incorrect hydrophobic interactions
• Allows protein to be correctly folded

Question 21

Where are chaperone proteins are found?

Answer 21

In endoplasmic reticulum

Question 22

What is the genetic code?

Answer 22

Sequence of base triplets (codons) in mRNA which code for specific amino acids

Question 23

Name 3 qualities of the genetic code

Answer 23

1. Non-overlapping
2. Degenerate
3. Universal

Question 24

Describe how genetic code is non-overlapping

Answer 24

Each base triplet is read in sequence only once

Question 25

Describe how genetic code is degenerate

Answer 25

Some amino acids are coded by more than one base triplet

e.g. tyrosine can be coded for by UAU or UAC

Question 26

Describe how genetic code is universal

Answer 26

Same specific base triplets code for same amino acids in all living things

e.g. UAU codes for tyrosine in all organisms

Question 27

What are start/stop signals (or codons)?

Answer 27

Triplets that tell the cell when to start/stop production of a protein

Question 28

Transcription Makes _______ Products in Eukaryotes and Prokaryotes

Answer 28

Different

Question 29

What does transcription make in eukaryotes?

Answer 29

pre-mRNA

Question 30

What is pre-mRNA?

Answer 30

mRNA strands containing introns and exons

Question 31

Why does pre-mRNA contain introns and exons?

Answer 31

Introns and exons are both copied into mRNA during transcription

Question 32

Name and describe the process how pre-mRNA is made into mRNA

Answer 32

Process called splicing occurs: introns are removed and exons join together = forming mRNA strands

Question 33

Where does splicing occur?

Answer 33

In nucleus

Question 34

What does transcription make in prokaryotes?

Answer 34

mRNA - it’s produced directly from DNA without splicing

Question 35

Transcription

What enzyme separates the strands of DNA in prokaryotes?

Answer 35

RNA polymerase

Question 36

Explain why the percentage of bases from the middle part of the chromosome and end part are different (2)

Answer 36

• Different genes
• Have different base sequences
• Codes for different proteins

Question 37

Different sequences of bases code for different proteins. Explain how. (2)

Answer 37

• Protein made up of (chain of) amino acids
• Each amino acid has its own base code

Question 38

Explain how copying bases more than once may give rise to differences in the protein (2)

Answer 38

Changes base sequence of later triplets/amino acid codes