Protein Synthesis

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课程笔记

Transcription: Making mRNA

Transcription is the first stage of protein synthesis, occurring in the nucleus.
Part of the DNA molecule unwinds and hydrogen bonds between complementary bases break, exposing the gene to be transcribed.
Free RNA nucleotides pair with complementary bases on the template strand of DNA.
RNA polymerase bonds the sugar-phosphate groups of RNA nucleotides to form the mRNA backbone.
The mRNA molecule is a single-stranded copy of the gene, complementary to the template strand.
After transcription, hydrogen bonds between mRNA and DNA break, and the DNA double helix re-forms.
mRNA leaves the nucleus through a nuclear pore and moves into the cytoplasm.
DNA is too large to leave the nucleus, so mRNA acts as a messenger carrying the genetic code.

Protein synthesis

Complementary Base Pairing in Transcription

In RNA, adenine pairs with uracil (not thymine), and cytosine pairs with guanine.
The DNA strand that carries the genetic code is the coding strand; the opposite strand is the template strand.
The template strand is transcribed to produce mRNA, which is complementary to it.
Example: DNA template strand TAC GGA AGA CTT GGG produces mRNA AUG CCU UCU GAA CCC.
RNA polymerase is the enzyme for transcription; DNA polymerase is for DNA replication — do not confuse them.

Translation: Synthesising Polypeptides

Translation takes place in the cytoplasm and uses the genetic code on mRNA to build a polypeptide.
mRNA attaches to a ribosome, which is made of a large and small subunit and ribosomal RNA (rRNA).
The mRNA binds to the small subunit; two tRNA molecules can bind to the large subunit simultaneously.
tRNA molecules carry specific amino acids to the mRNA on the ribosome.
The anticodon on tRNA pairs with a complementary codon on mRNA.
A peptide bond forms by condensation between adjacent amino acids, requiring ATP.
The ribosome moves along the mRNA one codon at a time until a stop codon is reached.
The completed amino acid chain is released from the ribosome and forms the final polypeptide.

Codons, Anticodons and tRNA Structure

A triplet is a sequence of three DNA bases that codes for a specific amino acid.
A codon is a sequence of three mRNA bases that codes for a specific amino acid.
An anticodon is a sequence of three tRNA bases complementary to a codon.
The anticodon is located at the bottom of the tRNA molecule and consists of three exposed RNA bases.
tRNA molecules bind to their specific amino acids before participating in translation.
tRNA-activating enzymes attach amino acids to their corresponding tRNA molecules.
Complementary base pairing between mRNA codons and tRNA anticodons ensures the correct amino acid sequence.
Example: if an mRNA codon is CAG, the complementary tRNA anticodon is GUC.

Features of the Genetic Code

The genetic code is a triplet code: each sequence of three bases codes for one amino acid.
There are 20 different amino acids used to make proteins.
The code is non-overlapping: each base is read only once as part of a codon.
There are 64 possible codons (4³) but only 20 amino acids, so the code is degenerate (multiple codons can code for the same amino acid).
The genetic code is universal: almost all organisms use the same code, enabling genetic engineering.
Some triplets code for start (TAC – methionine) and stop signals, marking where genes begin and end.
Degeneracy can limit the effect of mutations because a change in the third base may still code for the same amino acid.

Deducing Amino Acid Sequences

To deduce an amino acid sequence from a DNA coding strand, first work out the template strand using complementary base pairing (A-T, C-G).
Then work out the mRNA codons, complementary to the template strand (A-U, C-G).
Use an mRNA codons and amino acids table to identify the amino acid for each codon.
Example: DNA coding strand TTC GAG CAT TAC GCC gives mRNA UUC GAG CAU UAC GCC, which codes for Phe-Glu-His-Tyr-Ala.
The number of amino acids can be calculated by dividing the number of mRNA nucleotides by 3.

Gene Expression and DNA Stability

The human genome contains approximately 20,000 protein-coding genes.
Not every protein is needed in every cell; gene expression switches genes on or off.
Genes that are expressed are 'switched on' and undergo transcription and translation.
Genes that are not expressed are 'switched off' or silenced.
Transcription is a key stage at which gene expression can be controlled.
DNA is very stable due to hydrogen bonds between bases and strong phosphodiester bonds between nucleotides.
This stability allows DNA strands to act as reliable templates over generations of cell replication.

Protein Structure and Mutations

A gene mutation is a change in the base sequence of DNA, which may result in a new allele.
Mutations occur randomly and are copying errors during DNA replication in S phase of interphase.
Mutations in body cells are not inherited; mutations in gametes are inherited by offspring.
Most mutations are harmful or neutral, but some can be beneficial.
A point mutation alters one base and can change a single amino acid in the polypeptide.
Sickle cell disease is caused by a point mutation: DNA triplet GAG changes to GTG on the coding strand.
This results in mRNA codon GUG instead of GAG, so valine replaces glutamic acid at position 6 of the β-globin chain.
The altered haemoglobin (HbS) distorts red blood cells into a sickle shape, reducing oxygen capacity and blocking capillaries.

The sickle cell mutation

Effects of Sickle Cell Mutation

Sickle-shaped red blood cells have a limited oxygen-carrying capacity.
They can block capillaries and limit the flow of normal red blood cells.
People with sickle cell anaemia suffer from acute pain, fatigue and anaemia.
There is a correlation between the global distribution of sickle cell disease and malaria.
In areas with high malaria incidence, the sickle cell allele is more frequent, likely due to increased resistance to the malaria parasite in heterozygotes.

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练习题

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1.In which part of the cell does transcription occur?
Easy
- ANucleus
- BCytoplasm
- CRibosome
- DMitochondria
2.Which enzyme is responsible for catalysing the formation of the sugar-phosphate backbone of mRNA during transcription?
Easy
- ADNA polymerase
- BRNA polymerase
- CtRNA-activating enzyme
- DHelicase
3.Which of the following correctly describes the role of the ribosome in protein synthesis?
Easy
- AIt catalyses the formation of peptide bonds between amino acids
- BIt transcribes DNA into mRNA
- CIt carries amino acids to the mRNA
- DIt unwinds the DNA double helix
4.A tRNA molecule has the anticodon UAC. Which mRNA codon will it pair with?
Medium
- AAUG
- BTAC
- CUAC
- DATG
5.Which of the following statements about the genetic code are correct? (select all that apply)
Medium
- AIt is degenerate, meaning multiple codons can code for the same amino acid.
- BIt is non-overlapping, meaning each base is read only once.
- CIt is universal, meaning almost all organisms use the same code.
- DIt is overlapping, meaning each base is read multiple times.
- EIt contains 64 codons, each coding for a different amino acid.
6.The genetic code is degenerate, meaning that a single amino acid can be coded for by more than one codon.
Easy
True or false?
7.During translation, the mRNA molecule is used as a template to synthesise a polypeptide chain.
Easy
True or false?
8.Match each term with its correct definition.
Medium
- Codon
- Anticodon
- Triplet
- A sequence of three mRNA bases that codes for a specific amino acid
- A sequence of three tRNA bases that are complementary to a codon
- A sequence of three DNA bases that codes for a specific amino acid

更多题目

9.Place the following stages of protein synthesis in the correct order.
Medium
- mRNA binds to the small subunit of a ribosome
- tRNA molecules bring specific amino acids to the ribosome
- Peptide bonds form between adjacent amino acids
- The polypeptide chain is released when a stop codon is reached
10.A DNA coding strand has the sequence TAC GGA AGA CTT GGG. What is the sequence of the mRNA transcript?
Medium
- AAUG CCU UCU GAA CCC
- BTAC GGA AGA CTT GGG
- CATG CCT TCT GAA CCC
- DUAC GGA AGA CUU GGG
11.Sickle cell disease is caused by a point mutation in the β-globin gene. The DNA triplet GAG on the coding strand is changed to GTG. What is the resulting change in the amino acid sequence?
Medium
- AGlutamic acid is replaced by valine at position 6
- BValine is replaced by glutamic acid at position 6
- CGlutamic acid is replaced by lysine at position 6
- DValine is replaced by leucine at position 6
12.During transcription, the template strand of DNA is the one that is transcribed to form mRNA.
Easy
True or false?
13.Which of the following are features of the genetic code? (select all that apply)
Medium
- AIt is a triplet code.
- BIt is overlapping.
- CIt is universal.
- DIt is degenerate.
- EIt contains 20 codons in total.
14.Which of the following correctly describes the role of tRNA-activating enzymes?
Medium
- AThey attach amino acids to tRNA molecules.
- BThey transcribe DNA into mRNA.
- CThey form peptide bonds between amino acids.
- DThey unwind the DNA double helix.
15.Match each molecule with its correct description.
Medium
- mRNA
- tRNA
- rRNA
- Carries the genetic message from DNA to the ribosome
- Brings amino acids to the ribosome
- Forms part of the structure of the ribosome
16.Place the following events of transcription in the correct order.
Medium
- DNA unwinds and hydrogen bonds break
- Free RNA nucleotides pair with exposed bases on the template strand
- RNA polymerase bonds the sugar-phosphate groups together
- mRNA leaves the nucleus via a nuclear pore
17.The DNA coding strand sequence is TTC GAG CAT TAC GCC. Using the rules of base pairing, what is the sequence of the template strand?
Medium
- AAAG CTC GTA ATG CGG
- BTTC GAG CAT TAC GCC
- CUUC GAG CAU UAC GCC
- DAAG CUC GUA AUG CGG
18.Mutations in gametes can be inherited by offspring.
Easy
True or false?
19.Which of the following statements about the process of transcription are correct? (select all that apply)
Medium
- ATranscription occurs in the nucleus of the cell.
- BRNA polymerase bonds the sugar-phosphate groups of RNA nucleotides together.
- CThe template strand of DNA is transcribed to form mRNA.
- DFree RNA nucleotides pair with exposed bases on both strands of DNA.
- EThymine in DNA pairs with adenine in the new RNA strand.
20.Which of the following are roles of the ribosome in translation? (select all that apply)
Medium
- AIt binds the mRNA molecule at the small subunit.
- BIt allows two tRNA molecules to bind simultaneously at the large subunit.
- CIt catalyses the formation of peptide bonds between amino acids.
- DIt synthesises mRNA from DNA.
- EIt carries amino acids to the mRNA.
21.Which of the following statements about tRNA are correct? (select all that apply)
Medium
- AtRNA molecules bind with specific amino acids in the cytoplasm.
- BThe anticodon is a sequence of three bases on tRNA that is complementary to an mRNA codon.
- CtRNA-activating enzymes attach amino acids to tRNA molecules.
- DtRNA contains the codon that codes for an amino acid.
- EtRNA is made of two strands held together by hydrogen bonds.
22.A DNA coding strand has the sequence TAC GGA AGA CTT GGG. Which of the following statements are correct? (select all that apply)
Hard
- AThe template strand sequence is ATG CCT TCT GAA CCC.
- BThe mRNA transcript is AUG CCU UCU GAA CCC.
- CThe mRNA transcript is UAC GGA AGA CUU GGG.
- DThe first amino acid coded for is methionine.
- EThe DNA template strand is identical to the mRNA transcript.
23.Match each molecule or structure with its correct description.
Medium
- mRNA
- tRNA
- Ribosome
- RNA polymerase
- Carries the genetic message from DNA to the cytoplasm
- Brings specific amino acids to the ribosome
- Catalyses the formation of peptide bonds
- Links RNA nucleotides to form the sugar-phosphate backbone
24.Match each stage of protein synthesis with its correct location.
Medium
- Transcription
- Translation
- DNA replication
- Nucleus
- Cytoplasm
25.Place the following events of translation in the correct order.
Medium
- mRNA binds to the small subunit of the ribosome
- tRNA molecules bind with specific amino acids in the cytoplasm
- Anticodons of tRNA pair with complementary codons on mRNA
- Peptide bonds form between adjacent amino acids
- A stop codon is reached and the polypeptide is released
26.Place the following steps of transcription in the correct order.
Medium
- Hydrogen bonds between complementary DNA bases break and the DNA unwinds
- Free RNA nucleotides pair with exposed bases on the template strand
- RNA polymerase bonds the sugar-phosphate groups of RNA nucleotides together
- Hydrogen bonds between mRNA and DNA break and the DNA double helix re-forms
- mRNA leaves the nucleus through a pore in the nuclear envelope
27.The enzyme RNA polymerase is involved in transcription, while DNA polymerase is involved in DNA replication.
Easy
True or false?
28.During translation, the mRNA molecule moves along the ribosome one codon at a time.
Medium
True or false?
29.The genetic code is overlapping, meaning that each base is read as part of more than one codon.
Medium
True or false?
30.Mutations that occur in gametes can be inherited by offspring.
Easy
True or false?
31.A DNA template strand has the sequence 3'-TAC GGA AGA CTT GGG-5'. Which mRNA transcript is produced from this template strand?
Medium
- AAUG CCU UCU GAA CCC
- BTAC GGA AGA CTT GGG
- CUAC GGA AGA CUU GGG
- DATG CCT TCT GAA CCC
32.In a section of DNA the coding strand reads 5'-GCCGTAAC-3'. Which mRNA sequence is transcribed from this gene?
Medium
- A5'-GCCGUAAC-3'
- B5'-CGGCATTG-3'
- C5'-GCCGTAAC-3'
- D5'-CGGCAUUG-3'
33.Myoglobin is a eukaryotic protein consisting of a single polypeptide chain of 153 amino acids. Calculate the minimum number of DNA bases needed to code for myoglobin.
Hard
- A459
- B153
- C306
- D51
34.A patient has a mutation that changes the mRNA codon GAG to GUG. Which amino acid change occurs in the polypeptide?
Medium
- AGlutamic acid is replaced by valine
- BValine is replaced by glutamic acid
- CGlutamic acid is replaced by glycine
- DNo amino acid change occurs
35.The diagram shows part of the mRNA sequence and its corresponding amino acid sequence for cytochrome c in two species. The amino acid isoleucine (Ile) is coded for by three different codons. What property of the genetic code does this demonstrate?
Challenge
- ADegeneracy
- BUniversality
- CNon-overlapping
- DComplementary base pairing
36.A tRNA molecule has the anticodon 5'-UAC-3'. Which mRNA codon will it pair with?
Hard
- A5'-AUG-3'
- B5'-UAC-3'
- C5'-GUA-3'
- D5'-CAU-3'
37.A student is investigating transcription in a eukaryotic cell. Which of the following correctly identifies the enzyme responsible for catalysing the formation of the sugar-phosphate backbone of mRNA, and the location where this process occurs?
Medium
- ARNA polymerase; nucleus
- BDNA polymerase; nucleus
- CRNA polymerase; cytoplasm
- DDNA polymerase; cytoplasm
38.Which of the following statements about the experimental technique of using a DNA sequencing machine are correct? (select all that apply)
Medium
- AIt can sequence 5.5 × 10⁸ base pairs per hour.
- BIt is used to determine the order of bases in a DNA molecule.
- CIt requires the use of RNA polymerase to synthesise new DNA strands.
- DThe human genome is approximately 3 billion base pairs long.
- EIt directly produces a polypeptide chain.
39.The genetic code is non-overlapping, meaning each base is read only once as part of a single codon.
Easy
True or false?
40.Match each molecule or structure with its correct role in protein synthesis.
Medium
- RNA polymerase
- Ribosome
- tRNA
- mRNA
- Catalyses formation of sugar-phosphate backbone of mRNA
- Provides the site and catalyses peptide bond formation
- Carries specific amino acids to the ribosome
- Carries the genetic code from DNA to the cytoplasm
41.Complete the sentence about the role of tRNA.
Easy
During translation, the ____ on tRNA pairs with the complementary ____ on mRNA.
42.Name the enzyme that is responsible for catalysing the process of DNA replication.
Easy
43.Myoglobin is a protein consisting of a single polypeptide chain of 153 amino acids. Calculate the minimum number of DNA bases needed to code for myoglobin.
Medium
44.Complete the sentence about the genetic code.
Easy
The genetic code is described as ____ because multiple codons can code for the same amino acid.
45.A gene has an mRNA molecule consisting of 180 nucleotides. Calculate the number of amino acids that will be coded for by this gene.
Medium
46.Identify the type of bond that joins two amino acids together in a polypeptide.
Medium
47.Name the enzyme that catalyses the formation of the sugar-phosphate backbone of mRNA during transcription.
Medium
48.The human genome is approximately 3 billion (3 × 10⁹) base pairs long. A DNA sequencing machine can sequence 5.5 × 10⁸ base pairs per hour. Calculate the number of days it would take to sequence 1500 genomes. Give your answer to the nearest day.
Challenge
49.An mRNA molecule contains 300 nucleotides. Calculate the number of amino acids coded for by this mRNA molecule.
Easy
50.A polypeptide consists of 250 amino acids. Calculate the minimum number of nucleotides on the mRNA molecule that codes for it.
Medium
51.A gene contains 1800 DNA base pairs. If the entire gene is transcribed and translated, how many amino acids would the resulting polypeptide contain?
Medium
52.A DNA sequencing machine can sequence 5.5 × 10⁸ base pairs per hour. Calculate the number of days, to the nearest day, it would take to sequence 1500 human genomes, each approximately 3 × 10⁹ base pairs long.
Medium
53.Describe the role of transfer RNA (tRNA) in the process of protein synthesis.
Medium
54.Outline why two different alleles result in different expressions of a gene.
Hard
55.Describe the changes that occur between polypeptide synthesis and the formation of the functional protein.
Medium
56.Explain why the protein produced as a result of a point mutation may not function as it should.
Hard
57.Outline the process that allows the amino acids to be added to the polypeptide in the correct order.
Hard
58.Myoglobin is a protein consisting of a single polypeptide chain of 153 amino acids. Calculate the minimum number of DNA bases needed to code for myoglobin.
Medium
- A459 bases
- B413 bases
- C505 bases
- D551 bases
59.A gene has an mRNA molecule consisting of 180 nucleotides. Calculate the number of amino acids that will be coded for by this gene.
Medium
- A54 amino acids
- B72 amino acids
- C66 amino acids
- D60 amino acids
60.The human genome is approximately 3 billion (3 × 10⁹) base pairs long. A DNA sequencing machine can sequence 5.5 × 10⁸ base pairs per hour. Calculate the number of days it would take to sequence 1500 genomes. Give your answer to the nearest day.
Challenge
- A307 days
- B375 days
- C409 days
- D341 days
61.An mRNA molecule contains 300 nucleotides. Calculate the number of amino acids coded for by this mRNA molecule.
Easy
- A90 amino acids
- B100 amino acids
- C120 amino acids
- D110 amino acids
62.A polypeptide consists of 250 amino acids. Calculate the minimum number of nucleotides on the mRNA molecule that codes for it.
Medium
- A675 nucleotides
- B825 nucleotides
- C750 nucleotides
- D900 nucleotides
63.A gene contains 1800 DNA base pairs. If the entire gene is transcribed and translated, how many amino acids would the resulting polypeptide contain?
Medium
- A540 amino acids
- B600 amino acids
- C660 amino acids
- D720 amino acids
64.A DNA sequencing machine can sequence 5.5 × 10⁸ base pairs per hour. Calculate the number of days, to the nearest day, it would take to sequence 1500 human genomes, each approximately 3 × 10⁹ base pairs long.
Medium
- A341 days
- B375 days
- C307 days
- D409 days

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