DNA replication and protein synthesis: flashcards, quiz and summary

Flashcards

1. 1What does semiconservative mean for a newly made DNA molecule?

   Each new DNA molecule contains one original strand from the parent molecule and one newly synthesized strand.

2. 2Why does DNA polymerase only add nucleotides in the 5' to 3' direction, and how does that affect the two strands?

   DNA polymerase can only add nucleotides in the 5' to 3' direction. Because the strands run in opposite directions, the leading strand can be made continuously, but the lagging strand must be made in short Okazaki fragments.

3. 3How do the leading and lagging strands differ?

   The leading strand is built continuously toward the replication fork. The lagging strand is built in short pieces away from the fork, and DNA ligase joins those pieces together.

4. 4What is the central dogma of molecular biology?

   DNA is transcribed into RNA, and RNA is translated into protein. Transcription links DNA to RNA, and translation links RNA to protein.

5. 5Why does RNA have uracil instead of thymine, and how does this change base pairing?

   RNA uses uracil in place of thymine, so adenine in DNA pairs with uracil in RNA, rather than with thymine.

6. 6What does pre-mRNA processing add or remove in eukaryotes, and why?

   A 5' cap and poly-A tail are added to protect the mRNA. Introns are cut out and exons are joined by splicing, so the mRNA carries only the coding sequence.

7. 7How do codons and anticodons work together during translation?

   The ribosome reads codons on mRNA. Each tRNA has an anticodon that pairs with a codon and carries the matching amino acid, and the ribosome links these amino acids into a chain.

8. 8What does it mean that the genetic code is redundant?

   Most amino acids are specified by more than one codon. Because of this, some base changes do not change the amino acid that is added.

9. 9Why are frameshift mutations usually more harmful than point mutations?

   A frameshift changes the reading frame, so every codon after the insertion or deletion is read differently. A point mutation usually changes only one codon.

10. 10How do silent, missense, and nonsense mutations differ?

    A silent mutation changes a base but keeps the same amino acid. A missense mutation changes one amino acid, as in sickle cell anemia. A nonsense mutation creates an early stop codon.

11. 11Where do transcription and translation take place in eukaryotic cells?

    Transcription takes place in the nucleus, where RNA polymerase makes pre-mRNA from DNA. Translation takes place on ribosomes in the cytoplasm.

Practice quiz

Pick an answer to check it.

1. 1Which enzyme unwinds the DNA helix and breaks the hydrogen bonds at the replication fork?

   1. Helicase
   2. DNA polymerase
   3. Primase
   4. DNA ligase
   Show the answer

   A. Helicase Helicase unwinds the helix and breaks hydrogen bonds to open the replication fork. DNA polymerase adds nucleotides, primase makes the RNA primer, and DNA ligase joins Okazaki fragments.

2. 2The template DNA strand reads TAC GGA TTC. Which mRNA is transcribed from it?

   1. TAC GGA TTC
   2. UAC GGA UUC
   3. ATG CCT AAG
   4. AUG CCU AAG
   Show the answer

   D. AUG CCU AAG Each template base pairs with its complement: A becomes U, T becomes A, G becomes C, and C becomes G. This gives AUG CCU AAG. The other options copy the template directly or use thymine, which RNA does not contain.

3. 3Which codon is the start signal for translation?

   1. UGA
   2. AUG
   3. UAG
   4. AAG
   Show the answer

   B. AUG AUG is the start codon and codes for methionine. UGA and UAG are stop codons, and AAG codes for lysine.

4. 4A mutation changes a codon so that it becomes a stop codon early in the gene. What type of mutation is this?

   1. Silent
   2. Frameshift
   3. Nonsense
   4. Missense
   Show the answer

   C. Nonsense A nonsense mutation creates a premature stop codon. A missense mutation changes the amino acid but does not stop translation, and a frameshift involves an insertion or deletion, not a single substitution.

5. 5One base is deleted from the middle of a gene. Why is this usually more harmful than a single base substitution?

   1. DNA polymerase cannot proofread the change
   2. Every codon after the deletion is read in a shifted frame
   3. It removes an intron from the pre-mRNA
   4. It always creates a stop codon right away
   Show the answer

   B. Every codon after the deletion is read in a shifted frame A deletion that is not a multiple of 3 shifts the reading frame, so every codon after it changes. A single substitution usually changes only one codon, so the damage is often smaller.

6. 6Where does translation take place in a eukaryotic cell?

   1. In the nucleus, alongside RNA polymerase
   2. At the replication fork during the S phase
   3. On ribosomes in the cytoplasm
   4. Along the lagging strand of DNA
   Show the answer

   C. On ribosomes in the cytoplasm Translation happens on ribosomes in the cytoplasm, where mRNA is read and amino acids are linked into a polypeptide. The other options describe steps of DNA replication or transcription.

Test traps

What teachers like to test on this, and the mistakes students make.

• Swapping transcription and translation: transcription makes RNA from DNA in the nucleus, and translation makes a protein from mRNA on ribosomes.
• Copying the template strand directly when writing mRNA instead of pairing each base with its complement, and writing T in RNA instead of U.
• Putting codons on tRNA or anticodons on mRNA. The codon is on mRNA and the anticodon is on tRNA.
• Thinking the lagging strand is made in one continuous piece, or forgetting that Okazaki fragments are joined by DNA ligase.
• Forgetting that DNA polymerase only builds in the 5' to 3' direction, which explains why the two strands are made differently.
• Assuming every mutation is harmful: silent mutations keep the same amino acid, missense mutations change one amino acid, and nonsense mutations create a stop codon.

Summary

• DNA is a double helix of two antiparallel strands, with adenine paired to thymine and guanine paired to cytosine by hydrogen bonds.
• DNA replication is semiconservative, so each new molecule keeps one original strand and gets one new strand.
• Helicase unwinds the DNA, primase lays down an RNA primer, and DNA polymerase adds nucleotides only in the 5' to 3' direction while checking for mistakes.
• The leading strand is made continuously, while the lagging strand is made in Okazaki fragments that DNA ligase joins.
• In the central dogma, transcription makes RNA from DNA and translation makes a polypeptide from mRNA.
• In eukaryotes, pre-mRNA gets a 5' cap and a poly-A tail, and introns are removed by splicing before it leaves the nucleus.
• Ribosomes read mRNA codons, and tRNA anticodons bring the matching amino acids, which are linked by peptide bonds.
• Point mutations can be silent, missense, or nonsense, while frameshift mutations from insertions or deletions that are not multiples of 3 usually do more damage.

Key terms

Semiconservative replication

The process in which each new DNA molecule contains one original strand and one newly made strand.

Helicase

The enzyme that unwinds the DNA helix and breaks the hydrogen bonds between base pairs at the replication fork.

DNA polymerase

The enzyme that adds new nucleotides to a growing DNA strand in the 5' to 3' direction and checks for mistakes.

Okazaki fragment

A short piece of DNA made on the lagging strand, which is later joined to the others by DNA ligase.

Codon

A group of three bases on mRNA that codes for one amino acid or signals a stop.

Anticodon

A three-base sequence on tRNA that matches a codon on mRNA and carries the correct amino acid.

Intron

A noncoding section of pre-mRNA that is cut out during splicing.

Frameshift mutation

An insertion or deletion that is not a multiple of 3 bases, which shifts the reading frame so every codon after it changes.

How to study it

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Questions