2024 Biol101 Test2 feedback Tut - G2P

1.     What was the primary aim of the Hershey-Chase experiment, where bacteriophage DNA and bodies were labelled with radioactive phosphorus and sulphur, respectively?

• To determine the structure of DNA.

• To elucidate the role of proteins in genetic inheritance.

• To investigate the replication mechanism of DNA.

• To confirm that DNA, not protein, is the genetic material of bacteriophages.

• To study the role of RNA in gene expression

Answer explanation

We have covered a few experiments in Genes to Proteins, and the Hershey Chase experiment was one of them. I have given you a clue in the question: "bacteriophages" that will discriminate this from Avery-McLeod's experiments.

What is the difference between the 5' and 3' end of a polynucleotide?

• The 5' end has a hydroxyl group, while the 3' end has a phosphate group.

• The 5' end has a purine base, while the 3' end has a pyrimidine base.

• The 5' end has a pyrimidine base, while the 3' end has a purine base.

• The 5' end is involved in transcription, while the 3' end is involved in translation

• The 5' end has a phosphate group, while the 3' end has a hydroxyl group

Answer explanation

Remember, the 5' end of a nucleotide is the end where the phosphate group is attached to the C5, and the 3' end where the OH group is linked to the C3. When nucleotides attach to each other to form a POLYnucleotide, this polarity is kept and the polynucleotide has polarity and we also refer to it as the 5' and 3' ends. See the image too. Remember, RNA has OH groups on both the C2 and C3; DNA has an OH group on C3 and H2 on C2. The other options in this question were very obviously wrong, but the one option had the 3' and 5' ends reversed - please read your answer options very carefully.

An RNA nucleotide consists of ______.

• a ribose sugar, a phosphate group and a nitrogenous base

• a sugar and a phosphate group

• a deoxyribose sugar, a phosphate group and a nitrogenous base

• a deoxyribose sugar and a nitrogenous base

• a phosphate group and a nitrogenous base

Answer explanation

Remember, a nucleotide always consists of a sugar, a phosphate group and a nitrogenous base. The question specifically asked for the components of an RNA nucleotide, which will then be a ribose sugar, phosphate group and nitrogenous base.

All the other options had one component missing, except for one, but in that one the sugar was DEOXYribose sugar, which is not part of RNA, but DNA.

The resulting daughter molecules in DNA replication contain one strand of the original parental DNA and one new strand. What is the explanation for this phenomenon?

• DNA replication is conservative.

• DNA replication is not conservative.

• DNA replication is semiconservative.

• RNA synthesis is conservative.

• RNA synthesis is semiconservative.

Answer explanation

You could have already disregarded the answers with RNA, because the question dealt with DNA REPLICATION. You could also have disregarded the option that said "DNA replication is not conservative" as this is not a relevant option. The Meselson-Stahl experiment (see image here) only tested if DNA replication is conservative, dispersive or semiconservative, and their results showed that it is semiconservative. This is simply a definition you needed to learn even if the experiment was difficult to understand: Semiconservative means that after DNA replication the two daugher DNA molecules have one parent strand and one daughter (new) strand.

Select the reason why the leading and lagging strands are synthesized differently during DNA replication.

• The origins of replication occur only at the 5′ end.

• Helicases and single-strand binding proteins work at the 5′ end.

• DNA polymerase can join new nucleotides only to the 3′ end of a pre-existing strand, and the strands are antiparallel.

• DNA ligase works only in the 3′ → 5′ direction.

• Primase can only bind one primer on the leading strand

Answer explanation

Lets see which options you could have discarded and why: 1) The origin of replication at 5' is nonsense; 2) Helicases and SBBs work on both ends, because it splits and stabilises the two strands on BOTH sides of a replication bubble. 3) DNA ligases have nothing to do with the building of the DNA - they only "glue" together the already-built strands at the end of replication. 4) Primase can bind multiple primers, but only one is NEEDED to build the leading strand.

The reason that the leading and lagging strands are built (synthesized) differently during DNA replication is that the polymerase can ONLY bind the nucleotides to the 3' end and not the 5' end. So, when the two DNA strands separate at the replication fork, it can build the leading strand in one, continuous strand, but the lagging strand must be built in bits and pieces (also called Okazaki fragments, remember?).

Give the label for A on the image

Answer explanation

You must know that this is a replication fork at DNA replication - this is something you need to LEARN. I have given you the same image as on the Prezi and in the textbook, but I don't have to... Remember, we have also told you MANY, many times in class that you need to know these components and the processes.

You can, in the image, clearly see that A is the enzyme that builds the new strand, so it must be POLYMERASE. It builds the new DNA strand (there is no replication fork in transcription, there is also none of the other structures given on this image). The structure must therefore be DNA POLYMERASE

Give the label for B on the image

Answer explanation

In the image you can clearly see that B is the structure that breaks the two parent DNA strands apart - so it must be HELICASE.

Give the label for C on the image

Answer explanation

There is another structure that looks like C on the other DNA strand too. They don't seem to do anything other than bind to the strands - they also will block the two strands from binding to each other again once helicase has separated them.... they are thus SINGLE STRAND BINDING PROTEINS

Give the label for D on the image

Answer explanation

On this image D consists of nucleotides, but 1) they are a different colour than the other DNA nucleotides and 2) this is the beginning of building a new strand. Remember, in DNA replication, a new DNA strand cannot be built if an RNA primer has not started the process.... so this is an RNA PRIMER

Give the label for E on the image

Answer explanation

So, this structure clearly builds the RNA primer, so it is PRIMASE, the enzyme responsible for building the primer.