BIO120: Lecture #3 Quiz

List three differences between DNA and RNA structure (not function!)

1. DNA is double-stranded, RNA is single-stranded. 2. DNA contains a pentose sugar Deoxyribose, RNA contains the pentose sugar Ribose. A pentose is a 5-carbon sugar molecule. 3. DNA is limited to the nucleus, RNA is made in the nucleus, but can travel outside of it. 4. DNA has a nitrogenous base called Thymine, but RNA doesn't. Instead, RNA has Uracil. In DNA thymine pairs with adenine, but in RNA uracil pairs with adenine. 5. There is only one type of DNA but 3 kinds of RNA (messenger, transfer and ribosomal RNA).

A certain DNA sample is found to have a makeup consisting of 22% thymine. Use Chargaff's rules to fill in the percentages for the other three nitrogenous bases.A DNA strand has 22% thymine. The percentages for adenine, guanine, and cytosine are blank.

Adenine: 22% Thymine: 22% Guanine: 28% Cytosine: 28% Using Chargaff's rules, we know that since Adenine & Thymine pair up, there must be an equal percentage of both found in this DNA sample. If Thymine makes up 22% of the DNA sample, then so must Adenine. Together, the pair makes up 44% of the sample. This leaves Guanine & Cytosine to make up the remaining 56% of the DNA sample. Since there must be an equal amount of both present, we can simply divide 56 in half, and therefore each of these nitrogenous bases make up 28% of the sample.

Define: codon, anticodon, and nonsense codon.

Codon - a sequence of three nucleotides which together form a unit of genetic code in a DNA or RNA molecule. anticodon - a sequence of three nucleotides forming a unit of genetic code in a transfer RNA molecule, corresponding to a complementary codon in messenger RNA. Nonsense codon - terminate the translation process since there are no corresponding tRNA molecules with anticodons for them. They are also called stop codons and there are three of them: UAA, UAG, UGA.

Why is DNA with a high GC content more difficult to denature than that with a low GC content?

DNA with high GC content has additional hydrogen bonding between the C=G base pair, making it harder to denature. DNA with a higher GC content is more difficult to denature because G-C bonds have three hydrogen bonds, whereas A-T bonds only have two. Therefore, G-C bonds are more stable and have a greater resistance to denaturation.

You are a world-renowned virologist! To study COVID-19, you label the virus with both P32. Then you infect a variety of human cell types in culture. Explain how you can use P32 to determine which human cell types are infected with the virus. Could you use S35? Why or why not? Hint: review the lecture slide that shows the discovery of DNA in the 1950s...

P32 or radioactive phosphorus is used to label DNA within the bacteriophage, or can be used to label the DNA within COVID-19, similar to what Hershey and Chase did. By using P32 as an identifier as to what DNA is being passed on, we should be able to identify which type of cells are being infected with the disease. S35 would not be able to be used during the experiment because it only labels the protein/protein capsules. After reviewing the Hershey and Chase experiment, it's clear the S35 can't be used due to it not providing any identifiable information for the radioactive activity, or transference of the DNA for the virus, like P32. A bacteriophage ONLY injects it's DNA into the cell it infects, so only P32 can be used. However, animal viruses are taken up completely by the infected cell, DNA, capsid, and sometimes the envelope too! So both proteins and DNA would get into the cell, meaning that both P32 and S35 would work for an animal virus!

What is the role of the "promoter", "terminator" and mRNA in transcription?

Promoter: The starting site on a DNA strand for transcription of RNA by the RNA polymerase. Terminator: The site on a DNA strand at which transcription ends. mRNA: The type of RNA molecule that directs the incorporation of amino acids into proteins.

What is the advantage of having a "degenerate" genetic code?

The degeneracy of the genetic code made it possible for organisms to prosper on Earth. Organisms, which did not use a degenerate genetic code, would extinguish from this planet. This is one significant point of the genetic code.

List 4 different "jobs" that enzymes perform during the synthesis of new DNA (DNA replication).

During the synthesis of DNA replication, there are enzymes that perform the synthesis of new DNA strands. The enzyme Helicase unzips the DNA double helix that results in the replication of a "replication fork" or two strands. The enzyme Primase starts the process of making a new DNA strand by making a small RNA strand called the "primer," which is the start of the new DNA strand. The enzyme DNA Polymerase then binds to the "primer" and starts to add DNA bases in one direction from the 5' prime end to the 3' prime end. Lastly, the enzyme DNA ligase seals up the fragments of DNA in both strands to form a continuous double strand.

Why can translation begin before transcription is complete in prokaryotes, but not eukaryotes?

In prokaryotic cells, the translation of mRNA into protein can begin even before transcription is complete. Because mRNA is produced in the cytoplasm in prokaryotes, the start codons of an mRNA being transcribed are available to ribosomes before the entire mRNA molecule is even made. In eukaryotic cells, transcription takes place in the nucleus. The mRNA must be completely synthesized and moved through the nuclear membrane to the cytoplasm before translation can begin.

Describe the process of "initiation", "elongation" and "termination" in translation

Initiation - in this stage, the ribosome gets together with the mRNA and the first tRNA so translation can begin. Elongation - in this stage, amino acids are brought to the ribosome by tRNAs and linked together to form a chain. Termination - in the last stage, the finished polypeptide is released to go and do its job in the cell.

Explain (use pictures if it helps!) why Okazaki fragments form on the lagging strand during DNA replication

The start of a new DNA strand starts with a small RNA strand called the "primer," by which the enzyme DNA polymerase binds to and starts to add nucleotide bases in one direction from the 5' prime end to the 3' end. The leading strand results in continuous synthesis of DNA, but the lagging strand results in discontinued synthesis because it runs in the opposite direction from the 3' prime end to 5' prime end. Because DNA can't be synthesized from 3' prime to 5' prime, DNA is synthesized in Okazaki fragments from 5' prime to 3' prime. Each fragment starts with an RNA primer, DNA polymerase then adds a short row of DNA bases. Once this fragment is done, another RNA primer is laid down and this repeats again discontinuously.

Explain why the genetic code is a "triplet" code using math! :-)

There are 20 amino acids but only 4 nucleotides! A triplet code could make a genetic code for 64 different combinations (4x4x4) genetic code and provide plenty of information in the DNA molecule to specify the placement of all 20 amino acids. (4x4) or (4) could not provide as many combinations.

In transcription...what is copied? What is made?

Transcription is the process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA).