2.1 Learning Objectives

DNA Structure and Chromosomes

Draw a generalized nucleotide and contrast the structure of an RNA nucleotide versus a DNA nucleotide.

RNA Nucleotide	DNA Nucleotide
Contains a Sugar, Phosphate Group, and Nitrogenous Base	Note “De-OXY-ribose”, The main difference is that on the sugar one of the bonds is “H” instead of OH. Remember this one by it being “De-OXY-fied”

Contrast the structure of a DNA versus RNA polymer.

RNA Polymer	DNA Polymer
Phosphate Groups covalently bonds to the -OH on the sugar to create “backpbone”. Nitrogenous base is unbonded to anything but the exisiting carbon sugar.	Phosphate group bonds the same way but each nitrogenous base then hydrogen bonds with another base on a different strand
Single Strand	Double Strand

Identify the bond between two nucleotides and the 5’ and 3’ ends of a DNA and RNA polymer.

Although prevously mentioned as a Covalent Bond, this is a specific type called a “Phospodiester Bond”

To find the 5’ and 3’ of both DNA and RNA, look at where the phosphate group is in relation to its sugar and base. The phosphate group of each will be poitned in the 5’ direction.

Therefore, the 5’ side of the chain will have the Phosphate Group and the 3’ side will have the -OH of the sugar.

Note: We read DNA from the 5’ end to the 3’ end.

Draw and describe the structure of DNA, identify the bonds which hold the two strands together, and define antiparallel.

DNA Structure
As mentioned before the Phosphate group of each nucleotide bonds to the -OH of the sugar of the next. Then, the bases bond together with a hydrogen bond. The two strands are “antiparallel” and the whole strucure then ‘twists’ into a double helix.

The bond that holds these together is a Hydrogen bond between the two nitrogenous bases.

These strands are AntiParallel because the 5’ end of one is parallel to the 3’ end of the other. They’re “opposite” and “flipped”.

Draw and describe how DNA is packaged into chromosomes in the nucleus (including the terms chromatin, histones and nucleosomes) and explain the importance of DNA packaging.

DNA is wrapped around Histone proteins. This forms structures known as “nucleosomes”

These will appear to be “beads on a string”.

The string of these Histone proteins that have DNA wrapped around them is called chromatin.

This chromatin is then folded and looped and packed tightly into a chromasome.

Note: This describes the process for Eukaryotic Chromosomes only. Recall that Prokaryotic Chromosomes are circular.

Contrast purines and pyrimidines, categorize nitrogenous bases (A, T, G, C, U) into these categories, and list the base pairing rules.

Purines	Pyrimidines
Guanine (G), Adenine (A)	Cytosine (C), Uracil (U) [RNA Only] , Thymine (T) [DNA Only]
Double Ring nitrogenous base structure	Single Ring nitrogenous base structure

Note: You can literally see the ring difference. You can identify the categories easily this way.

Base	Pairs With	How can you tell?
Cytosine (C)	Guanine (G)	Look for 3 hydrogen bonds
Adenine (A) [In DNA]	Thymine (T)	Look for 2 hydrogen bonds in DNA
Adenine (A) [In RNA]	Uracil (U)	Look for 2 hydrogen bonds in RNA

Predict the proportion of nitrogenous bases in a molecule of DNA using the base pairing rules.

These problems can be daunting but are pretty straight forward once you get the hanf of them.

Example:

Given a DNA molecule is 20% Adenine, what percentage is Guanine?

We know since it’s 20% Adenine and it’s DNA, each of those Adenine bases pairs with a Thymine. This means that another 20% is Thymine.

Adding those together, we know 40% of the molecule is Adenine and Thymine.

That leaves 60% we still don’t know.

We do know, however, that the remaining 60% is made of Cytosine and Guanine.

Since we know that this must be split evenly between them since they’re paired, we know that 30% of it is Guanine

Trace the flow of information from the DNA to a functional protein (central dogma).

DNA → mRNA → Protein