PART THREE Genetics
DNA and RNA: Structure of Nucleic Acids
As we learned earlier, the building block of life is the cell. A cell must be able to perform its particular function, and it must be able to divide and create more cells. All the information a cell needs to do its job is contained within its deoxyribonucleic acid (DNA). Without DNA, there cannot be cells; without DNA, there can be no life.
The four nitrogenous bases provide the language for the genetic code.
The sequence of nitrogenous bases in a gene codes for the synthesis of a protein.
All of the collective DNA held within a critter (bacterium, protist, fungus, plant, or animal) is called its genome. Prokaryotic cells have it easy; their genome is one single loop of DNA (along with tiny “bonus” loops of DNA called plasmids). Eukaryotic cells have way more DNA, and it’s organized into separate linear segments called chromosomes. The number of individual chromosomes depends on the species; humans have 46 chromosomes, and each cell’s nucleus (with the exception of the egg and sperm … more on that later) contains the exact same collection of 46 chromosomes. But don’t assume that all other critters must have fewer than 46 chromosomes because, you know, humans are so special and complex. The number of chromosomes doesn’t directly correlate with a species’ complexity:
The number of chromosomes doesn’t tell you how many individual genes there are. For example, even though chickens have more chromosomes than humans, they still have about the same number of genes as us (20,000—25,000). A gene is a segment of DNA within a chromosome that codes for a certain protein. And no matter the life-form, we all share a similar DNA language; all DNA consists of the same selection of individual monomers, called nucleotides.
Your genome is like a library, and each individual book in that library is a chromosome. Each book (chromosome) is filled with letters (nucleotides), and when the letters are grouped, they spell individual words (genes). The Must Know concept is to understand that the four different kinds of DNA nucleotides create the language for our genetic code. The DNA alphabet only needs four different “letters” to create everything from humans to chickens, bacteria to toadstools.
Even though you are quite different from a chicken, your DNA is composed of the same selection of nucleotides as your friend the fowl. As we learned in a previous chapter, there are four different nucleotides, each defined by the type of nitrogenous base it carries: adenine, thymine, guanine, or cytosine. These four nitrogenous bases provide the language for all life’s genetic code. The language of ribonucleic acid (RNA) is also composed of four nitrogenous bases: three of which are shared by DNA (adenine, guanine, and cytosine) and one of which is unique to RNA (uracil).
The nucleic acids (both DNA and RNA) are large carbon-based polymers. Recall that a polymer is a large molecule that is composed of smaller, repeating subunits called monomers. The monomer of DNA and RNA is a nucleotide:
Basic structure of a nucleotide (either DNA or RNA)
Each nucleotide has three components to it: a 5-carbon sugar (ribose in RNA, deoxyribose in DNA), a phosphate group, and a nitrogen-containing base. Recall that the must know concept focuses on the four different options for nucleotides, and the difference is found in the nitrogenous base. The sugar and phosphate groups of the nucleotide never change. This is a good thing, because when monomers link together to create the larger polymer, the sugar and phosphate groups hold on to each other by forming strong covalent bonds. This forms the sugar-phosphate “backbone” of the DNA molecule (as seen in the following figure).
Structure of double-stranded DNA. Notice the location of strong covalent bonds along the sugar-phosphate backbone, and the weaker hydrogen bonds between the adenine-thymine and guanine-cytosine pairing.
Author: Messer Woland. https://commons.wikimedia.org/wiki/File:DNA-structure-and-bases.png.
The nitrogenous base is hanging off the sugar part of each nucleotide, and is not part of the alternating sugar-phosphate backbone. If you are talking about the double-stranded structure of DNA, the base is pointing “inward” toward the center of the double helix, and is gently holding on to the second strand of the double helix. This bond is not the strong covalent bond of the backbone, but instead a special intermolecular bond called a hydrogen bond. Hydrogen bonds occur along the length of the entire DNA molecule, holding the two strands together in what is called a base pairing. The hydrogen bonds occur between specific pairings of nitrogenous bases of the two strands:
DNA Base-Pairing Rules
In DNA, adenine always pairs up with thymine, and guanine always pairs up with cytosine. There are, in fact, two reasons for this specific pairing. One has to do with the different shapes of the two types of nucleotides: purines versus pyrimidines. Think of it this way: structurally speaking, why wouldn’t it be a good idea to pair up [adenine + guanine] and [cytosine + thymine]? Look at the adenine and the guanine … those are both the big, double-ringed purine bases. That leaves the two smaller pyrimidines (cytosine and thymine) to base-pair. Depending on the pairing, your DNA molecule would not have a consistent width!
The second reason the base-pairing rules exist has to do with the number of hydrogen bonds formed between each pair (look at the third column of the previous table). Adenine and thymine make a great pair because it consists of one large base and one small base, and they both like to form two hydrogen bonds. The guanine and cytosine pairing also consists of one large base and one small base, plus they both want to form three hydrogen bonds.
Note that, in order to maintain a consistent width, there must always be a single-ringed nucleotide paired with a double-ringed nucleotide. The second reason it must be A-T and G-C pairing is due to the numbers of hydrogen bonds each base wants to make.
Structure of RNA
Even though DNA tends to get the most attention, RNA is just as important. It is very similar in structure, and our must know concept applies here, too: the four different bases of RNA provide the language for its genetic code. There are differences, however, between the structures of RNA and DNA:
1. Instead of the sugar deoxyribose, RNA nucleotides contain the sugar ribose.
2. RNA is single stranded instead of double stranded, like DNA.
3. RNA has the nitrogenous base uracil instead of thymine. If base pairing occurs between DNA and RNA, the uracil will partner up with adenine (see below):
There are two very important types of RNA that will play a pivotal role in gene expression: messenger RNA (mRNA) and transfer RNA (tRNA). We’ll definitely talk about them later. But for now, the most important thing to focus on are these nitrogenous bases. These four letters create the genetic language. Even though there are only four options (small, when compared to the 26 letters of the English alphabet), it is enough to create an unlimited number of different proteins! But first, let’s talk about how the cell organizes your genetic library.
DNA Chromosome Structure
The chromosomes in your nuclei aren’t just floating around like strewn-about strands of spaghetti. In order to enable 46 total chromosomes to fit into a single cell’s nucleus, the DNA needs to be efficiently packed.
If you stretch out all the DNA from a single cell’s chromosomes and line them up end-to-end, it would be six-feet long!! That’s right … from a single cell. Six feet of DNA. Whoa.
The DNA of a chromosome is packaged into tight little bundles by wrapping it around special proteins called histones, sort of like a thread being wrapped around a spool. This mixture of DNA and histone proteins is called chromatin. This phenomenon of tightly wrapping the DNA around histones will be significant later, when we talk about transcription and gene expression.
1. Rank the following terms from smallest to largest: chromosome, gene, genome, nucleotide.
2. The upright portion of the DNA backbone is composed of alternating _______________ and _______________ portions of the nucleotides. This backbone of the DNA is very strong because it is linked by _______________ bonds. The two strands of the double helix hold onto one another because of _______________ bonding between the _______________ of complementary nucleotides.
3. What would be the impact on DNA structure if base pairing occurred between cytosine-thymine and guanine-adenine?
4. List the three structural differences between DNA and RNA.
5. What would be the RNA sequence complementary to the DNA sequence ACTGACA?
6. What are the two reasons for the base-pairing rules (G-C and A-T)?
7. What is the name of the monomer of DNA? _______________ Which part of the monomer is the basis for the genetic language? _______________
8. Based on the base-pairingrules of DNA, write the complementary sequence to GGACACTT.
9. A strand of DNA is wound around special proteins called _______________. This mixture of DNA and protein is called _______________.