2.6 Structure of DNA and RNA Flashcards
The basic structures of DNA and RNA are very ___
Similar as they are both made of monomers called nucleotides.
Structure of DNA and RNA
DNA and RNA are composed of three parts, namely a pentose sugar (meaning that it has five carbon atoms), a phosphate group, and a nitrogenous base.
Differences between the structure of DNA and RNA
- A DNA nucleotide makes up the basic unit of DNA and RNA; although in RNA, the pentose sugar is ribose instead of deoxyribose.
- DNA and RNA also differ in their bases. DNA has four nitrogenous bases: guanine (G), adenine (A), thymine (T), and cytosine (C).
- RNA has the same bases except for thymine, which is replaced by uracil (U).
How are bases classified?
- Bases can be classified based on the number of rings present in their structure.
- Purines are bases that have two rings in their structure, while pyrimidines contain only one ring.
- Thymine, cytosine, and uracil are examples of pyrimidines, while adenine and guanine are classified as purines.
Structure of a DNA molecule
- Contains the sugar deoxyribose
- Bases involved are adenine, thymine, guanine, and cytosine.
- Is a double-stranded molecule.
Structure of an RNA molecule
- Contains the sugar ribose.
- Bases involved are adenine, uracil, guanine, and cytosine (thymine has been replaced by uracil).
- Is a single-stranded molecule
Diagram of the simplified structure of a DNA nucleotide.
Drawing diagrams of single nucleotides of DNA and RNA
You should be able to draw simple diagrams of the structure of single nucleotides of DNA and RNA, using circles, pentagons, and rectangles to represent phosphates, pentoses, and bases, respectively.
The nucleotide units link together through a ___
- Phosphodiester bond (a covalent bond) to form a single strand, a polynucleotide.
- The phosphodiester bond is always formed between the phosphate group attached to the 5’-C of one sugar and the hydroxyl (OH) group attached to the 3’-C of another sugar.
What do 3’ and 5’ refer to?
- 3’ and 5’ refer to the carbon position on the sugar in DNA.
- Carbon 1 is attached to the base and from there on carbon atoms are counted in a clockwise direction.
An important fact about DNA is that it is made up of two ___
Strands held together by hydrogen bonds to form a double helix, while RNA is only a single-stranded molecule.
Diagram of DNA (double-stranded)
How is the DNA structure usually described?
Double helix
What is the name of the pentose present in DNA molecules?
Deoxyribose sugar
RNA consists of uracil instead of ___
Thymine
DNA is the ___
Genetic material of living organisms.
Guanine always pairs with
Cytosine
Adenine always pairs with ___
Thymine
The complementary base pairing rule
- Guanine always pairs with cytosine.
- Adenine always pairs with thymine.
How are two nitrogenous bases held together?
- The two strands are held together by hydrogen bonds between G and C (3 hydrogen bonds) and between A and T (2 hydrogen bonds).
- In order for bases to be facing each other, the two strands must run in opposite directions (i.e. they are antiparallel ) to each other.
- Thus, one strand of DNA runs from 5’ to 3’, and the opposite strand runs from 3’ to 5’. The 3’ and 5’ refer to the exposed carbon (on the sugar) at the ends of the DNA chains
Diagram of DNA double helix
Crick and Watson (Using a model to elucidate the DNA structure)
- Crick and Watson built on what was known about the chemistry of DNA to determine its actual structure.
- They used the information from the DNA X-ray diffraction patterns produced by Rosalind Franklin and Maurice Wilkins to deduce that the DNA molecule must have a regular double helix structure.
- Additionally, they integrated Chargaff’s base ratio, a finding that showed A is always equal to T and C always equal to G in the DNA molecule to work out the complementary base pairing of A to T and C to G.
- Once they had all the information they built a 6-foot (about 1.8 m) model out of metal scrap and, as expected, everything clicked into place.
- One very important aspect of using a model is the ability to visualise the molecule and to quickly see how well it fits the available evidence.
If a DNA sample were composed of 10% thymine, what would be the percentage of guanine?
40
If a DNA sample contains 10% thymine that means there would have to be 10% adenine as well, as thymine pairs with adenine. That leaves 80% for cytosine and guanine combined; so the correct answer is half of 80 (which is 40)
Which method was used in the discovery of the DNA double helix?
X-ray
In a DNA deoxyribose sugar of a single nucleotide, which carbon is connected to the base?
The first carbon
The 5th carbon makes up the 5’ end and the 3rd carbon makes up the 3’ end of a nucleotide. The carbons are counted clockwise starting from the carbon attached to the base and finishing on the carbon attached to the phosphate.
What are the three main components of a DNA nucleotide?
Phosphate, Deoxyribose sugar, Nitrogenous Base
Which molecule is found in both DNA and RNA?
Phosphate
Define “nucleic acid backbone”
Nucleic acids are composed of a backbone of sugar molecules and phosphate groups. The nitrogen bases are attached to the sugars.
Which base replaces thymine in RNA?
Uracil
Watson and Crick- making DNA models
Using trial and error, Watson and Crick were able to assemble a DNA model that demonstrated the following:
- DNA strands are antiparallel and form a double helix
- DNA strands pair via complementary base pairing (A=T; C=G)
- Outer edges of bases remain exposed (allows access to replicative and transcriptional proteins)
As Watson and Crick’s model building was based on trial and error, a number of early models possessed faults:
- The first model generated was a triple helix
- Early models had bases on the outside and sugar-phosphate residues in the centre
- Nitrogenous bases were not initially configured correctly and hence did not demonstrate complementarity
The final construction of a correct DNA molecule owed heavily to the X-ray crystallography data generated by Franklin
- This data confirmed the arrangement of the DNA strands into a helical structure
- The data was shared without Franklin’s knowledge or permission and contributed profoundly to the final design
- Hence, Franklin is now recognized as a key contributor to the elucidation of DNA structure
