2.6 - Structure of Dna and Rna Flashcards
Elucidation of the structure of DNA using model making
describe how dna structure was found
- While others worked using. an experimental basis, Watson and Crick used a stick and ball model to test their ideas on the possible structure of dna. This allowed them to visualize the molecule and to see if it fit with available evidence
Their first model, a triple helix, was rejected because:
- The ratio of Adenine to Thymine was not 1:1 (chargaff)
- It needed too much magnesium (Franklin)
From their setbacks they realized:
- Dna must be a double helix
- The relationship between the bases and base pairing
- The strands must be anti-parallel to allow base pairing to happen
The visual nature of their second model suggested:
- Possible mechanisms for replication
- information was encoded in triplets of bases
Watson and Crick won nobel prizes for their work, but their work relied on Rosalind Franklin and Maurice Wilkins work with X-ray diffraction.
What is a nucleotide
A single unit of nucleic acid
Nucleic acid
Nucleic acids are very large molecules that are constructed by linking together nucleotides to form a polymer.
there are two types of nucleic acids: DNA and Rna
how do nucleotides form polymers?
Nucleotides are linked into a single strand via a condensation reaction and covalent bond
- Bonds are formed between the phosphate of one nucleotide and the pentose sugar of the next.
- The phosphate group (attached to the 5’C of the sugar) joins with the hydroxyl group attached to the 3’C of the sugar.
- This results in a phosphodiester bond between the two nucleotides and the formation of a water molecule
- Successive condensation reactions between nucleotides results in the formation of a long single strand.
Dna differences from rna
RNA: Uracil, ribose, single stranded and often but no always linear.
DNA: Thymine, Deoxyribose. two anti-parallel complementary strands form a double helix.
How is the double helix structure maintained
- Sugar phosphate backbone is hydrophilic, so it is positioned on the outside.
- Nitrogenous bases are very reactive so are protected on the inside.
- Hydrogen bonds hold adjacent sections together
- Hydrogen bonds hold complementary base pairs together
- Complementary base pairing ensures that mistakes are not made in the replication or transcription of DNA
-Polynucleotides are anti-parallel.
Summary of DNA structure
- Each polynucleotide strand consists of a chain of nucleotides bonded covalently
- Two polynucleotide chains of DNA are held together by hydrogen bonds between complementary base pairs
- A pairs with T via 2 hydrogen bonds
- G pairs with G via 3 hydrogen bonds - In order for bases to be facing each other and pair, the two strands have to be antiparallel.
- As the polynucleotide chain lengthens, the atoms that make up the molecule will arrange themselves in a n optimal energy configuration. This position of least resistance results in the double-stranded DNA twisted to form a double helix with 10-15 bases per twist.
Key points about DNA structure
- DNA is a double helix
- Each strand is made of single units called nucleotides
- Antiparallel strands of polynucleotides
- Purine (AG) with pyrimidine (TC)
- It has a sugar phosphate backbone
- Bases join the two strands together by hydrogen bonds
- complementary base pairing
- Each strand of DNA can be millions of base pairs in length and coil up to make chromosomes.
Analysis of experiment providing evidence that DNA is genetic material.
- In the mid 20th century, scientists were unsure as to whether proteins or chromosomes were the genetic material of cells.
- Hershey and Chase wanted to solve this problem by finding out if protein of DNA was the genetic material of viruses.
Viruses infect cells and transform them into virus producing factories:
- Viruses inject their genetic material into cells
- The non-genetic part of the virus remains outside
the cell
- Infected cells produce large numbers of the virus
- The cell bursts releasing the copied virus
Hershey and Chase chose to study the T2 bacteriophage, which infects E.Coli bacterium, because of its very simple structure consisting of:
- Protein coat (capsid)
- DNA inside the coat
Amino acids containing radioactive isotopes were used to label the virus
- Sulfur for the protein coat (capsid)
- Phosphorus for the DNA
The experiments combined T2 bacteriophage with E. Coli bacteria. At the end of the experiment a centrifuge was used to separate them:
- The smaller virus remained in the supernatant (liquid)
- The bacteria formed a pellet.
Hershey and Chase deduced that DNA therefore was the genetic material used by viruses because DNA was being transferred into the bacteria.
Franklin and Wilkins investigation of DNA structure by x-ray diffraction.
- When X-rays are directed at a material, most of it passes through and some is scattered by the particle sin the material diffraction
- For X-ray diffraction to work well the material ideally should be crystallised so that the repeating pattern causes diffraction to occur in a regular way.
- DNA cannot be crystallised but the molecules were arranged regularly enough for the technique to work
The image allowed Franklin to make deductions about DNA:
- Cross is center - molecule is helical in shape
- Angle of the cross shape - showed the steepness of the helix
- Distance between the horizontal bars - showed turns of the helix to be 3.4 nm apart
DNA structure suggested a mechanism for DNA replication. Outline the evidence that supports complementary base pairing.
DNA replication and mechanism by which it can happen are implied by complementary base pairing.
- (X-ray diffraction showed that) the DNA helix was both tightly packed and regular: therefore pyrimidines need to be paired with purines
- The electrical charged of adenine and thymine are compatible and opposite thus allowing two hydrogen bonds to form between them
- The pairing of cytosine with guanine allows for three hydrogen bonds to form between them.
- The logical deduction is that if an adenine base occurs on one strand then the opposite only possibly base is thymine and vice versa. If an Guanine base occurs on one strand then the opposite only possibly base is Cytosine and vice versa.
Describe how nucleosomes help super coil DNA
- Eukaryotic DNA supercoiling is organised by nucleosomes.
- Nucleosomes both protect DNA and allow it to be packaged, this in turn allows it to be supercoiled
- Nucleosomes are formed by wrapping DNA around 8 histone proteins and an H1 locking protein.
Why does eukaryotic DNA need to be supercoiled?
Supercoiling is when a DNA strand has been wound back on itself multiple times so that the molecule becomes compacted.
Why?
- It is essential to pack genetic material into the nucleus.
- To organise DNA to allow cell division to occur
- To control gene expression - supercoiled DNA can not be transcribed
- Allows cells to specialise by permanently supercoiling DNA (heterochromatin)
- Transcription of active chromatin (euchromatin) can be promoted or inhibited by the associated histones.
The facts:
- The length of DNA in a human (eukaryotic) cell is 2m
- Eukaryotic chromosomes are 1-20 um in length
- The nucleus in a eukaryotic cell has a diameter of 10 um.
