2.6-7 Flashcards
(38 cards)
Outline the structure of nucleosomes
Nucleosomes consist of DNA wrapped around 8 histoine proteins and held together by another histoine protein
What does a DNA nucleotide contain?
A sugar ( deoxyribose) and nitrogenous base ( adenine, thymine, cytosine or guanine) and a phosphate group.
What are nucleotides?
The building blocks of DNA
What are the two types of nucleic acid?
DNA and RNA
What is the sugar in nucleotides?
Deoxyribose- which has five atoms, so is a pentose sugar
How are the parts of a nucleotide linked together?
By covalent bonds
What are nucleic acids?
Very large molecules that are constructed by linking together nucleotides to form a polymer
Why are the base sequences in nucleic acids important?
The base sequence is the store of information and the sugar phosphate backbone ensures that the store is stable and secure
Differences between DNA and RNA
- sugar in DNA is deoxyribose and in RNA is ribose
- two polymers of nucleotides in DNA but only one in RNA
- DNA bases: adenine, cytosine, thymine and guanine. RNA bases: adenine, cytosine, guanine and uracil
In a diagram of DNA and RNA, how can phosphates, sugars and bases be drawn?
- circles for phosphates
- pentagons for pentode sugar
- rectangles for bases
What are the two ends of a nucleotide polymer referred to as?
The 3’ terminal and 5’ terminal, the 3’ being linked to the C3 atom and the 5’ being linked to the phosphate
How are the complementary base pairs joined?
By hydrogen bonds
What can be said about the direction of DNA strands?
They are parallel, but run in opposite directions so are known as anti parallel. One strand is oriented in the direction 5’ to 3’ and the other 3’ to 5’
What were the stages in Watson and cricks discovery of DNA structure?
- First model was a triple helix, with based on the outside of the molecule and magnesium of holding the two strands together with ionic bonds to the phosphate groups on each strand
- Rejected as Rosalind franklin said there would not be enough magnesium to form cross links between the strands. Also didn’t take into account chargaffs finding that the amount of adenine equals thymine and cytosine equals guanine.
- Formed equal lengths of bases to fit between backbones. Realised two strands must be anti parallel.
How are new strands of DNA formed?
By adding nucleotides one by one and linking them together- resulting in 2 DNA molecules, both composed of an original and newly synthesised strand
Why is DNA replication semi conservative?
The result is 2 new DNA molecules, both composed of an original and newly synthesised strand
How does the base sequences of the template strand affect replication?
It determines the base sequence of the new strand; only a nucleotide carrying a base that is complementary to the next base on the template strand can successfully be added.
How was semi conservative replication justified?
1958- Medellin and stahl- cultured bacterium E. coli for 14 generations in a medium of 15nitrogen. In centrifugal, the less dense material rises, the presence of 15N was shown in this gradient and when culturing the E. coli further in 14nitrogen and 13nitrogen, it became even less dense and rose.
What is the function of helicase?
Helicase unwinds the double helix and seperates the two strands by breaking hydrogen bonds
What are helicases?
A group of enzymes that use energy from ATP. Consists of 6 globular peptides arranged in a donut shape.
How does helicase seperate the double helix?
They assemble with one strand of the DNA molecule passing through their donut shape and and the other outside it. Energy from ATP is used to move long the DNA molecule, breaking the hydrogen bonds between the bases and parting the two strands
What is the function of DNA polymerase?
It links nucleotides together to form a new strand, using the pre existing strand as a template
How does DNA polymerase form a new strand?
It moves along the Template strand in the same direction, adding one nucleotide at a time. Free nucleotides with different bases are available where DNA is being replicated. DNA polymerase brings nucleotides into the position where hydrogen bonds between the bases can form. Once this happens, DNA polymerase links it to the end of the new strand through a covalent bond between the phosphate group of the free nucleotide at the 5’ terminal and the pentode sugar at the existing end of the new strand, the 3’ terminal
What happens in a polymerase chain reaction?
- Separates DNA by heating to 95’ for 15 seconds
- Quickly cools to 54’- reannealing to form double strands
- Primers are present to prevent reannealing of parent strands and continue replication
- Heated to 62’ and parent strands are copied from primers
