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Flashcards in 2.6-7 Deck (38)
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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?



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?

1. 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

2. 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.

3. 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?

1. Separates DNA by heating to 95’ for 15 seconds
2. Quickly cools to 54’- reannealing to form double strands
3. Primers are present to prevent reannealing of parent strands and continue replication
4. Heated to 62’ and parent strands are copied from primers


What is a polymerase chain reaction?

Rapid DNA replication. A cycle takes 2 minutes. 30 cycles take less than an hour and amplify the DNA by a factor of a billion


What is transcription?

The synthesis of mRNA copied from the DNA base sequences by RNA polymerase


Why does transcription only occur along one of the two strands of DNA?

Because RNA is single stranded


What are the stages of transcription?

- RNA polymerase bonds to a site on the DNA at the start of a gene

- RNA polymerase moves along the gene separating DNA into single strands and pairing up RNA nucleotides with complementary bases on one strand of the DNA. Thymine pairs with adenine.

-RNA polymerase forms covalent bonds between the RNA nucleotides

- the RNA separates from the DNA and the double helix reforms

- transcription stops at the end of the gene and the completed RNA molecule is released


What is a sense strand?

The DNA strand with the same base sequence as the RNA


What is an antisense strand?

The template strand that has a complementary base sequence to both the RNA and sense strand