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Flashcards in DNA structure and replication Deck (48)
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What are chromosomes composed of?

- Both DNA (genes lie across DNA in linear fashion) and proteins (proteins known as histones).


Describe the structure of DNA.

- Double helix. Consists of a phosphate and sugar backbone held together by four nitrogenous bases (A, T, C and G).


Name the four bases that make up the genetic code?

- Adenine.
- Thymine.
- Guanine.
- Cytosine.


How are base pairs held together?

- Hydrogen bonds.


Explain Chargaff's ratio.

- If you isolate DNA and analyse the chemical composition, the number of A's ALWAYS equals the number of T's and the number of C's ALWAYS equals the number of G's. This is due to complementary base pairing.


What is the constant diameter of DNA?

- 2 nanometers.


What is the difference between a purine base and a pyrimidine base?

- A purine base is a double ring structure (e.g. Adenine or Guanine).
A pyrimidine base is a single ring structure (e.g. Cytosine, Thymine and Uracil).


How do purines and pyrimidines pair?

- A purine will always pair with a pyrimidine.


How many hydrogen bonds hold A and T together?



How many hydrogen bonds hold C and G together?



What does a nucleotide consist of?

- A nitrogenous base, a phosphate and a deoxyribose sugar (five carbon sugar).


How many carbon atoms does a deoxyribose sugar contain?



By convention in which direction is DNA sequence written?

5' to 3'.


An element located at the 5' end of a sequence is said to be ...

- Upstream.


An element located at the 3' end of a sequence is said to be ...

- Downstream.


On which strand are Okazaki fragments formed during DNA replication?

- Lagging strand.


How many nucleotide pairs in an ecoli genome?

- 4.7x10(6).


Difference between prokaryotic and eukaryotic replication?

- Although both semi-conservative replication they are slightly different.
- Prokaryotic replication: begins at a fixed location called the replication origin and replicates about 1000 nucleotides per second. It's polymerase includes a proof reading function and so there are very few errors. Circular.
- Eukaryotic replication: many places were replication can begin and replicates about 50 bp per second. The human genome is much bigger than that of prokaryotes and their replication is much slower.


What is chromatin?

- It is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells.


What does a nucleosome consist of?

- Made up of a protein core with DNA wound around the surface. 8 proteins in the core of the nucleosome - 2 copies of histones H2A, H2B, H3 and H4.
- Histone H1 acts as a linker between nucleosomes and helps old them together.
- Each nucleosome contains between 180 and 200 bases of DNA depending on species.


Explain the central dogma of molecular biology?

- DNA --> (transcription/RNA synthesis) --> RNA --> (mRNA moves from the nucleus to the cytoplasm where translation/protein synthesis occurs) --> Protein.


What is a hybrid?

- Offspring of two plants or animals of different species e.g. A mule/a plant that inherits one gene for yellow seeds (Y) and one gene for green seeds (y).


Most cells are diploid, what is the complement of germ cells (sperm and egg)?

- Haploid.


Is linkage between genes complete?

- No: chromosomes can exchange material during meiosis leading to recombination between linked genes.


What is the frequency of recombination?

- Between two linked genes: this depends on the distance between them on the chromosome. Genes that are close together recombine less frequently than genes located further apart.


How does PKU come about?

- Results from a genetic defect in metabolism of the amino acid phenylalanine.


How frequently does the helix turn in DNA?

- Every 3.4nm.


What is the distance between adjacent bases?

- 0.34nm: so there are 10 bases per turn of the helix.


What is the diameter of the helix?

- 2nm (this is how they knew DNA was composed of 2 chains and not just 1).


What are Okazaki fragments?

- Pieces of DNA that result from interrupted synthesis of lagging strand of DNA during replication.


In DNA how many nucleotides per helical turn?



Which carbon atom in the sugar found in DNA carries the nitrogenous base?



Which carbon atoms in deoxyribose participate in the phosphodiester bond of the DNA backbone?

3' and 5'.


If 34% adenine what % of guanine?



Guanine and cytosine are help together by how many h bonds?



Adenine and thymine are held together by how many h bonds?



Which carbon atoms in deoxyribose participate in the phosphodiester bond of the DNA backbone?

3' and 5'.


In which direction is DNA synthesised on leading and lagging strands during DNA replication?

5' to 3'.


Name the two pyrimidines.

- T and C (they all have y in them).


Name the two purines.

- A and G.


What is required for replication of the lagging strand?

- DNA lipase, polymerase and helicase.


When DNA is separated into single stranded DNA what keeps the strands from re-annealing?

- ssDNA binding proteins.


What is DNase?

- A DNA digesting enzyme.


What are the levels of phosphorous and sulphur in DNA and proteins?

- DNA: high in phosphorous atoms (P) but has no sulphur (S).
- Proteins: contain sulphur atoms but have no phosphorous.


What does CpG describe?

- this describes a C and G next to each other in the sequence.
- p = phosphate group between two sugars, therefore this C and G are on the same strand.
- if written as CG this usually refers to a base pair meaning these are on opposite strands of DNA.


Name the three possible ways in which DNA could replicate.

- conservative replication (original DNA molecule maintained and completely new copy generated).
- dispersive replication (molecule replicated but both strands of the resulting molecules are a mixture of old and new DNA).
- semi conservative replication (one strand of DNA from old molecule is incorporated into new one, therefore one old and one new). This is how DNA is actually replicated.


Describe what happens in preparation for DNA replication.

- DNA is unwound by a helicase enzyme.
- another enzyme called RNA primase generates a small RNA molecule which is complementary to the DNA.
- this acts as a primer for the DNA polymerase enzyme which will synthesise the new DNA chain.


Describe what happens during DNA replication.

- 2 strands held together by hydrogen bonds (antiparallel).
- DNA unwinds (by helicase enzyme) as hydrogen bonds are broken.
- now have two ssDNA molecules.
- this is not as stable as double stranded and so ssbinding proteins attach to stabilise DNA and stop it being degraded.
- molecule of DNA polymerase binds to DNA and replicates new strand (this binds and moves in 3'-5' and so new molecule is built in 5'-3' direction.
- a second type of DNA polymerase binds the other template strand. This must synthesise discontinuous segments of polynucleotides (these sequences are called OKAZAKI FRAGMENTS).
- because these fragments can only be built 5'-3' this process is limited by how far the original DNA has been unwound.
- Okazaki fragments result from the lagging strand.