7.1 : DNA Structure & Replication

Question 1

how is DNA packaged within the nucleus? what role do nucelosomes play?

Answer 1

• DNA –> wrapped around an octomer (8 histones), to form a nucleosome –> nucleosomes coil and stack together to form chromatin (fibre) –> loop and fold with the help of additional proteins to form chromosomes
• nucleosomes help supercoil the DNA :
• -> makes DNA more compact = better storage
• -> protects DNA from damage = less exposed
• -> assists in cell division = greater mobility

Question 2

what binds the dna to the histone octamer?

Answer 2

H1 histone (packaging protein) binds DNA to histone 'bead', so DNA double helix coils twice around histone core
--> histone (positive charge) & DNA (negative charge) = bind & form neutral charge

Question 3

what occurs with the chromatin fibre during nuclear division?

Answer 3

nucleosome coils up to form a chromatin fibre –> which is coiled again –> coils are looped around a ‘scaffold’ protein fibre (non-histone protein) = supercoiled into a much condensed chromosome.

Question 4

what is the direction of a DNA molecule?

Answer 4

phosphate groups along each strand –> form bridges between the carbon 3 of one sugar molecule and carbon 5 of another (phosphate connects them)
–> two strands are antiparallel & complimentary (one chain from 5’ to 3’ and other is 3’ to 5’)

Question 5

what is the dideoxiribonucleotide chain termination method?

Answer 5

ddntp (dideoxiribonucelotide triphosphate) –> added to a growing chain of DNA, where DNA polymerase can’t add more nucleotides (picks up a tagged ddntp instead of another) so the chain growth stops.
–> each type of ddntp (depending on nitrogenous base attached to sugar) is tagged with a marked fluorescent tag, so the identity of the nucleotide that ends each strand is identified, which is the complementary nucleotide to the last one in the known sequence.

Question 6

what is the human genome project?

Answer 6

a project that determined the base pairs that make up the human D?A, and mapping all the genes in the human genome.

Question 7

Non-gene regions of eukaryotic chromosomes are DNA with no roles, what are introns?

Answer 7

• non coding sequences of DNA (removed by RNA splicing)

- interrupts exons (coding sequences) of eukaryotic genes.

Question 8

what are telomers?

Answer 8

• special nucleotide sequences which consist of many repetitions of one short nucleotide sequence.
• occur near ends of DNA molecules & ‘seal’ the ends of linear DNA together.
• prevent erosion of genes.

Question 9

what is tRNA?

Answer 9

• transfer RNA : genes that transfer RNA
• parts of DNA template that code for short lengths of RNA that transfer amino acids from a pool of amino acids to a growing polypeptide in a ribosome.

Question 10

in what phase does DNA replication take place?

Answer 10

interphase

Question 11

what is the leading strand?

Answer 11

• the exposed 5’ to 3’ strand

- where DNA polymerase adds nucelotides to the free 3’ end of the exposed strand.

Question 12

what is the lagging strand? what occurs here?

Answer 12

• where replication is discontinuous
• RNA primer is formed by primase then DNA polymerase III adds nucleotides to it –> forming an okazaki fragment.
• -> DNA polymerase I replaces RNA nucleotides at the start of each fragment, with DNA nucleotides –> ligase joins okazaki fragments together (these DNA fragments are synthesised and joined together)

Question 13

how has it been found that the DNA contains genetic information and not proteins in chromosomes?

Answer 13

during the replication of a bacteriophage (phage)

1) phage attaches to the bacterial wall, and injects the virus with a DNA.
2) virus takes DNA to host’s synthesis location
3) new viruses are assembled & escape to repeat the infection cycle.
4) found out that the DNA inside the protein head of the phage remained inside the virus and infected the virus’ DNA, not the protein.