W2 Organisation of DNA in chromosomes

Question 1

Differences between Purines and Pyrimidines

Answer 1

Purines:
-9-membered double ring
-Adenine & Guanine

Pyrimidine:
-6-membered single ring
-Cytosine and Thymine

Question 2

DNA Supercoiling

Answer 2

-Amount and type of supercoiling controlled by topoisomerases
-Introduces negative supercoils or convert negatively supercoiled DNA
into relaxed DNA
(Negative supercoil = Underwound)
(Positive supercoil = Overwound)

Question 3

Bacterial DNA Looping

Answer 3

Bacterial DNA is organised into looped domains:
-Tenfold compaction of DNA

Question 4

Chromosome Terminology:

Answer 4

https://gyazo.com/00afd22eb95cc93a2865f4870c8c16c7

Question 5

Chromosome Morphology: Hs17q21.1

Answer 5

Hs = H. sapiens
17 = Chromosome 17
q = Q arm (long arm)
21.1 = Band number

Question 6

Histone structure:

Answer 6

-Histone core is an octamer of 4 histones x 2 (H2A, H2B, H3, H4)
-Small basic proteins with net +ve charge (So can bind to -ve DNA)

Question 7

Nucleosome:

Answer 7

-Histone core + ~147bp segment of DNA wound about 1.65 times
-Condenses DNA by ~6x
-Histone H1 lies outside core

Question 8

‘Beads on a string’ model:

Answer 8

-Individual nucleosomes connected by linker DNA
-Binding of H1 condenses the ‘beads on a string’ structure

Question 9

Solenoid structure:

Answer 9

-The binding of H1 causes nucleosomal DNA to condense further into a 30nm compact structure (called the 30-nm chromatin fiber)

Question 10

Types of chromatin:

Answer 10

Euchromatin:
-Light stain
-Less condensed
-Actively transcribed
-Coding DNA

Constitutive heterochromatin:
-Dark stain
-Condensed
-Transcriptionally inactive
-Repetitive DNA

Facultative heterochromatin:
-Varies in state in different cell types
-Condensed
-Transcriptionally inactive
-Non repetitive DNA

Question 11

Centromeres:

Answer 11

-Contain blocks of repetitive non-coding (‘satellite’) DNA
-Form point of attachment of spindles fibers
-Responsible for accurate segregation of chromosomes during cell division
-Same function in all eukaryotes but no common DNA sequence
-Different chromatin structure and packing than other chromosome regions

Question 12

Telomeres:

Answer 12

-Required for replication and stability of the chromosome
-If telomeres are removed, chromosomes fuse end to end
-Vertebrate telomeres share a common simple telomeric sequence (5’-TTAGGG-3’)
-Length maintained by telomerase
-Telomerase adds single strand at the 5’ end, polymerase adds the complementary strand