structure & organization of the eukaryotic genome

Question 1

what does the term ‘genome’ refer to?

Answer 1

genome refers to the entire genetic material of the organism

Question 2

what is the proportion of genes and non-coding sequences in most multicellular eukaryotes?

Answer 2

• only a small proportion of all the DNA in a cell consists of genes
• most of the DNA is made up of non-coding sequences

Question 3

why is there a need to pack DNA?

Answer 3

• this is because a eukaryotic DNA molecule is very long, hence, DNA packing is required to compact & fit all the DNA molecules into the nucleus to prevent breakage of DNA

Question 4

how is DNA packed?

Answer 4

• eukaryotic DNA is combined with a large number of proteins to form chromatin fibres in a non-dividing cell
• the chromatin then goes through multiple levels of packaging, resulting in condensed metaphase chromosomes in cells undergoing cell division

Question 5

what does the level of DNA packing depend on?

Answer 5

• whether the cell is undergoing cell division in both interphase & dividing cells, or only dividing cells!!

Question 6

what are the levels of DNA packing if the cell is undergoing cell division in both interphase & dividing cells?

Answer 6

1. DNA wraps around proteins called histones to form nucleosomes
2. the chain of nucleosomes coil to form the 30nm chromatin fibre
3. the 30 nm chromatin fibre forms looped domains attached to a protein scaffold, forming the 300nm chromatin fibre

Question 7

what is the level of DNA packing if the cell is only undergoing dividing of cells?

Answer 7

• the looped domains coil & fold further to give rise to highly condensed chromosomes

Question 8

give an overview of the 4 different levels of chromatin packing in multicellular eukaryotic cells

Answer 8

1. formation of 10 nm chromatin fibre via wrapping of DNA around histones to form nucleosomes
2. formation of 30 nm chromatin fibre by coiling of 10 nm chromatin fibre
3. formation of looped domains by 30 nm chromatin fibre, which are attached to a protein scaffold, making up a 300 nm chromatin fibre
4. coiling & folding of looped domains to further compact the chromatin fibre to form the metaphase chromosome

Question 9

what is a nucleosome?

Answer 9

• a nucleosome is the basic unit of DNA packing

Question 10

what does each nucleosome consist of?

Answer 10

• each nucleosome consists of DNA wound twice around a histone octamer (8 histone proteins)

Question 11

what are the type & number of molecules present in the histone octamer?

Answer 11

• 4 types, 2 of each type

Question 12

what is the DNA between nucleosomes called?

Answer 12

linker DNA

Question 13

what do nucleosomes form?

Answer 13

the 10 nm chromatin fibre

Question 14

what is the structure of the 10 nm chromatin fibre?

Answer 14

it resembles beads on a string, where each ‘bead’ is a nucleosome core particle

Question 15

what are histones?

Answer 15

they are the major structural proteins of chromosomes

Question 16

why are histones able to bind tightly to the negatively charged DNA?

Answer 16

this is because histones have a high proportion of positively charge amino acids, mainly lysine & arginine, hence, they are able to bind tightly to the negatively charged DNA by ionic attractions

Question 17

what is the 30 nm chromatin fibre?

Answer 17

the nucleosomes & linker DNA coil to form the 30 nm chromatin fibre

Question 18

how many nucleosomes are assembled in 1 turn of 30 nm chromatin fibre?

Answer 18

about 6

Question 19

what are the molecules involved to stabilize the 30 nm chromatin fibre?

Answer 19

the histone proteins of nucleosomes, linker DNA & H1 histone interact to stabilise the 30 nm chromatin fibre

Question 20

what are looped domains?

Answer 20

they are the loops formed by 30 nm chromatin fibre

Question 21

what are looped domains attached to?

Answer 21

they are attached to a protein scaffold, resulting in a 300 nm chromatin fibre

Question 22

when is the metaphase chromosome observed?

Answer 22

ONLY IN DIVIDING CELLS

Question 23

what is the metaphase chromosome?

Answer 23

the metaphase chromosome is formed by the further coiling & folding of the looped domains on the protein scaffold to compact the 300 nm chromatin

Question 24

what are the 2 forms of interphase chromatin?

Answer 24

1. heterochromatin

2. euchromatin

Question 25

what is heterochromatin?

Answer 25

heterochromatin is highly condensed DNA that usually contain DNA that are not transcriptionally active

Question 26

why does heterochromatin contain genes that are not transcriptionally active?

Answer 26

this is because heterochromatin is too highly condensed, hence, general transcription factors & RNA polymerase cannot bind to the TATA box on the promoter of the gene

Question 27

what is euchromatin?

Answer 27

euchromatin is less condensed DNA that contain genes that are transcriptionally active, as general transcription factors & RNA polymerase is able to bind to the TATA box in the promoter of the gene

Question 28

why does euchromatin contain genes that are transcriptionally active?

Answer 28

this is because euchromatin is less condensed, hence, the general transcription factors & RNA polymerase are able to readily bind with the TATA box in the promoter of the gene, initiating transcription