DNA Packag. a Chroma. Struc

Question 1

Why is DNA packed?

Answer 1

• to fit in the cell
• if the DNA molecule in a diploid cell were laid out end to end, the total length of DNA would be approximately 2 meters.
• the diameter of a typical cell nucleus is only 10 μm
• To be protected
• Having higher levels of organisation to prevent itself from becoming an unmanagable tangle (unüberschaubares Gewirr)
• To regulate which genes are accessed and when
• To control the accessibility of enzymes of replication and repair

Question 2

How does it (packing) happen in procaryotes?

Answer 2

• Prokaryotic cells do not contain nuclei or other membrane-bound organelles
• The nucleoid is simply the area of a prokaryotic cell in which the chromosomal DNA is located
• prokaryotes compress their DNA into smaller spaces is through supercoiling

Question 3

What kind of chromosomes do Prokaryotic/ Eukaryotes have?

Answer 3

• many prokaryotes contain a single circular chromosome

- eukaryotes contain multiple linear chromosomes

Question 4

How are prokaryotic chromosomes and eukaryotic chromosomes condensed?

Answer 4

• Prokaryotic chromosomes are condensed in the nucleoid via DNA supercoiling and the binding of various architectural proteins.
• eukaryotic chromosomes are condensed in a membrane-bound nucleus via histones

Question 5

How many copy of each genes do prokaryotes/ eukaryotes have?

Answer 5

• Most prokaryotes contain only one copy of each gene (i.e., they are haploid)
• Most eukaryotes contain two copies of each gene (i.e., they are diploid)

Question 6

What kind of DNA do prokaryotic/eukaryotic genomes contain?

Answer 6

• Prokaryotic genomes are efficient and compact, containing little repetitive DNA
• Eukaryotes contain large amounts of noncoding and repetitive DNA

Question 7

How is a condensed chromosome constructed?

Answer 7

-Telomere, centomere, Arm

Question 8

What are nucleosomes?

Answer 8

• DNA and histones are organized into repeating subunits
• Each nucleosome contains a disk-shaped core complex consisting of two copies each of histone H2A, H2B, H3, and H4 assembled into an octamer. The remaining histones H1 is found associated with the outer surface of the core particle

Question 9

What are core and linker histones? How many base pairs of DNA helix does one nucleosome contain?

Answer 9

• Four core histones in nucleosome
• Two of each of H2A, H2B, H3 & H4
• Fifth histone, H1 is the linker histone
• > One nucleosome contains 146 base pairs of DNA helix

Question 10

What are histone proteins?

Answer 10

-Histone proteins

basic (contain of mostly positively charged amino acids such as: lysine & arginine) bind DNA backbone

Question 11

What holds the DNA and histones together?

Answer 11

• DNA is negatively charged and histones are positively charged
• Around 140 hydrogen bonds are formed between DNA and the histone core in each nucleosome
• Hydrophobic interactions and salt linkages
• The type of bending of DNA

Question 12

With what does nucleosome dissociate? And into what?

Answer 12

-dissociates with high salt concentration into octameric histone core and 146- nucleotide-pair DNA double helix

Question 13

What digests the linker DNA?

Answer 13

-nuclease

Question 14

What do nucleosomes form?

Answer 14

• they join to form 30nm fiber (faser) (->more compact structure)
• > H1 (histone) play a role in this compaction

Question 15

What is chromatin?

Answer 15

-a complex of DNA and chromosomal proteins
~ twice as much protein as DNA
-construct chromosome

Question 16

Nam two mayor types of proteins

Answer 16

-Histone: abundant, basic proteins with a positive charge that bind to DNA
->5 main types: H1, H2A, H2B, H3, H4
~equal in mass to DNA
highly conserved (erhalten) across species
-Non-histones: all the other proteins associated with DNA
-differ markedly in type and structure
-amounts vary widely
-not highly conserved across species

Question 17

Which four levels appear doing the process of constructing a chromosome?

Answer 17

• Level 1: Nucleosomes or ‘beads on a string’
• Linker DNA and Histone proteins form a complex
• Level 2: Chromatin fibre
• > Nucleosomes coil and stack
• Level 3: Looped fibres:
• > Chromatin fibre loops (ziehen) along the protein scaffold (Eiweißgerüsts)
• Level 4: More folds to make the most compacted chromosome
• > Looped chromatin fibre folds

Question 18

What does tails have?

Answer 18

-histones

Question 19

What is chromatin made of?

Answer 19

-DNA, histone proteins, non histone proteins in equal mass…

Question 20

What is the difference between chromatin and chromosome?

Answer 20

• During cell division and DNA replication, chromatin are condensed into whole chromosomes
• chromosome consists of the highest condensed structure of the DNA double­helix for the proper separation of the genetic material between daughter cells

Question 21

What happens before cell division?

Answer 21

• the chromosomes are duplicated before cell division

- >This make two sister chromatids attached from the centromere

Question 22

Which proteins are needed for chromatid attachment?

Answer 22

• Cohesin and condensin are two ring shaped proteins
• Cohesin holds two sister chromatids together
• Condensin is involed in condensations and configures duplicated chromosome for seperation

Question 23

Describe the different steps in chromosomes during mitosis

Answer 23

-end of S phase -> G2 (uncondensed sister chromatids, arms are cohered)
-beginning of prophase (condensed sister chromatids arms are cohered)
-Middle of prophase (condensed sister chromatids, arms are free)
(cohesion along chromosomes arms are released)
-Anaphase (condensed chromatids have separated) (cohesion at centromer is degraded)