DNA Packing

Question 1

Organization of DNA in most prokaryotes

Answer 1

Circular chromosomes
dsDNA (double stranded)
May also contain plasmids (small DNA molecules that can replicate independently of chromosomal DNA)
Contained in nucleoid

Question 2

Relaxed circular DNA

Answer 2

1 strand’s backbone has been nicked (cut): allows for relaxation
Neutral form of DNA

Question 3

Negative supercoiling

Answer 3

Untwisting DNA
Causes contortion and makes DNA more compact
Major form of supercoiling

Question 4

Positive supercoiling

Answer 4

Further twisting DNA

Causes contortion and makes DNA more compact

Question 5

Topoisomerases

Answer 5

Enzymes that control supercoiling
Twist and unknot DNA in a way that prevents breakage
Type 1: makes nick in 1 strand of DNA
Type 2: cuts both strands

Question 6

Organization of DNA in eukaryotes

Answer 6

Contained primarily in nucleus, but also in mitochondria and chloroplasts

Question 7

C value

Answer 7

Amount of haploid DNA in base pairs in organism

Question 8

C value paradox

Answer 8

Increased c value doesn’t necessarily mean increased complexity of organism

Question 9

Karyotype

Answer 9

Complete set of chromosomes

Question 10

Chromatin

Answer 10

DNA + protein in the nucleus

Question 11

Histones

Answer 11

Proteins that help pack chromatin into nucleus
Positively charged: DNA is negatively charged
Consists of H1, H2A, H2B, H3, and H4

Question 12

Nonhistones

Answer 12

Proteins associated with DNA

Various types: DNA replication, repair, transcription, recombination

Question 13

How DNA is packed

Answer 13

Double helix is wound around histones to form nucleosomes (10 nm fiber) -> 10 nm fiber is condensed and wound into 30 nm fiber -> 30 nm fiber is attached to protein scaffold

Question 14

30 nm fiber

Answer 14

Chromatin fiber of packed nucleosomes

Comprises 300 nm fiber

Question 15

10 nm fiber

Answer 15

Nucleosome and linker DNA (“beads on a string”)

Comprises 30 nm fiber

Question 16

Nucleosome

Answer 16

DNA wound around histone core

Question 17

Euchromatin

Answer 17

Chromatin that exhibits normal packing and unpacking in cell cycle
Contains genes that are actively being transcribed (loosely packed)
Typically devoid of repetitive sequences

Question 18

Heterochromatin

Answer 18

Packed tightly

Not being actively transcribed

Question 19

Constitutive heterochromatin

Answer 19

Present in all cells in same location on every chromosome

Ex- centromere

Question 20

Facultative heterochromatin

Answer 20

DNA condenses or decondenses depending upon status of cell
Changes from euchromatin into heterochromatin
Ex- X chromosome into Barr body

Question 21

Unique-sequence DNA

Answer 21

Occurring once or only a few times in genome

Ex- genes

Question 22

Repetitive DNA

Answer 22

Occurs from a few times to 1,000,000 times in genome
Packed tightly
Ex- telomeres, centromeres

Question 23

Dispersed repeated sequences

Answer 23

Distributed at irregular intervals in genome

Question 24

Tandemly repeated

Answer 24

Sequence that repeats itself many times in a row

Question 25

Centromere

Answer 25

Constitutive heterochromatin
Packed tightly: constricts chromosome
Mitotic spindle attaches to centromere in order to pull chromosomes apart during cell division

Question 26

Telomeres

Answer 26

Tandemly repeated DNA at the ends of linear chromosomes

Protects integrity of DNA from exonucleases (chew up ends of DNA: defense against foreign DNA)

Question 27

Satellite DNA

Answer 27

Form of highly repetitive DNA

Comprises centromeres