Bacterial Chromosomes
This region of a bacterial cell contains its highly compacted chromosome.
Nucleoid
Most cellular DNA has this type of supercoiling.
Negative supercoiling
These structures make up the “beads” in the beads-on-a-string model.
Nucleosomes
This type of chromatin is less condensed and is frequently transcribed.
Euchromatin
This chromosome region attaches to the kinetochore.
Centromere
Unlike most eukaryotic chromosomes, the E. coli chromosome has this overall shape.
Double-stranded closed circle/circular
Negative supercoiling makes these two DNA processes easier by promoting strand separation.
Replication and transcription
Name the four histones that make up the nucleosome core.
H2A, H2B, H3, H4
These enzymes add acetyl groups to lysines on histone tails.
Histone acetyltransferases
These structures act as protective caps at the natural ends of chromosomes.
Telomeres
Bacterial DNA is compacted by being highly folded into a series of these structures.
Twisted loops
A DNA molecule has 300 bp and 20 complete rotations; classify it as relaxed, positively supercoiled, or negatively supercoiled.
Positively supercoiled because relaxed DNA has ~10 bp/turn, so 300 bp would normally have ~30 turns; 20 means it is underrotated.
If a cell contains 50 nucleosomes, how many copies of H2B are present?
100 H2B molecules because each nucleosome contains two H2B proteins.
Histone acetylation generally causes chromatin to become more accessible or less accessible to transcription?
More accessible to transcription
What happens during mitosis to a chromosome fragment that lacks a centromere?
It is lost during mitosis.
Give two major structural differences between a typical bacterial chromosome and a human chromosome.
Bacterial chromosomes are typically circular and relatively simple/compact, whereas human chromosomes are multiple, linear, and extensively associated with histones/proteins.
Besides facilitating strand separation, what is the other major advantage of negative supercoiling described in the chapter?
It allows DNA to be packed into a smaller space.
Approximately how many base pairs of DNA are wrapped around each histone octamer, and what histone acts as a clamp at the linker DNA?
145–147 bp; H1
Predict what would happen to transcription if histone deacetylase activity increased.
145–147 bp; H1
Why do telomeres become progressively shorter in most somatic cells?
Because most somatic cells do not sufficiently maintain their chromosome ends with telomerase, so telomeres shorten with successive cell divisions.
A mutation prevents bacterial DNA from forming its normal folded and twisted-loop organization; predict the major problem the cell would face regarding its chromosome.
The chromosome would have difficulty compacting into the small bacterial cell/nucleoid.
Two identical DNA molecules differ only in supercoiling: one is relaxed and one negatively supercoiled; which should require less energy to initiate strand separation, and why?
Negatively supercoiled DNA, because underwinding facilitates separation of the two DNA strands.
Put these structures in order from least to most condensed: metaphase chromosome, nucleosome, 30-nm fiber, DNA double helix, looped chromatin.
DNA double helix → nucleosome → 30-nm fiber → looped chromatin → metaphase chromosome
A gene becomes highly sensitive to DNase I after chromatin remodeling; what does this suggest about its chromatin structure and transcriptional activity?
DNA double helix → nucleosome → 30-nm fiber → looped chromatin → metaphase chromosome
A mutation eliminates telomerase in a rapidly dividing germ-cell lineage; predict what will happen to its telomeres over successive divisions and why.
The telomeres would progressively shorten with successive divisions because telomerase normally lengthens telomeres and prevents chromosome shortening in germ cells.