BIO 142 Week 8 Readings - Chapter 11 - DNA within the cell, DNA classes in eukaryotes, modes of replication - Genetics : A Conceptual Approach.
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11.1 Large Amounts of DNA Are Packed Into a Cell
● Tertiary structure of DNA allows it to be packed into the confined space of the cell
○ High-order folding
Supercoiling
● Takes place when the DNA helix is subjected to strain by being overwound or underground
○ B-DNA is the lowest-energy, most relaxed state of DNA; a stretch of 100bp of DNA would
assume ~10 turns
○ Overrotated molecules exhibit positive supercoiling
○ Underrotated molecules exhibit negative supercoiling
○ Partial solution to the cell’s “storage problem”; supercoiled DNA occupies less space than relaxed
DNA
● Supercoiling relies on topoisomerases
○ Topoisomerases: enzymes that add or remove rotations from the DNA helix by temporarily
breaking the nucleotide strands, rotating the ends around each other, and then rejoining the broken
ends
■ Can both induce and relieve supercoiling
● Most DNA found in cells is negatively supercoiled - two advantages over relaxed state:
○ Makes the separation of the 2 DNA strands easier during replication and transcription
■ Underrotated → separate of the 2 strand is faster and requires less energy
○ The supercoiled DNA can be packed into a smaller space than relaxed DNA
The Bacterial Chromosome
● Bacterial genomes consist of a single circular DNA molecule that forms a series of twisted loops
○ Bacterial DNA is associated with a number of proteins that help to compact it (unlike eukaryotic
DNA, which uses histone proteins)
● Nucleoid: the distinct clump that appears when bacterial DNA is viewed with a microscope
Eukaryotic Chromosomes
● Each eukaryotic chromosome consists of a single extremely long linear molecule of DNA
○ Tremendous packing and folding is required to make the DNA fit into the nucleus
○ Extent of packing and folding changes throughout the cell cycle
, ● Interphase: the chromosomes are in an elongated, relatively uncondensed state
○ “Relatively uncondensed” = still highly condensed, just less so than the DNA of mitotic
chromosomes
● Throughout the cell cycle, chromosomes progress from a highly packed state to a state of extreme
condensation (necessary for chromosome movement in mitosis and meiosis
● DNA packing changes locally during replication and transcription, when the 2 strand have to unwind
○ Packing of eukaryotic DNA is not static but changes regularly in response to cellular processes
Chromatin
● The complex of DNA + proteins = chromatin
○ Euchromatin: undergoes the normal process of condensation and decondensation in the cell cycle
■ The majority of the chromosomal material
■ Where most transcription takes place
○ Heterochromatin: remains in a highly condensed state throughout the cell cycle (even interphase)
■ Permanent heterochromatin located at the centromeres and telomeres of chromosomes
● Called constitutive heterochromatin
■ General lack of transcription
■ Absence of crossing over
■ Replication late in S phase
■ Facultative heterochromatin appears at certain developmental stages
Characteristic Euchromatin Heterochromatin
Chromatin condensation Less condensed More condensed
Location Chromosome arms Centromeres, telomeres, other
specific places
Type of sequences Unique Repeated (only constitutive
heterochromatin)
Presence of genes Many Few (only constitutive
heterochromatin)
When replicated Throughout S phase Late S phase
Transcription Often Infrequent
Crossing over Common Uncommon
● Most abundant proteins in chromatin = histones
● Histones:
○ Small positively charged proteins
○ 5 major types:
■ H1
■ H2A
■ H2B
■ H3
■ H4
○ Have a high percentage of arginine and lysine → give positive charge
○ Positive charge attracts the negatively charged phosphates of DNA; this attraction holds the DNA
in contact with histones
● Variant histones can be incorporated into chromatin in place of 1 of the major types; these alter chromatin
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