Chapter 11 Flashcards
Telomeres prevent chromosomes from
being degraded at some level.
However, chromosomes become shorter with each round of replication
Evidence shows that telomeres
shorten with age, and premature shortening are associated with aging features.
Hardships encountered early in life can play a part in
telomere shortening!
Children in orphanages had significantly shorter telomeres
than those in foster homes!
E. coli –
has a single molecule of DNA (approximately 4.6 million base pairs).
Stretched out straight – it would be 1,000 times as long as the cell!
Human cells contain more than 6 billion base pairs of DNA!
Over 6 feet if stretched end to end!
Even the DNA in the smallest human chromosome would stretch 14,000 times the length of the cell nucleus!
Relaxed B-DNA has
~ 10bp per turn of helix. (100 bp of DNA would assume ~ 10 turns)
Supercoiling
- occurs when energy is used to add or remove any turns.
Strain on the molecule, causes the helix to twist on itself!
Positive supercoiling
Over-rotated molecules
Negative supercoiling
Under-rotated molecules
Topoisomerases –
enzymes that
add or remove rotations from the
DNA helix.
Break nucleotide strands, rotate
ends, and rejoin!
Tertiary structure: Supercoiling
Bacterial and eukaryotic DNA usually fold into loops stabilized by proteins
Prevents free rotation of the ends.
Supercoiling takes place within the loops.
Most DNA found is cells is _______supercoiled!
negativity
- Supercoiled DNA can be packed into smaller space than relaxed DNA.
- Makes the separation of strands easier for replication and transcription (requires less energy and is faster!)
A DNA molecule 300 bp long has 20 complete rotations. This DNA molecule is
negativity
The Bacterial Chromosome
Single circular DNA molecule
Bacterial DNA is not attached to histone proteins, but is associated with other proteins that help compress.
DNA appears as a clump called the nucleoid
Eukaryotic Chromosomes
Each chromosome –consists of a single long linear DNA molecule.
Extreme folding required!
The tertiary chromosomal structure of eukaryotic DNA is not static!
Changes regularly in response to various cellular processes (Cell cycle stages, meiosis, replication, transcription, etc)
Chromatin =
complex of DNA and proteins
Euchromatin:
undergoes normal process of condensation and decondensation in the cell cycle
Heterochromatin:
remains in a highly condensed state throughout the cell cycle, even during interphase.
constitutive heterochromatin
Permanent heterochromatin at the centromeres and telomeres; and the Y chromosome as well
facultative heterochromatin
May also occur at certain developmental stages;
Facultative heterochromatin occurs along one entire X chromosome in female mammals when that X becomes inactivated!
Histones
The most abundant proteins in chromatin
Small, positively charged proteins, 5 major types (H1, H2A, H2B, H3 and H4). - 2 of each except H1
Positive charges attract the negative charges on the phosphates of DNA !
Chromatin Structure
DNA double helix
Cont.
Neutralizing their positive charges would have which effect on the histone proteins?
Changes in Chromatin Structure
Dynamic nature was observed in polytene chromosomes.
Giant unusual chromosomes that arise when repeated rounds of DNA replication take place without accompanying cell division.
Chromatin Modifications
Methylation of DNA (Ch. 10)
Use of variant histone proteins in the nucleosome
Binding of nonhistone proteins to DNA and chromatin
Although these changes don’t alter DNA sequence, they can have major changes on expression of genes.
epigenetic changes (epigenetics)
Stable alterations of chromatin structure that may be passed down to descendant cells or individuals
Nucleosome
~146 bp DNA + eight histone proteins (histone core)
Chromatosome
~ 166 bp DNA + histone core + H1
