DNA, Chromosomes & Genomes II Flashcards
What are two views that challenge the theory that histones are more than just structural proteins?
Mammalian chromatin contains equal mass of histone and non-histone proteins.
Histones are highly conserved.
How does heterochromatin affect DNA?
It silences the genes it packages without regard to sequence and is directly inherited by daughter cells.
You are studying genetics in a laboratory. You examine a condensed chromatin that is highly concentrated around the centromeres and telomeres. Further testing shows that the chromatin contains few genes. What type of chromatin are you studying?
Heterochromatin
What type of chromatin is less condensed?
Euchromatin
What is position effect variegation?
Breakage events in the genes that bring heterochromatin near active genes tends to silence the genes.
What are three histone modifications?
Acetylation of lysines
Mono, di and tri-methylation of lysines
Phosphorylation of serines.
When are major histones synthesized?
During S-phase. They are assembled into nucleosomes on daughter DNA helices just behind replication fork.
When are variant histones synthesized?
During interphase. They are inserted into already-formed chromatin.
What is the purpose of the histone code?
It determines how/when DNA is packaged in a nucleosome. The code is also read by the code reader complex.
Reading a histone code invovles joint recognition of marks. Where are these marks found?
On the nucleosome and the histone tail. The meanings of the codes are largely unknown.
What is the purpose of code reader-writer enzymes?
They spread the marks (signals) over chromosomes.
What is the purpose of chromatin remodeling proteins?
They either condense or decondense long stretches of chromatin as the reader moves along.
How is remodeling contained to one area of the chromosome?
Physical barriers
Enzymatic barriers
HS4 region (protects the beta globin locus from silencing and contains a cluster of histone acetylase binding sites).
What is the purpose of centromeric heterochromatin?
It contains H3 histone CENP-A and other proteins that pack the nucleosomes into dense arrangements to form the kinetochore, a structure required for attachment of the mitotic spindle.
What is satellite DNA?
Centromere sequences in humans that consist of short repetitive DNA sequences.
What are neocentromeres?
Centromeres that form spontaneously on fragmented chromosomes, some of which lack alspha satellite DNA.
What does de novo centromere formation require?
A seeding event on allpha satellte DNA.
How are histones inherited?
H3 - 4 tetramers are directly inherited by the daughter strands at the replication fork.
What are the larges chromosomes known?
Lampbrush chromosomes.
It is a series of large chromatin loops emanating from a linear chromosome axis.
How are different genetic activities localized in the nucleus?
Different “neighborhoods” have effects on gene expression.
Distinct biochemical environments have high local concentrations of enzymes and molecules for specific processes.
What is the final step in chromosome packaging?
Condensing the mitotic chromosomes.
How are sister chromatids formed and how are they held together?
Two daughter DNA molecules replicated in interphase are separately folded to produce two sister chromatids. They are held together at their centromeres.
What are the two purposes of condensation?
Disentanglement of sister chromatids to allow separation for cell division.
Protection of fragile DNA molecule as separation occurs.
Compaction of DNA is aided by what proteins?
Condensins.
ATP hydrolysis coiles the DNA molecules into chromatids.
They are the major structural component of the core of every metaphase chromosome.
