DNA & Chromosomes II (Lec 2) Flashcards
What are some examples of inheritance superimposed on DNA (epigenetics)?
DNA methylation
Chromatin Structure
Histone modification
What are some of the challenges to the view that histones were thought to be just involved in packaging DNA?
mammalian chromatin contains equal mass of histone and non histone proteins
histones are highly conserved
one form of chromatin silences the genes it packages without regard to sequence and is directly inherited by daughter cells
True or False?
Heterochromatin is very condensed.
true
Where is heterochromatin highly concentrated at?
centromeres and telomeres
True or False?
Heterochromatin contains a large number of genes
False, very few number of genes
What is the position effect?
activity of a gene depends on position on chromosome; will be silenced if relocated near heterochromatin
The less condensed form of chromatin is called?
euchromatin
What part of histones are subject to a variety of covalent modifications?
Amino acid side chains
Variants exist for each core histones except for __
H4
When and where are major histones synthesized?
during S-phase and assembled into nucleosomes on daughter DNA helices just behind replication fork
When and where are variant histones synthesized?
during interphase and inserted into already-formed chromatin. (requires histone exchange process catalyzed by chromatin remodeling complex)
What does the histone code tell you?
determines how/when DNA is packaged in nucleosome
What reads the histone code?
code reader complex; involves joint recognition of histone tail
What does reading histone code entail?
involves joint recognition of marks at other sites on nucleosome along with tail recognition
How does modified chromatin spread?
after modifying enzyme marks one or few neighboring nucleosomes, chain reaction can ensue; code reader-writer enzymes spread the mark over chromosome
How does chromatin remodeling occur?
Complexes with ATP-dependent chromatin remodeling protein work together to either condense or decondense long stretches of chromatin as reader moves along
What stops the spread of chromatin modifications?
barrier sequences: several mechanisms (physical barriers, enzymatic barriers), HS4 region which contains cluster of histone acetylase
Why is centromeric Heterochromatin special?
contains centromere-specific H3 histone, CENP-A, and other proteins that pack the nucleosomes into dense arrangements to form the Kinetochore.
defined by assembly of proteins, not DNA sequence
How is centromeric chromatin organized?
2 alternating types of chromatin: nucleosomes with CENP-A on the outside fold, nucleosomes with normal H3 on the inside
What binds to the kinetcohore?
CENP-A
What chromatin structures are directly inherited by the daughter strands at the replication fork?
H3-H4 tetramers
What are the largest chromosomes known?
lampbrush chromosomes from amphibian oocytes
In regards to chromatin movement, how does heterochromatin move?
preferentially associates with the nuclear lamina
True or False?
The interior of the nucleus is very homogenous
false, heterogeneous
Are mitotic chromosomes highly condensed?
yes
Chromatids are held together at ____
centromeres
What is the purpose of condensation?
disengagement of sister chromatids to allow separation for cell division, protection of fragile DNA molecule as separation occurs
What are condensins?
proteins that aid in condensation; use ATP hydrolysis to coil the DNA molecules into chromatids
What are homologues?
genes that are similar in both sequence and function due to common ancestry
True or False?
Gene sequences are more tightly conserved than genome structure
true
How do genomic changes occur?
occur as mistakes in DNA replication and repair (1/1000)
In regards to evolution of globin gene family, duplication and mutation gave rise to what?
beta and alpha genes
translocation moved alpha to separate chromosome, further duplication and mutation resulted in more specialized beta molecules
What are pseudogenes?
duplicated gene that has become irreversibly inactivated by multiple mutations
True or False?
Histone modifications are transient, not permanent.
true
What happens in gene duplication and divergence?
both copies remain functional while diverging in sequence and pattern of expression
