1.2 - Chromosomes and Chromatin Flashcards
non coding DNA sequences (3)
- genes containing introns
- regulation (cell specific expression)
- junk DNA - repeated sequences; transposons, simple repeat and duplications (often associated with heterochromatin)
distinct sequence elements required for a functional chromosome (3)
- telomeres
- replication origins
- centromere
telomeres (telomeric repeat sequences) (3)
- found at ends of chromosomes
- provide buffer between gene sequences and end of chromosome
- bound by protein that help protect ends from exposure
multiple origins of replication (4)
- at least one per arm
- after replication there are 2 identical sister chromatids
- each chromatid contain one old/one new strand (semi-conservative replication)
- sisters held together until chromosome segregation during mitosis
centromeres (3)
- normally associated with arrays of repeated DNA sequences
- in humans - alpha satellite repeats are AT-rich and vary slightly from one another in their DNA sequence
- flanking centric heterochromatin contains DNA sequences composed of different types of repeats
kinetochore (4)
- positioned within inner repeats of centromere
- consists of inner and outer plate
- formed by set of kinetochore proteins
- spindle microtubules attach to the kinetochore in M phase of cell cycle
chromosome organisation in interphase nuclei (cell cycle)
chromosomes are distributed throughout the nucleoplasm
chromosome organisation at mitosis
chromosomes condense - individual chromosomes can be seen with the light microscope
chromosome organisation in interphase nuclei (2)
- during interphase - individual chromosomes are not obviously structured in a stable conformation, but still partially resolved from one another across nuclei
- arrangement minimises tangles between chromosomes and helps condensation at mitosis
where does the Rabl configuration occur?
in some cell types (especially plant cells)
The Rabl configuration
chromosomes adopt configuration with the centromeres clustered at one end, telomeres abutting (adjacent) the nuclear envelope at opposite pole reflecting arrangement of the chromosomes at anaphase
how condensed can chromosomal DNA be?
mitotic chromosomes can be up to 10,000x more compact than the length of the DNA
- how the DNA is packaged is highly variable
how is DNA packaged
compacted by association with proteins -> chromatin
at the simplest level how are nucleosomes arranged on DNA
like beads on a string
structure of nucleosome (3)
- nucleosome core particle
- protein core
- octameric histone core
nucleosome core particle
consists of 147 base pairs of a DNA wound around a protein core
protein core
made up of small highly basic proteins called histones
octameric histone core
contains 2 molecules of each of 4 histones (H2A, H2B, H3, H4)
histone H1
binds to both the DNA and nucleosome in these are where the DNA exits the nucleosome, stabilising chromatin fibre
higher order packaging of chromatin (2)
- DNA wound around histones
- additional histone (H1) arranges nucleosomes into 30nm fibre
30nm nucleosome fibre during mitosis
further condensed at 2 additional levels to achieve packaging observed in mitotic chromosomes
what varies in different genome regions during interphase?
extent of packaging DNA into chromatin
forms chromatin in nucleus appears in under electron microscope (2)
- euchromatin
- heterochromatin
euchromatin (2)
- transcriptionally active
- relatively loose nucleosome arrangement
heterochromatin (2)
- transcriptionally inactive
- associated chromatin configurations that are highly compacted (inactive genes)
which regions of genome are associated with euchromatin?
transcriptionally active
which regions of genome are associated with heterochromatin?
repressed
histone remodelers
promote sliding of histone DNA to promote wither euchromatin or heterochromatin (ATP dependant)
typically what state of chromatin are telomeres and centromeres?
heterochromatin
histone tail
each histone has a tail that extends out of the nucleosome core
how can histone tails be modified?
by methylation and acetylation to influence chromatin structures and thus gene expression
how can histone modification effect chromatin organisation and thus gene expression?
can change overall chromatin structure by either directly affecting how nucleosomes pac together by promoting/ inhibiting binding of proteins that can alter chromatin organisation
what does heterochromatin spreading require?
reader-writer coupling
reader-writer coupling (2)
- one protein recognises and binds to the modified histone
- bound protein then recruits histone modifying enzyme which modifies neighbouring histones
how is heterochromatin established?
DNA-binding proteins (DBPs) can recruit histone modifying enzymes (HME) to chromatin
how do ribonucleoproteins and RNA-binding proteins recruit modify chromatin
a nascent transcript generated by RNA polymerase can contain recognition motifs for either ribonucleoprotein (RNP) or RNA-binding proteins (RBP) which recruit histone modifying enzymes (HME) to chromatin
X chromosome inactivation (3)
- to ensure equal gene expression between XX and XY individuals the extra X is transcriptionally silenced in early development
- silenced heterochromatin state is the inherited throughout rest of development
- established in mammals via expression of non-coding RNA Xist on the X chromosome that will be inactivated
what can happen if different X chromosomes have lightly different genes
differences of random X left activated are maintained in clonal offspring of embryonic cell
