BB1720 DNA packaging Flashcards
How much DNA is in a human cell?
6 billion base pairs
0.6 billion turns
each turn is 3.4nm
= 2m
How is DNA packaged in the cell?
- by histones/non histone chromosomal proteins that bind and fold DNA
- occurs in chromosomes
- nucleosomes begin the process
What do histones do?
short regions of DNA wrap around histones to form chromatin
role of non histone proteins?
support DNA structure
what is a chromosome?
long linear DNA molecule associated with proteins
how is a nucleosome formed?
when 8 separate histone protein sub units attach to the DNA molecule
functional unit of DNA packaging
which proteins are involved in the formation of the nucleosome?
102-135 amino acids that interact with DNA
2x H2A, H2B, H3 and H4
what is the difference between H2A H2B H3 and H4?
H2A, H2B, H3 and H4 form the octamer
they are part of a histone family
similar structures and amino acid sequences
varients of histones with roles in gene regulation
How much and how far does DNA wrap around the histone?
146bp
1.65x
How does the nucleosome form the chromosome?
- adjacent nucleosomes connect via spacer DNA
- histone 1 attaches and stacks two nucleosomes to form a chromatosome
- DNA wraps around the histone and compresses the structure
- the string of beads coil and nucleosomes stack to form a chromatin fibre
- chromatin fibre is condensed and folded by addition of scaffold proteins
- all join and form a mitotic chromosome
How are chromosomes in non dividing cells organised?
- very fine and tangled
- DNA packed in folded chromatin fibres
how does the cell access tightly packed DNA to make proteins?
- scaffold proteins
- 30nm chromatin loop fibre attaches to central scaffold
- similar loop and scaffold structures occur in interphase but not visible
- lampbrush chromosomes
what are lampbrush chromosomes?
- transcriptionally active chromosomes found in the germinal vesicle of large oocytes of many animals
- the cells are in prophase of first meiotic division
what happens to chromosomes after DNA replication?
more condensed
non dividing cells in interphase
certain insects have visible chromosomes during interphase
what does it mean if there is a break in dna pattern
translation and trascription are occuring
what are the different types of chromatin in interphase?
euchromatin
heterochromatin
features of euchromatin?
less condensed
doesnt strongly stain
major genes involved in transcription
unmodified by enzymes
features of heterochromatin?
highly condensed
visible by light microscope in eukaryotes
gene activity suppressed
modified by enzymes
where are chromosomes contained in eukaryotes
the nucleus
what happens to chromosomes during interphase?
highly organised
regulated
why is the rate of transcription lower in the presence of histones?
machinary cannot access DNA
why are chromosomes not visible during interphase?
cell has a relaxed structure
no access to cell
How many types of histone molecules are commonly found in eukaryotic cells?
5
which charge amino acids do histones contain?
Histones have a high content of positively charged amino acids.
what is the association of DNA and histone is mainly mediated by?
hydrogen bonds
whats the features of histone H1 DNA packaging
H1 binding gives nucleosome a defined angle
H1 binds to two DNA helices at once
H1 induces compaction (“30 nm fibre”)
H1 stabilises higher-order chromatin structures
what is the state of chromosomes in interphase cells?
not highly compacted and are clearly visible
Transcription and other aspects of nucleic acid metabolism are?
active in euchromatin
repressed in heterochromatin
A picture in which all of the homologous chromosomes of an organism have been stained and organised is called?
karyogram
how are chromosomes in the nucleus are highly organised?
all chromosomes are in defined locations in the nucleus
what is a core nucleosome is composed of?
two copies each of histone H2A, H2B, H3 and H4 and 146 bp of DNA
how does in vitro, the rate of transcription from DNA differ depending on histones?
is lower when histones are present than when histones are absent
