Chromosome Strucutre and Chromatin Flashcards
What is chromatin?
the material of which chromosomes are made of
What is a chromatid?
one of the two copies of a replicated chromosome that is joined at the centromere to the other copy
What is the centromere?
the chromosomal region that holds sister chromatids together and where the kinetochores forms
What is a kinetochore?
a protein complex that forms on chromosome centromeres during M phase that binds microtubules and directs chromosome movement in mitosis
What are telomeres?
the ends of chromosomes
Where are the chromosomes?
the nucleus
What proteins form chromatin?
histones
What are genes transcribed to produced?
the RNA molecules needed to programme translation in the cytoplasm
What encode proteins and genetic traits?
the sequence of bases in the DNA
What is genetic material split into in eukaryotic cells?
individual long pieces of double stranded DNA (chromosomes)
How many chromosomes in humans?
46
Why does DNA take up little space in the nucleus?
DNA is a naturally helical molecule and is supercoiled using specialised enzymes
When are chromosomes active?
all the time
How many histone pieces are in a nucelosome?
8
Metaphase chromosomes:
(in mitosis) highly condensed and compacted
Interphase chromosomes:
visible less distinct and exhibit variable levels of compaction depending on their activity
Euchromatin:
largely de-compacted and potentially active in gene expression
Heterochromatin:
highly compacted and transcriptionally inactive(in general)
What do centromeres and telomeres contain?
repetitive DNA sequences
What des electron micro graphing show?
‘unfolded’ chromatin: ‘beads on a string’
‘Beads on a string’:
beads are nucleosomes and string is DNA
Why is the wrapping loose in euchromatin?
so that the raw DNA may be accessed
What process allows chromatin to become more compact?
condensation
What does protein does chromatin condensation involve?
histone H1
Where do individual chromosomes tend to concentrate?
within discrete territories with limited intermingling
What types of chromatin is visible during interphase?
- euchromatin
- heterochromatin
Describe euchromatin
diffuse and light staining; actively transcribed genes; tend to concentrate to the middle of the nucleus
Describe heterochromatin
condensed, dark staining, genes not transcribed; repetitive sequences; tend to concentrate at the peripery
What is a method by which gene expression can be controlled?
DNA accesibility
Why must gene expression be regulated?
to ensure proper timing and location of protein production
Can regulation occur at multiple points?
yes
What is chromatin remodeling?
the alteration of chromatin structure
Why do nucleosomes contain DNA and histones in a tight complex?
it compacts, making DNA less accessible
Can transcription factors bind to DNA in the nucleosome?
yes
What do eukaryotic cells have a large variety of?
histone modifying enzymes that covalently modify chromatin structure and ATP-dependant chromatin remodelling complexes, to permit DNA replication and transcription
What do each histone have at it N terminus?
a positively charged amino acid “tail”
What can histone acetyltransferases(HATs) do?
change their tails charge by adding acetyl groups to lysine residues (HDAC’s remove them)
Possible mechanisms for increased local access include?
- nucleosome sliding
- nucleosome displacement
- partial histone displacement
- replacement of octamer subunits with histone variants
