Chapter 12 Flashcards
Bacterial Chromosomes
Circular double stranded DNA compacted into necleoid
Readily replicated and transcribed
Viral Chromosomes
Either DNA or RNA Single or double stranded Circular or linear Inert until released into host cell Able to package long DNA into small volume
Supercoiling
Facilitates compaction of DNA
Closers circular molecules
More compact and sediment more rapidly than linear forms
Topoisomerases
Enzymes that cut one or not DNA strands.
Wind or unwind helix before resealing ends.
Supercoiled DNA and topoisomerases are found:
In eukaryotes
Prokaryote and eukaryote DNA
Replication and transcription create supercoils downstream as double helix unwinds.
Polytene Chromosomes
Represent pairs homologs
Are unusual;not found in eukaryotes
Found in various tissues:
Salivary, rectal, & midgut
Paired homologs
Polytene chromosomes
Puff regions
DNA of paired homologs
Undergoes many rounds of replication without strand separation or cytoplasmic division.
Puff regions
Bulges where DNA is uncoiled and are visible manifestations of high level gene activity.
Lampbrush Chromosomes
Large with extensive DNA looping
Easily isolated from oocytes
Sites of gene activity
Similar to chromosome puffs
Chromatin
@ interphase eukaryotic Chromosomes uncoil and decondense into chromatin
Dispersed in nucleus
During cell division chromatin could and condenses back into visible Chromosomes
Histones
Positively charged proteins associated with chromosomal DNA in eukaryotes
5 main types of histones
H1 H2A H2B H3 H4
Nucleosomes
A length of DNA coiled around a core of histones
Condensed several times to form intact chromatids
Chromatin remodeling:
To allow replication and gene expression DNA protein interactions, chromatin must:
Relax compact structure
Expose regions of DNA to regulatory proteins
Have a reversal mechanism for inactivity
Chemical modifications are important to
Genetic function
What histones tails provide potential
Targets along chromatin fiber for chemical modifications
Acetylation
Methylation
Phosphorylation
Acetylation
Addition of acetyl group to positively charged amino group on side chain.
Changes net charge of protein by neutralizing positive charge.
Methylation
Adds methyl group to arginine and lysine residues in histones
Phosphorylation
Enzyme kinase
Adding phosphates groups to hydroxyl groups of amino acids serine and histidine
Euchromatin
Uncoiled and active
Appears unstained during interphase
Heterochromatin
Mostly inactive Appears stained during interphase Replicates in S phase Telomere maintains chromosome integrity Centromere involved in chromosome movement
Chromosome binding techniques
Differential staining along longitudinal axis of mitotic chromosome
