Bio 212, Chap 12
How does a polytene chromosome differ from a typical eukaryotic chromosome?
Polytene chromosomes are found in a variety of tissues in the larvae of some flies and several species of protozoans and plants. A polytene chromosome contains banding patterns and is large because of repeated replications of DNA without nuclear division.
In which of the following ways do polytene chromosomes differ from other chromosomes?
Polytene chromosomes are replicated but not separated.
What is unusual about the amino acid composition of histones? How is the function of histones related to the amino acid composition? Of which histones are nucleosomes composed?
Histones contain large amounts of positively charged amino acids such as lysine and arginine. Thus, they can bind electrostatically to the negatively charged phosphate groups of nucleotides. Nucleosomes are composed of all histones except H1.
Histone acetyltransferases (HATs) are capable of remodeling chromatin by adding acetyl groups to various lysine residues in histones that comprise the nucleosome. Following this modification, the lysine residue no longer has a positive charge. Which statement is true?
Histones in general have a net positive charge that allow them to bind to DNA. Acetylation of histones, decreases their positive charge and weakens the histone-DNA interaction.
What genetic process is occurring in a puff of a polytene chromosome?
RNA transcription
Referring to the image, there appears to be a span of 13 "beads" present. Ignoring the DNA before the first bead and after the 13th bead, how much DNA is present in the photo?
Since there are 13 nucleosome core particles connected by 12 linker regions: (13 x 146) + (12 x 54) = 2546 bp
That some organisms contain much larger amounts of DNA than are apparently "needed" and that some relatively closely related organisms may have vastly different amounts of DNA is more typical in ________.
eukaryotes than in prokaryotes
All EXCEPT which of the following chromosomes can be discerned by light microscopy?
interphase chromosomes
What is a "puff" in a polytene chromosome?
site of gene transcription
Assume that the DNA associated with a nucleosome core particle plus the DNA in the linker adds up to 200 bp. Approximately how many base pairs are found in the linker region?
~54
Which of the following are examples of heterochromatin?
-Barr body -telomeric DNA -repetitive DNA -centromeric DNA
Chromatin structure is dynamic. In regions of highly condensed chromatin, such as the centromere, the boundary between heterochromatin and euchromatin is variable. Genes that are near this boundary region can be influenced by either type of chromatin in what is referred to as position effects.
-When heterochromatin spreading does not reach the new location of the w + allele, the gene will be transcribed, producing red eye patches. -Even though eye color phenotypic variegation exists within the eye, all cells have the same genotype. -When heterochromatin spreading encompasses the new location of w + allele, the gene is not transcribed, producing white eye patches.
The condensed areas in polytene and lampbrush chromosomes are referred to as ________.
chromomeres
In E. coli, the genetic material is composed of ________.
circular, double-stranded DNA
Which of the following are examples of heterochromatin?
-centromeric DNA -telomeric DNA -Barr body -repetitive DNA
This figure represents supercoiled, circular, double-stranded DNA: If this molecule of DNA originated as a linear molecule with a linking number (L) of 30, which was then circularized and unwound, the L for the unwound circular molecule would be ________.
28
How many total polypeptide chains would be present in a single nucleosome core particle?
8
What is the first order of chromatin packing?
Coiling around nucleosomes
Sort each characteristic into the appropriate bin.
Euchromatin- less chromatin condensation, many expressed genes Heterochromatin- condensed chromatin, few expressed genes
Sort each characteristic into the appropriate bin.
Euchromatin- many expressed genes, less chromatin condensation Heterochromatin- condensed chromatin, few expressed genes
Contrast the major differences between the organization of DNA in viruses and bacteria versus eukaryotes. Drag the appropriate characteristics to their respective bins.
Eukaryotic genomes- contain large amounts of DNA, consist mostly of noncoding D N A sequencesconsist mostly of noncoding DNA sequences, contain a mixture of both unique and repetitive D N A sequencescontain a mixture of both unique and repetitive DNA sequences Viral or bacterial genomes- consist of mostly unique DNA sequences, Short, circular or linear D N A moleculesshort, circular or linear DNA molecules, consist mostly of coding D N A sequencesconsist mostly of coding DNA sequences
The second order of chromatin packing occurs when nucleosomes coil together to form a fiber that is 300 nm in diameter.
False
Which histone helps stabilize the solenoid structure?
H1
In the formation of nucleosomes, one histone class, H1, is not directly involved, yet it does associate with DNA to form higher level chromosomal structures. Where does this histone (H1) associate?
It binds to linker (spacer) DNA between nucleosomes.
What makes up the protein component of a nucleosome?
Two tetramers of histone proteins
How do we know this experimentally?
by autoradiography (incorporation of radioactive ribonucleotides)
Eukaryotic chromosomes contain two general domains that relate to the degree of condensation. These two regions are ________.
called heterochromatin and euchromatin
