Chromosome Organization

BIO 360 Exam 3

What is a nucleosome? How much DNA is found on a nucleosome?

A nucleosome is the repeating structural unit within eukaryotic chromatin which consists of DNA wrapped around histones. It contains 146-147 bp

For what are DNA sequences important?

Synthesis of RNA and cellular proteins

·       Replication of chromosomes

·       Proper segregation of chromosomes

·       Compaction of chromosomes

Illustrate the levels of compaction leading to a metaphase chromosome. Be sure to label key complexes and/or chromosome states.

A radial loop can be anywhere from [...] to [...] base pairs long

25k to 250k

In metaphase chromosomes are [...] condensed or [...] and have an overall diameter of [...]

In metaphase chromosomes are highly condensed or heterochromatic and have an overall diameter of 1400nm
Allows for easy separation

What additional compaction steps take place to ultimately form a metaphase chromosome?

Formation of radial loop domains: Within the nuclear matrix the 30nm fiber binds to a nuclear matrix fiber/protein and forms a radial loop (25k – 250k bp).

What are the different types of repetitive sequences? What is the significance of each?

1. Unique/non-repetitive: Structural genes and codes for proteins

2. Moderately repetitive: Genes for RNA & histones, origins of replication, and regulate gene expression and translation

3. Highly repetitive: For structural functions such as degradation prevention (telomeres) or centromeres for cell division

What is the significance of histone protein H1 and other proteins that bind to the linker region between nucleosomes?

Histone H1 along with nonhistone proteins within the linker region form a more compact structure called the 30nm fiber.

What does the term “beads on a string” refer to?

It refers to the overall structure of a nucleosome, that is, DNA wrapped around an octamer of histone proteins.

What enzymes regulate supercoiling in bacteria?

DNA Gyrase (aka DNA topoisomerase II): Introduces negative supercoils and untangles intertwined DNA molecules (relaxer)

DNA Topoisomerase I: Introduces positive supercoils (winder)

*Regulated by the drugs quinolones and coumarins

What is a viral envelope?

A membrane stolen from the infected host cell. Used to trick the host into thinking it is one of its own.

What is the difference between positive and negative supercoiling?

Positive: Overwinds DNA to the RIGHT

Negative: Unwinds DNA to the LEFT

*Both induce stress causing bulb formation as a relief mechanism

What are three major functions of DNA supercoiling?

1. Can create single-stranded DNA by utilizing tension to release a DNA strand
2. Compacts chromosomes
3. Regulates transcription by compacting (reducing) or expanding (enhancing)

What are the different forms of a viral genome?

1.      DNA or RNA

2.      Single or Double-Stranded

3.      Circular or linear

What are viruses composed of?

Non-enveloped: Nucleic acid, capsid (protein coat)

Enveloped: All the above with a membrane and spike proteins

What is the nucleoid?

A non-membrane bound region where chromosomes are found in bacterial cells

When are viral particles assembled?

During an infection. Process is spontaneous
-Particles include genetic materials and capsid proteins

To fit within the bacterial cell, the chromosomal DNA must be compacted about a [...]-fold

1000-fold

What are the two types of heterochromatin and where are these types found on a chromosome?

Constitutive heterochromatin: Always heterochromatic/compacted and transcriptionally inactive. Contain highly repetitive sequences – Centromere and telomere (ends)

Facultative heterochromatin: Interconvert between euchromatin and heterochromatin and thus vary in transcription states. – In between telomeres and the centromere where it is less dense

Is a virus a cell? Is a virus alive?

Viruses are not a cell, rather small infectious particles containing nucleic acid surrounded by a capsid of proteins. They are not alive because they rely on host cells for replication, as they lack the internal machinery to do so themselves.

What is the difference between euchromatin and heterochromatin?

Euchromatin: Less condensed and transcriptionally active

Heterochromatin: Tightly compacted and transcriptionally inactive (generally)

What molecule(s) are able to remove the bonds between the proteins/scaffold and DNA in a chromosome?

SALT! Leaves a scaffold behind

How much do the nucleosomes alone compact the chromosomal DNA?

Seven-fold (divide by seven)

What functions does supercoiling serve?

1. Compact bacterial chromosomes further than loop domains. Allows lots of genetic info to fit in a tiny bacterial cell.

2. Can create single-stranded DNA by utilizing tension to release a DNA strand
3. Regulates transcription by compacting (reducing) or expanding (enhancing)

Genome size and shape of virus, bacteria, and eukaryotes

Thousands/hundreds of thousands & circular or linear
Millions, & circular or linear
Billions & Linear

Compaction of the nucleosomes on each other creates what structure? How much does this shorten the chromosomal DNA?

30nm fiber which shortens chromosomal DNA seven-fold (in addition to the nucleosome compacting [seven-fold], so 50 fold compaction so far).

What is a nucleosome? How much DNA is found on a nucleosome?

A nucleosome is the repeating structural unit within eukaryotic chromatin which consists of DNA wrapped around histones. It contains 146-147 bp

What is a histone and how many are found in a nucleosome?

A histone is a protein that DNA wraps around. In a nucleosome there are eight histone proteins composed of two copies each of four different histone proteins (octamer).

For what are DNA sequences important?

Synthesis of RNA and cellular proteins

·       Replication of chromosomes

·       Proper segregation of chromosomes

·       Compaction of chromosomes

What role do condensin and cohesin play in organizing chromosomes?

*Both work to help form and organize metaphase chromosomes in the following ways:

• Condensin: Chromosome condensation

• Cohesin: Sister chromatid alignment

What are the different forms of a viral genome?

1.      DNA or RNA

2.      Single or Double-Stranded

3.      Circular or linear

What is the difference between positive and negative supercoiling?

Positive: Overwinds DNA to the RIGHT

Negative: Unwinds DNA to the LEFT

*Both induce stress causing bulb formation as a relief mechanism