Genetics - Chapter 12

Cohesin

A complex of proteins that holds sister chromatids together after replication and before anaphase.

Condensins

A complex of proteins that may act to further condense chromosomes for mitosis.

Kinetochore

A specialized structure composed of DNA and proteins that is the site at which chromosomes attach to the spindle fibers.

What defines chromatin loops

AT-rich sequences associate with nonhistone proteins

Evidence for the role of Xist in inactivation

An X chromosome that does not contain Xist cannot be inactivated, deletion of the Xist gene abolishes a chromsome's capacity for X inactivation and cells carrying one X chromsome that lacks the Xist gene must inactivate the other X.

Linker DNA

An additional 40 base pair connector that connects one nucleosome with the next.q

Duplication of Chromosomal DNA Requires starting points and special ends

As the chromosomes decondense for copying during replication, certain DNA sequences that do not encode proteins regulate the timing and accuracy of the process.

The job of DNA polymerase during replication

Assemble a new string of nucleotides according to a DNA template, linking about 50 nucleotides per second in a typical human cell.

How do proteins access bases within the genome to preform chromosomal packaging?

Chromatin is dynamic and can cahnage to allow access of specific proteins.

Constitutive Heterochromatin

Chromsomal regions that remain condensed in heterochromatin at most times in all cells

Action of Xist Gene

For example, the heterchromatin formation that inactivates an X chromosome in cells of female mammals to form Barr Bodies

Nucleosome

H1, H2A, H2B, H3, and H4 form the core of the most rudimentary DNA packaging unit.

Three Major Mechanisms that Control Chromatin Patterns

Modifications of histones are signals for other proteins to interact and cause changes in the level of compaction.

Repetitive Sequences of Telomeres

Not only contain no protein-encoding genes, they also prevent the transcription of genes brought into their vicinity.

Nonhistones

Play a purely structural role, helping to package DNA into more complex structures. The proteins that form the structural backbone, or scaffold of the chromosome.

The job of telomeres

Protect unprotected chromosomal ends from fusing to each other.

Shugoshin Protein

Protects the cohesin at the centromere from cleavage during meiosis I by interacting with the unique centromere components of meiotic cohesin.

Position-Effect Variegation

Rearrangements silence gene expression in some cells and not others

Facultative Heterochromatin

Regions of chromosomes that are heterochromatic in some cells and euchromatic in other cells of the same organism

Histones

Relatively small proteins with a preponderance of the basic, positively charged amino acids lysine and arginine. The histones' strong positive charge enables them to bind to and neutralize the negatively charged DNA throughout the chromatin.

What shape do the loops take on?

Rosette Shape

How may base pairs in a nucleosome?

Roughly 160 wrapped around a core of eight histones

G-Banding

Serves as the basis for most karyotyping because G-band preparations are very stable and require only a good light microscopic for detection.

Scaffold-associated Regions

Stretches of DNA that are found at the base of the chromatin loops

Replicon

The DNA running both ways from one origin of replication to the endpoints, where it merges with DNA from adjoining replication forks.

What happens when rearrangments place a segment of DNA next to a highly compacted heterchromatin near the centromere

The gene's expression may cease

Chromatin

The generic term for any complex of DNA and protein found in a cell's nucleus

Three Major Mechanisms that Control Chromatin Patterns (Part 2)

The pattern of nucleosomes in chromatin can be alteerd by remodeling complexes that change the accesibility of DNA sequences.

Specific Function of the Xist Gene

To produce an unusually large cis-acting RNA transcript.

Three Major Mechanisms that Control Chromatin Patterns (Part 3)

Variants of histone proteins can become incorporated into nucleosomes and cause different structures to form.

How do proteins access bases within the genome to preform chromosomal packaging? (Party 2)

Variations exist in the molecules making up the basic chromatin structure, and these variants recruit proteins that are necessary for chromosomal functions.