Molecular Structure of Chromosomes

What is a chromosome?

a discrete unit of genetic material

What is chromatin?

the complex of DNA and proteins that makes up eukaryotic chromosomes

What is a histone?

a group of proteins involved in the formation of nucleosomes

What is a nucleosome?

a structural unit of eukaryotic chromosomes composed of an octamer of histones wrapped with DNA

What are nucleosomes composed of?

146 or 147 base pairs

What is important to know about octamers?

• contains 2 molecules each of four types of histone proteins: H2A, H2B, H3, and H4

• very basic and are attracted to positive charges on histone proteins

What is the 30-nm fiber?

nucleosomes are organized into a more compact structure; H1 and other proteins are important in the formation of the 30-nm fiber

What are loop domains?

30-nm fiber is folded into loops

What is the mechanism of loop formation?

• The CCCTC binding factor

  • Binds to 3 regularly spaced repeats of the core sequence CCCTC

  • Two different CTCFs can bind to the DNA and bind to each other form a DNA loop

• SMC proteins

  • Forms a dimer than can wrap itself around 2 segments of DNA and form a loop

  • Prevents the loop from coming apart

What are the two compaction levels?

heterochromatin and euchromatin

What is heterochromatin?

the tightly compacted regions of chromosomes during interphase

What is euchromatin?

the less condense form of chromatin where the 30-nm forms loop domains

True or False: Chromosome scaffold contains a large amount of SMC proteins to help organize chromatin loops.

true

What is nuclease?

an enzyme that degrades (hydrolyzes) a phosphodiester linkage

What is exonuclease?

a nuclease that hydrolyzes nucleic acid from the end of a chain

What enzyme can proofread?

exonuclease

What is endonuclease?

an enzyme that hydrolyzes nucleic acid internally

What do topoisomerases do in bacteria?

nick, wind or unwind, and reseal DNA

True or False: Eukaryotes and archaea both have histones, but bacteria do not.

true

Where is euchromatin in interphase?

• Distributed throughout the nucleus

• Most in the form of 10-30 nm structures

Where is heterochromatin in interphase?

found at the periphery of the nucleus and in pockets elsewhere in the nucleus

What chromosome structures condense to form mitotic chromosomes?

30-nm fibers

What helps to anchor the 30-nm fibers?

scaffolding proteins

True or False: 30-nm fibers loop into 30-nm loops with the help of scaffold proteins.

true