organization of chromosomes--study guide and outline dna packaging—why and how if the dna in a...

Organization of Chromosomes--Study Guide and Outline

DNA Packaging—Why and How• If the DNA in a typical human cell were stretched out, what length would

it be? What is the diameter of the nucleus in which human DNA must be packaged?

• What degree of DNA packaging corresponds with “diffuse DNA” associated with G1? What kind of DNA packaging is associated with M-phase (“condensed DNA”)?

• What types of DNA sequences make up the genome? What functions do they serve?

• What are the differences between euchromatin and heterochromatin?• What types of proteins are involved in chromosome packaging?

– How do nucleosomes and histone proteins function in DNA packaging? – What is chromosome scaffolding?

Broad course objective: a.) explain the molecular structure of chromosomes as it relates to DNA packaging, chromosome function and gene expression

Necessary for future material on: Chromosome Variation, Regulation of Gene Expression

How much DNA do different organisms have?

DNA content does not directly coincide with complexity of the

organism. Any theories on why?

Organism haploid genome in bp

T4 Bacteriophage 168,900

HIV 9,750

E. colibacteria 4,639,221

Yeast 13,105,020

Lily 36,000,000,000

Amoeba 290,000,000,000

Frog 3,100,000,000

Human 3,400,000,000

Size measurements in the molecular world

• 1 mm (millimeter) = 1/1,000 meter• 1 mm (“micron”) = 1/1,000,000 of a

meter (1 x 10-6)• 1 nm (nanometer) = 1 x 10-9 meter

• 5 billion bp DNA ~ 1 meter• 5 thousand bp DNA ~ 1.2 mm

• 1 bp (base pair) = 1 nt (nucleotide pair)• 1,000 bp = 1 kb (kilobase)• 1 million bp = 1 Mb (megabase)

• Phage virus: 168 kb 65 nm phage head (~1,000 x length)

• E. coli bacteria: 1,100 mm DNA ~0.2 micron space nucleoid region (5,500 x)

• Human cell: 7.5 feet of DNA ~3 micron nucleus (2.3 million times longer than the nucleus)

Representative genome sizes

DNA packaging: How does all that DNA fit into one nucleus?

An organism’s task in managing its DNA:

1.) Efficient packaging and storage, to fit into very small spaces (2.3 million times smaller)

2.) Requires “de-packaging” of DNA to access correct genes at the correct time (gene expression).

3.) Accurate DNA replication during the S-phase of the cell-cycle.

(Equivalent to fitting 690 miles of movie film into a 30-foot room)

Chromosomal puffs in condensed Drosophila chromosome show states of de-condensing in expressed regions

Prokaryotic genome characteristics

How does the bacterial chromosome remain in its “tight” nucleoid without a nuclear membrane?

1. Circular chromosome (only one), not linear

2. Efficient—more gene DNA, less or no Junk DNA

3. One origin sequence per chromosome

(~ 40 kb)

Bacterial chromosome is normally supercoiled

Bacterial DNA released from supercoiling

Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

Bacterial DNA Supercoiling

Levels of DNA Packaging in Eukaryotes

Types of DNA sequences making up the eukaryotic genome

DNA type Function Number/genomeUnique-sequence Protein coding and non-coding 1

Repetitive-sequence Opportunistic? few-107

Centromere Cytoskeleton attachment 1 region/c’some

Telomere C’some stability Ends of c’some DNA

Centromere sequences

• Repeating sequences

• Non protein-coding

• Sequences bind to centromere proteins, provide anchor sites for spindle fibers


Reminder of function of kinetochores and kinetochore microtubules

Chromosome fragments

lacking centromeres are lost in mitosis

(Figure 11.10)

Telomere sequences function to preserve the length of the “ends”

Dolly: First successful cloned adult animal

Born on July 5, 1996, Dolly died on February 14, 2003.

Dolly suffered from lung disease, heart disease and other symptoms of premature aging.


Telomeres sequences may loop back andpreserve DNA-ends during replication

Major proteins necessary for chromosome structure

Protein type Function

Histone packaging at 11nm width, nucleosome formation

Linker proteins packaging at 11nm width, nucleosome formation

Scaffold “Skeleton” of the condensed mitotic c’some

Kinetochore Cytoskeleton attachment to centromere

Telomerase enzyme for preserving lengths of telomeres in stem cells (covered in DNA Replication chapter)

Telomere caps protects ends of linear chromosomes from degradation

Digestion of nucleosomes

reveals nucleosome

structure


Nucleosome structure

• Positively charged histone “tails” bind to DNA.

• Acetylation of histone proteins allows access to DNA -COCH3

-COCH3COCH3--

Trans-cription Factor


Arrangement of 30-nm chromatin fiber into looped domains

Eukaryotic DNA “released” from its tight packaging as a metaphase chromosome (only scaffold

remains)

Go over lecture outline at end of lecture

organization of chromosomes--study guide and outline dna packaging—why and how if the dna in a...

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