C h r om o s om e S t r u c t u r e , M i t o s i s a n d

M e i o s i s D r W e b b

DNA Packaging: Nucleosomes and Chroma7n

By: Anthony T. Annunziato, Ph.D. (Biology Department, Boston College) 2008 Nature

Educa7on

Each of us has enough DNA to reach from here to the sun and back, more than

300 7mes. How is all of that DNA packaged so 7ghtly into chromosomes and

squeezed into a 7ny nucleus?

DNA is packaged in Eukaryo=c Cells

hPp://www.nature.com/scitable/topicpage/dna-packaging-nucleosomes-and-chroma7n-310

DNA compac=on in Eukaryo=c Cells

DNA compac7on is very

complex and the DNA isnt just

crammed into the nucleus, it is

organized in a very orderly

fashion from the smallest unit -

the nucleosome, via loops, and

bands to the en7re

chromosome.

- Core DNA = 146 bp

- Linker DNA = 8-114 bp (usually 55bp)

- DNA turns 1 and 7mes around histone octamer.

The basic unit of DNA packing is the nucleosome

In electron micrographs Unfolded chroma7n has the appearance of beads on a string

Each bead is a nucleosome The basic unit of DNA packing

The Histones

The Histone Fold

Ribbon drawing

Simple.Conserved.

Adopted by all 4 core histones (H2A, H2B, H3 and H4).

Sequence schematic

Histone Octomer

Structure of the Nucleosome (1997)

First nucleosome structure with DNA (2.8 )

147 base pairs visible along with histone octamer (H1 was not present)

Timothy Richmond and co-workers

Histone tail sequences not visible in crystal

The 30 nm fibre -Solenoid

H1 stabilises the solenoid structure

Scaffold/Matrix aPachment regions

DNA Packing 300nm Fibre The 30-nm fiber, in turn

Forms looped domains, making up a 300-nm fiber

Protein scaffold

300 nm

DNA Packing 700nm Fibre

Chromosome

Chromosomes exist in two different states Before replica7on, chromosomes have one chroma=d Aier replica7on, chromosomes have 2 sister chroma7ds held together at the centromere

Homologues

Chromosomes exist in homologous pairs in diploid (2n) soma7c cells.

Excep7on: Sex chromosomes (X, Y) -haploid.

Karyotype

A karyotype is the complete set of all chromosomes of a cell of any living organism.

The chromosomes are arranged and displayed (oien on a photo) in a standard format: in pairs, ordered by size.

Upper right is a typical karyotype of a human male soam=c cell.

Lower right is a karyotype of a human sperm.

Diploid soma=c cell

Haploid sex cell

Chromosome Structure Euchroma=n-comprises of the genome and is transcrip7onally ac7ve Hetrochroma=n-highly condensed inac7ve chroma7n located at centromeres and telomeres

Centromere aPachment point for sister chroma7ds and spindle fibres Telomeres found at the ends of the chromosome. They are are made of TTAGGG repeats (500-3000 7mes) are are maintained by telomerases. Telomere shortening is important in aging

The wide range of histone modifica=on

Histone modifica=ons alter DNA packaging

Acetyla=on Ac7vate transcrip7on Silence telomeres DNA repair

Methyla=on Inac7ve transcrip7on

Phosphoryla=on DNA repair Mitosis

Ubiquityla=on Transcrip7onal ac7va7on

Sumoyla=on Transcrip7onal repression

General Outcomes of Histone Modifica=on

Unacetylated histones Acetylated histones

Histone acetyla=on Histone acetyla7on occurs on the surface of the nucleosome core on the histone tails.

Acetyla7on brings in a nega7ve charge that acts to neutralize the posi7ve charge on the histones, and decreases the interac7on of the N termini of histones with the nega7vely charged phosphate groups of DNA.

Acetyla7on of histone tails loosens chroma7n structure and permits access of the transcrip7on machinery

Chroma=n remodeling

-ATPase containing complexes -Examples SWI2/SNF2 Imita7on switch (ISWI); Mi-2 (CHD1) INO80.

Cell division

Mitosis

Soma7c cells divide by mitosis

Soma7c cells are diploid (2n) i.e two of each chromosome

During mitosis a 2n nucleus divides to produce daughter nuclei that are also 2n.

Mitosis maintains the number of chromosomes.

Results in cells such as internal organs, skin, bones, blood, etc.

Cell cycle has two parts:

(I) Growth prepara=on Interphase- 75% of cell life cycle G1: rapid growth S: DNA replicates; centrioles replicate. G2: cell prepares for cell division; microtubular structures form.

(II) Cell Division Mitosis (nuclear division) Cytokinesis (cytoplasm division)

Cell cycle-sequence of phases in the life cycle of the cell

How do cells divide?

Stages in Mitosis

Chromosomes shorten and become visible (early prophase)

Late in prophase, protein complexes known as kinetochores assemble on the centromeres.

centromere

chromosome

aster

Mitosis-Prophase

Kinetochore mediates a\achment to the spindle

Schematic drawing.Centromere (DNA segment) is at the

primary constriction. The kinetochore is a huge, complicated protein complex with several layers. The outer layer provides attachment sites for microtubules.

Mitosis-Prometaphase

Nuclear envelope disintegrates (late prophase).

The Kinetochores become fully matured The disrup7on of the nuclear envelope

allows for the mito7c spindles to gain access to the mature kinetochores.

Mitosis-Metaphase

Metaphase is a short res7ng period where the chromosomes are lined up on the equator of the cell, with the centrosomes at opposite ends and the spindle fibers aPached to the centromeres. Everything is aligned for the rest of the division process to occur.

Mitosis-Anaphase In anaphase, the centromeres divide. At this point, each individual chromosome goes from:

1 chromosome with 2 chroma7ds

to:

2 chromosomes with one chroma7d each.

Then the spindle fibers contract, and the chromosomes are pulled to opposite poles, towards the centrosomes.

Mitosis-Telophase

In telophase the cell actually divides. The chromosomes are at the poles of the spindle. The spindle disintegrates The nuclear envelope re-forms around the two sets

of chromosomes.

The cytoplasm is divided into 2 separate cells, the process of cytokinesis.

Mitosis-Cytokinesis

The organelles (other than the chromosomes) get divided up into the 2 daughter cells passively: they go with whichever cell they find themselves in.

Plant and animal cells divide the cytoplasm in different ways.

In plant cells, a new cell wall made of cellulose forms between the 2 new nuclei, about where the chromosomes lined up in metaphase. Cell membranes form along the surfaces of this wall. When the new wall joins with the exis7ng side wall, the 2 cells have become separate.

In animal cells, a ring of ac7n fibers (microfilaments are composed of ac7n) forms around the cell equator and contacts, pinching the cell in half.

Summary of Mitosis Prophase & prometaphase:

Chromosomes condense Nuclear envelope disappears centrosomes move to opposite sides of the cell Spindle forms and aPaches to centromeres on the

chromosomes

Metaphase Chromosomes lined up on equator of spindle centrosomes at opposite ends of cell

Anaphase Centromeres divide: each 2-chroma7d chromosome

becomes two 1-chroma7d chromosomes Chromosomes pulled to opposite poles by the spindle

Telophase Chromosomes de-condense Nuclear envelope reappears Cytokinesis: the cytoplasm is divided into 2 cells

Meiosis

Sex cells (gametes ) divide by meiosis

Sex cells are haploid

Aier cell division the chromosome number is halved

Results in gene=c varia=on by shuffling of maternal and paternal chromosomes.

No daughter cells formed during meiosis are gene7cally iden7cal to either mother or father.

Meiosis - Sex Cell (Gamete) Forma7on

In meiosis,

there are 2 divisions of the nucleus:

Meiosis I

&

Meiosis II

Mitosis vs. Meiosis

2n

Clone

Same gene7c informa7on in parent cell and daughter cell.

Give me another one just like the other one!

1n

Daughter cells different from parent cell and from each other.

Daughter cells have the number of chromosomes as soma7c cell.

Shuffling the genes (Mix it up!)