Spring 2009: Section 3 – lecture 1

Reading – Chapter 3

Chapter 10, pages 251 - 267

Cytology of Genetics

Eukaryotes

- replication

- mitosis

- meiosis

- recombination

Cytology of Genetics

Eukaryotes

- chromosome variation

- number of sets

- number of chromosomes is a set

- chromosome modifications

- speciation due to chromosome modifications

Eukaryotes - cells with linear chromosomes in true nuclei bounded by nuclear envelopes and that undergo meiosis.

DNA can be in three organelles in eukaryotic cells:

- nucleus - linear DNA, 96-98% of total DNA in the cell

- mitochondria - circular DNA, 1-2% of cellular DNA

- chloroplast - circular DNA, 1-2%of cellular DNA

Nucleus

- nuclear membrane

- nucleolus

- chromatin/chromosomes

Nuclear membrane:

- has openings, ‘pores’, that allows passage of material such as nucleotides, enzymes into the nucleus and mRNA, tRNA, and rRNA out of the nucleus.

Nucleolus:

- dark staining body in the nucleus, region of high transcription of rRNA.

Chromatin:

- relaxed form of linear DNA found at interphase. Chromatin exists in

two forms:

- euchromatin - stains lightly at interphase, regions of potential transcription.

- heterochromatin - stains dark at interphase, regions of inactive

or condensed DNA.

Chromosomes:

- condensed (tightly wound) DNA during mitosis and meiosis. The chromosome is actually a complex of DNA and proteins.

- metaphase chromosome

DNA – 15-20 %

protein – 65-75%

There are two primary types of proteins:

1. basic histones

2. acidic

Histones - basic proteins rich in the amino acids lysine and arginine. The positive charge allows the proteins to interact with the phosphate groups in the DNA.

They can be divided into two groups, those higher in the lysine and those higher in arginine

Histones high in lysine MW

H1 23,000

H2A 14,000

H2B 14,000

Histones high in arginine MW

H3 15,000

H4 11,000

The histone proteins H2A, H2B, H3, and H4 combine (2 copies of each) to form an octomer. DNA is coiled twice around an octomer in the nucleus. This gives the appearance of beads on a string. Each bead is called a nucleosome.

The nucleosomes facilitate condensing of the chromatin in interphase and to form chromosomes for mitosis and meiosis.

Reason for condensing of the chromatin before cell division - to insure each daughter cell gets all the genetic information.

The first step in condensation is the connecting of the nucleosomes by histone H1.

The next step is the coiling of the nucleosomes, further shortening the length of DNA. This coiled structure can exist during interphase.

The coiled DNA can be attached to proteins on the nuclear membrane during interphase forming looped domains.

nuclear membrane

looped domains

Prior to the start of either mitosis or meiosis the loops attach to a nonhistone (acidic) protein scaffolding. This further condenses the chromatin.

The scaffolding can also have folds resulting in a densely packed DNA = chromosomes

Proteins involved in the scaffolding:

- lamins

- scaffold proteins I, II, and III

some functions include:

- topoisomerase

- stabilize chromosome structure

Chapter 10 Fig. 10-21 pg. 263

DNA form Dia. Length Packing ratio

DNA 2 nm 40 m 1

nucleosomes 10-11nm 8 mm 6-7

solenoid fibril 25-30 nm 1 mm 40

looped domains 250 nm 50m 700

Radial loops & 850 nm 4 m 10,000

scaffolding (=.85 m)

Chromosome shape

primary constriction:

- determines length of arms of the chromosome. It is also the site for attachment of the microtubules during mitosis or meiosis. Other names for the primary constriction are centromere or kinetochore.

Based on the location of the primary constriction you can describe a chromosome as metacentric, acrocentric or telocentric.

metacentric acrocentric telocentric

arms = 1 arm > other arm only one arm

secondary constriction:

- on a few chromosomes there can be a second area that appears to be constricted. This is a region of

late transcribing DNA that codes for rRNA genes. It is related to the nucleolus and is called the NOR or nucleolar organizing region.