DNA & Chromosome structure I.DNA – a quick review II.Prokaryotes have “nucleoids” III.Eukaryotes – DNA is organized into Chromatin IV.Chromosome organization V.Banding Patterns & Karyotypes

I. DNA review 1.What are the criterion for genetic material? 2.How does DNA structure = function? 3.How was DNA discovered to be the genetic material? 4.Why is a primer needed for replication?

II. Chromosomes - Prokaryotes have “nucleoids” Bacterial nucleoid = DNA + protein DNA is in the form of loops, which are supercoiled and emerge from a dense protein containing structure: scaffold DNA-binding proteins – structurally similar to histones (found in eukaryotes), have positively charged amino acids that bond to phosphate groups HU HU H

Supercoiled DNA, packed tighter…

III. Eukaryotes – DNA is organized into Chromatin Eukaryotic DNA is highly organized by binding in a controlled manner to special proteins. DNA in 1 chromosome of a human cell = 19,000 to 73,000 um long (as compared to 1,200 um in bacteria) Histones & Nucleosomes are the key to packaging DNA into chromosomes. Chromatin = nucleoprotein structure, loose Chromatin = nucleoprotein structure, loose Chromsomes = only visible during mitosis, most packed state Chromsomes = only visible during mitosis, most packed state

Chromosomes Structures that contain the DNA for proper distribution of the genetic material during cell division

III. Chromosome organization there are several levels of organization – DNA 1.solenoid 2. chromatin fiber 3. scaffold 4. chromosome

1. Solenoid Hollow contact helix of nucleosomes (nucleosomes in one turn of the helix are in contact w/those of the next) 30nm fiber probably accounts for most of the chromatin in interphase.

2. chromatin fiber 3. scaffold 4. chromosome

Review: DNA is wrapped around spools called nucleosomes. Each nucleosome is composed of an octamer of proteins called histones. The DNA- nucleosome chain is further coiled and supercoiled into a 30nm structure known as a solenoid. The 30nm structure forms a series of looped domains that further condensed into the chromatin fiber 300 nm in diameter. The fibers are then coiled into the chromosome arms that constitute a chromatid.

A. Different types of chromatin: 1.Euchromatin: regions that stain more lightly because the genes are less compact 2.Heterochromatin: Dark stained regions, DNA is compact and thus genes are not transcribed

B. Chromatin Remodeling DNA – not static as chromatin, but moves between more condensed to less condensed state, for transcription purposes!

Review question How is prokaryotic DNA packaging different from eukaryotic DNA packaging? Why so different? Why similar?

IV. Banding Patterns & Karyotypes Cytogenetics – field of genetics that involves the examination of chromosomes. A.Banding: Certain chemical treatments of mammalian chromosomes yield differentially stained regions on chromosomes. The patterns obtained depend on the treatment used. C-banding stains centromeres. C-banding stains centromeres.centromeres R-banding is the reverse of C-banding and stains non-centromeric regions in preference to centromeres. R-banding is the reverse of C-banding and stains non-centromeric regions in preference to centromeres. G-banding is obtained with Giemsa stain. It yields a series of lightly and darkly stained bands G-banding is obtained with Giemsa stain. It yields a series of lightly and darkly stained bands Q-banding is a fluorescent pattern obtained using quinacrine for staining. The pattern of bands is very similar to that seen in G-banding Q-banding is a fluorescent pattern obtained using quinacrine for staining. The pattern of bands is very similar to that seen in G-banding Banding Differentiates regions along the chromosome: characteristic series of lateral bands in each member of the chromosome set… characteristic series of lateral bands in each member of the chromosome set… Used to identify homologous pairs of chromosomes

Banding – G banded Karyotype Chromosomes vary in size, centromere position and banding pattern!

Centromere – constricted region of chromosome Telomere – end of chromosome B. Visible chromosome landmarks

p arm – short arm of the chromosome; q arm – long arm of the chromosome Arrangement of genes on chromosomes genes are the functional regions along the DNA molecule that constitutes a chromosome – the regions that are transcribed to produce RNA. A chromosome represents large numbers of genes in a specific linear array & this array is different for different chromosomes. Genes vary enormously in size There are intergeneic segments There are repeating segments

Diploid # or 2n = 46, Haploid # or n = 23 C. Chromosome Number: Different species have highly characteristic numbers of chromosomes. Chromosome number is the product of two other numbers: The haploid number and the number of sets.

D. Sister chromatids & Homologous chromosomes Homologues = members of a pair of chromosomes for a diploid organism each type of chromosome is found in a homologous pair consisting of two parallel sister chromatids for a diploid organism each type of chromosome is found in a homologous pair consisting of two parallel sister chromatids If a particular gene is found on one chromosome; it is also found on the homologue If a particular gene is found on one chromosome; it is also found on the homologue