Iain McKillop, Ph.D. Biol 3111 AP BIOLOGY DNA Organization - Chromosomes and Chromatin

DNA? DNA – deoxyribonucleic acid – your DNA is simply a set of ‘protein-building instructions’ in the nucleus of your cells. (1 cell has 2 meters of DNA!) Those proteins then become, essentially, you. Gene – short sequence of DNA that codes for 1 protein. Chromatin – long strands of DNA + proteins. Diffuse and invisible. Chromosomes – tightly coiled, visible form your DNA takes during cell division.

DNA Packaging PROKARYOTES “Naked DNA” - i.e. no nuclear membrane DNA is circular - attached to membrane EUKARYOTES Much more complicated than prokaryotes Require much more genetic information (DNA) DNA must be folded and “packaged” to fit in the nucleus Utilize specialist proteins to bind to DNA - form CHROMATINS Iain McKillop, Ph.D. Biol 3111

Human Chromosomes Human cells contain: 46 Chromosomes - Diploid cells (2N) – body cells 23 Chromosomes - Haploid cells (1N) – sex cells Chromosomes are only visible during cell division - mitosis Iain McKillop, Ph.D. Biol 3111

Human Chromosomes Chromosome is a single DNA strand Human cells contain pairs of chromosomes - maternal and paternal HOMOLOGOUS CHROMOSOMES Human cells also contain sex chromosomes - X- maternal Y- paternal HETEROLOGOUS (Non-homologous) CHROMOSOMES Iain McKillop, Ph.D. Biol 3111

Iain McKillop, Ph.D. Biol 3111 Karyotype – a display of the chromosomes within a cell Human Karyotype

DOG KARYOTYPE

CAN YOU SPOT THE ABNORMALITY?

Trisomy 21 (1 in 660 births) Decreased muscle tone at birth Separated sutures Microcephaly Upward slanting eyes, unusual for ethnic group Small mouth with protruding tongue Broad short hands Single crease on the palm Retarded growth and development Delayed mental and social skills Iris lesion

Trisomy 21 = Down Syndrome

Other Trisomies and Monosomies Most are fatal The most common ones to be born include: –Trisomy 18 –Trisomy 13 –Trisomy 8 –Trisomy X –Monosomy X –XXY –XYY

Chromosomes have specific regions to allow accurate, rapid replication Replication origins - Points at which duplication begin Centromere - Allows each copy to be “pulled” to the new cell Telomere - Allow the ends of the chromosome to be replicated Chromosome Regions Protect the DNA from digestion by nucleases Iain McKillop, Ph.D. Biol 3111

Chromosome Regions Iain McKillop, Ph.D. Biol 3111

Chromosome Packaging Different levels of “packaging” have been identified 1. Nucleosomes 2. Solenoids 3. Loops 4. Rossettes 5. Chromosomes Iain McKillop, Ph.D. Biol 3111

The Nucleosome Nucleosomes consist of a complex of DNA (146 b.p.’s) and 8 proteins (HISTONES; 2 each of H2A, H2B, H3 & H4) Histones form a “protein core” around which DNA winds Leads to a “beads on a string” structure Iain McKillop, Ph.D. Biol 3111

The Nucleosome Histone Core Linker DNA Wound DNA Nucleosome Iain McKillop, Ph.D. Biol 3111

Solenoids Formation of chromatin allows the DNA to be packed into a solenoid Long solenoids then form LOOPS which are condensed (ROSSETTES) that make up a CHROMOSOME Iain McKillop, Ph.D. Biol 3111

Levels of Chromosome Structure

Regulating Mitosis and Cancer ( ) Anchorage Dependence – Cells need to be attached to a surface to divide. Density-Dependent Inhibition – When cells get crowded, they stop dividing. Too much competition for growth factor. Ex: cutting skin reduces skin cell density, so cell division occurs; this is “healing”

Cell Cycle Checkpoints The cell cycle will pause at certain points unless told to go forward by a mixture of inter- and intra cellular signals via signal proteins. –G 1 Checkpoint – most “important”. Usually if a cell passes this checkpoint it will divide. If not, it enters G o – a nondividing state. –G 2 Checkpoint – occurs before M phase –M phase checkpoint – occurs before cytokinesis

Cancer Cells Are Like Cellular Gangbangers Cancer Cells: – ignore the the rules of mitotic regulation –Steal (nutrients) from other cells –Spread to other cellular neighborhoods (tissues) – Quiz tomorrow on section 8.10