Differences in DNA Heterochromatin vs. Euchromatin Heterochromatin is DNA which tends to be highly compacted and dark staining. Euchromatin is not so compacted or dark. The number of genes in heterochromatin is generally small relative to euchromatin. Heterochromatin lacks genes or they are inactive Much heterochromatin is found in certain structural parts of the chromosomes: centromeres and telomeres. Also, much of Y chromosome. Move euchromatin to an area next to heterochromatin and it becomes heterochromatin: position effect.

Chromosome structure Arm http://www.med.uiuc.edu/m1/genetics/images/webun1/Chromosome.gif medic.med.uth.tmc.edu/.../ cellbio/hist-01.htm

More on Differences in DNA Base sequences are obviously different from one organism to another, but overall DNA composition can differ as well. In most eukaryotic organisms, DNA composition is not uniform across all the DNA in the cell: patches within the same cell where DNA composition is distinct from other regions.

More on Differences in DNA Base sequences differ from one organism to another Organisms are genetically different, so the base sequences, which determine traits, differ Overall DNA composition can differ as well. From one organism to another Also, within the same organism In eukaryotes, DNA composition is not uniform across all the DNA in the cell http://www.scq.ubc.ca/wp-content/uploads/2006/07/dna1.gif

Composition of DNA: % G+C The amount of A always equals the amount of T, and the amount of G and always equals the amount of C, but the percentage of G+C pairs and A+T pairs can be different among different organisms or in different sections of the DNA within the same cell..

Basepairs held together by H-bonds T-A base pairs are held together by 2 H-bonds G-C base pairs are held together by 3 H-bonds. Therefore G-C pairs require slightly more energy to separate. (also they are denser)

Analysis of DNA using spectrophotometry Max Abs: at 260 nm ss DNA absorbs more than ds DNA: hyperchromic shift

Measuring % G+C As DNA “melts”, becomes SS, absorbs more UV at 260 nm. Because G-C pairs have 3 H-bonds instead of two, DNA with more G+C is more stable, melts at higher temperature (blue).

Satellite DNA In prokaryotes, the %G+C base pairs is pretty much averaged out over the entire DNA; not so with eukaryotes. Density gradient ultracentrifugation can also be used to determine %G+C. G+C pairs are denser than A+T, migrate to a lower location (greater density) in the gradient. Fragmented eukaryotic DNA showed something odd…

Satellite DNA When the DNA was analyzed, a portion has a lower %G+C than the rest of the DNA, producing a “satellite band”. How could a portion of DNA have a different composition than the rest?

Repeated sequences If a section of DNA with a %G+C composition different from the rest of the DNA is repeated many times, DNA fragments from these regions of DNA would behave differently during the centrifugation.

Study of the Composition of DNA using DNA renaturation kinetics Break DNA into random fragments. Denature with heat (melt). Cool, allow strands to find their complements and go from ss to ds again (anneal/renature). Follow entire process using UV light absorption at 260 nm as DNA goes from ss to ds, Abs decreases.

Renaturation kinetics Kinetics: study of the rate of change. Major Point #1: the more copies of the complementary strands there are, the less time they will take to find each other the more DNA, the faster the process. In this fig., 2 different amounts of DNA from the SAME organism.

Renaturation kinetics-2 Major Point #2: Given equal amounts (same mass) of DNA, the bigger the total genome of the organism, the slower the renaturation. If the genome is bigger, and the amounts of DNA used in the experiment are the same, the organism with the bigger genome will have fewer copies of the complementary fragments, so annealing will take longer (see point #1).

Understanding genome size Imagine you have 20 playing cards. In one instance, you have these 5 cards, another 5 cards exactly the same, and 2 more sets of the Ace thru 10 but of diamonds. <Deck 1> In the second instance, you have ace thru 5 of hearts and also of diamonds. <Deck 2> In which case will you match up pairs of hearts and diamonds most quickly? The Deck 1 gets matched up quicker. http://www.skydiveelsinore.com/calendar/images/playing-cards-spread.jpg

Cot curves: Studying renaturation of DNA The amount of DNA affects the rate at which DNA fragments renature. To avoid the problem of comparing samples with different amounts of DNA, the change in ss DNA is graphed vs.the initial DNA concentration (Co) x the time (t): Cot Y-axis is the fraction or percent of the DNA that is ss (experiment starts by denaturing the DNA). X-axis is Cot which is a Log scale. www.cas.muohio.edu/.../gene2000/ lect7/fig9p8c.jpg

Satellite DNA and Cot curves When human DNA was analyzed this way, this was the result: Remember the card deck experiment: when there is only one of each card in the deck, they take longer to match up. So DNA that anneals quickly must be in multiple copies…

Cot curves and satellite DNA Categories variable among different organisms. Highly repetitive DNA, many complements, find each other quickly. Single copy (unique sequence) much slower. http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/eukarychrom/cot2.gif

Types of DNA Highly repetitive DNA: 5-45 % of DNA depending on species. In humans: ALU family: contains Alu I site. 300 bp long, appears 500,000 times, dispersed. 5% of DNA. SINEs = short interspersed elements transposable Alpha satellite DNA: tandem repeats of 170 bp occur 5,000-15,000 times; make up part of centromere. 6% L1 family (in humans), example of LINEs Long interspersed elements

DNA in fewer copies Moderately (middle) repetitive DNA: Tandem or interspersed repeats VNTRs, good for DNA fingerprinting Variable number tandem repeats 15 – 100 bp long, between or within genes Dinucleotide repeats (CA)N, also good for forensic work in maize and yeasts: transposons in large numbers. genes for rRNA, ribosomal proteins, histones Unique, “single copy”: typically 30-75% of DNA in most eukaryotes.

All your DNA codes for proteins? Sorry, not close Only 4% codes for proteins, in 30,000 genes 96% of DNA includes Introns, “junk” DNA within and around genes. Genes coding for rRNA and tRNA Junk DNA called repetitive sequences Pseudogenes; have sequences that look like genes but are never expressed, don’t work. We are related to everything else Our genes look like those from chimpanzees, bacteria.