Prokaryotes and Eukaryotes Differences in DNA replication and

Prokaryotes and Eukaryotes Differences in DNA replication and chromosome structure

Differences between Prokaryotic & Eukaryotic Cells • there is usually one replication origin in prokaryotic DNA, and more than one replication origin in eukaryotic DNA

Differences between Prokaryotic & Eukaryotic Cells • in prokaryotic cells, DNA polymerase I, II, and III function in replication and repair; in eukaryotic cells, there are more than 3 different types of DNA polymerase

Difference in Chromosome Structure • the human genome consists of 23 pairs of chromosomes • if all of the DNA was stretched out, it would measure 1. 8 metres in length • how does it fit into a typical human cell nucleus, 10 micrometres in diameter?

Hierarchical Organization • DNA is coiled around a group of 8 stabilizing proteins called histones, forming nucleosomes

Chromosome Structure • solenoid = a group of 6 nucleosomes coiled into chromatin fibres • chromatin fibres fold into final chromatin structure through supercoiling.

Chromosome Structure See animation: http: //www. biostudio. com/c_%20 education%20 mac. htm

Prokaryotic DNA • primary DNA is single circular chromosome • supercoiling also in prokaryotes • plasmids are other circular pieces of DNA contained in prokaryotes • plasmids can be exchanged through conjugation

Chromosome ends • Prokaryotes have circular DNA so when DNA replicates, it always completes the circle • Eukaryotes have linear chromosomes and the ends of the chromosome present a problem to DNA polymerase

The last Okazaki fragment • Since no DNA is next to the last primer that is removed, DNA polymerase can’t finish the last strand of DNA

• As a result, the chromosomes gets a little bit shorter after every replication • DNA has built in non-coding repeating sequences to combat this – called telomeres • Telomeres also keep chromosomes linear

• after many replications, telomeres will eventually disappear, allowing coding DNA to be damaged • cell senescence = cell aging signals cell death • Hayflick limit = total number of times a normal cell can divide (about 50 in humans)

Telomeres and Aging • Once cells reach the Hayflick limit, cell functioning declines -> senescence • As an organism ages, more of its cells have reached this limit

Telomerase • Enzyme that adds new telomere sequences maintaining their length and avoiding cell aging

Dolly the Sheep First mammal cloned Born in 1996 from a donor cell that was 6 years old Shortened telomeres would be reflected in its chromosomes Expected lifespan would have been 11 -12 years but died at the age of 6.

Telomerase and Anti-aging? ? • No proof that telomere length will affect the lifespan of organism • Even with Dolly • But hasn’t stopped industry from trying to exploit the use of telomerase

“Stop Aging Now TS-X Telomere Supplement”

Telomerase and Cancer • Cancer cells have been found to have lots of telomerase allowing them to continuously divide without the loss of the telomeres • Target for cancer treatment is targeting telomerase production or activity in cancer cells more research needs to be done

Classwork • Page 294: 1, 3, 4, 5, 8, 9 • Page 298: 1 -7