Zellbio 1 Flashcards
DNA as genetic material was disputed until Alfred Hershey and Martha Chase did their „blender experiment“ in 1952. How did that work?
They worked with T2 viruses, which are made entirely of protein or DNA. Each virus acts as a molecular syringe (Spritze), injecting its genetic material into a bacterium, the empty viral capsule remains attached to the outside of the cell. To determine whether the genetic material of the virus is protein or DNA, the researchers radioactively labelled the DNA in one batch of viruses with 32P and the proteins in a second batch of viruses with 35S. Because DNA lacks sulphur and the proteins lack phosphorus, these radioactive isotopes provided a handy way for the researchers to distinguish these two types of molecules. These labelled viruses were then allowed to infect E.coli, and the mixture was disrupted by brief pulsing in a Waring blender to separate the infected bacteria from the empty viral heads. When the researchers measured the radioactivity, they found that most of the 32P-labeled DNA had entered the bacterial cells, while the vast majority of the 35S labelled proteins remained in solution with the spent viral particles.
Why didn’t people believe Oswald Avery in 1944 and why did he never
get the Nobel prize? Speculate !!
The failure to award a Nobel Prize to Oswald T. Avery for the discovery of DNA as the genetic material can be used as one example. Avery undoubtedly discovered that DNA is the carrier of the genetic material when he showed that DNA from strains of bacteria with high pathogenicity could transform strains with low to high pathogenicity. His first publication on this topic appeared as early as 1944. Avery was nominated several times between 1932 and 1942 for work on polysacharide antigens. From 1945 he was nominated every year for his discovery concerning DNA. At the time many scientists thought of DNA, with its four different building blocks, as having too simple a structure to be the genetic material. Instead they favored the idea that proteins, with their 20 different amino acids, were more likely to be the genetic material and did not trust the enzyme digestions used by Avery to remove protein from his preparation of DNA. By the time the scientific community, including the Nobel Committee, had accepted Avery’s data, he had passed away.
Do you have an idea how chromosome painting works?
a) DNA-Hybridization. Chromosome painting is carried out by exposing the chromosome to a collection of human DNA molecules that have been coupled to a combination of fluorescent dyes. For example, DNA molecules derived from Chr. 1 are labelled with one specific dye combination, those from Chr.2 with another and so on. Because labelled DNA can form base pairs, or hybridize, only to its chromosome of origin, each chromosome is differently labelled. For experiments the chromosomes are treated so that the double-helical DNA separates into individual strands to enable base-pairing with the labelled single stranded DNA while keeping the chromosome structure relatively intact. These dyes mainly distinguish between DNA that is rich in A-T nucleotide pairs and DNA that is G-C rich, and they produce a striking and reliable pattern of bands along each chromosome.
b) Durch in situ Hybridisierung. Chromosomenspezifische DNA-Moleküle werden mit verschiedenen Fluoreszenz-Farbstoffen markiert. Ein DNA Moleküle eines Chromosoms wird mit einem anderen Fluoreszenz-Farbstoff markiert als ein DNA-Molekül eines anderen Chromosoms.
- Chromosomen werden denaturiert -> Auftrennung in Einzelstränge
- Zugabe der markierten DNA-Moleküle
- Hybridisieren-> gefärbte DNA-Moleküle hybridisieren spezifisch mit dem jeweiligen Chromosom
- Chromosomen sind unterschiedliche angefärbt
In Indian muntjacs chromosomes fused without large changes in gene number. How can this happen?
a) Nobody knows.
b) Chromosomen sind fusioniert, Gene sind erhalten geblieben und das Leseraster nicht verschoben. Das Ergebnis ist gleich ob Gene aus einem Chromosom oder aus unterschiedlichen Chromosomen exprimiert werden.
Gene number is only vaguely correlated with species complexity. Why?
Viele Gene können unwichtige Information tragen. Unterschiedliche Genexpression. Regulation. Alternatives Spleißen.
Is gene expression stalled during mitosis?
a) Ja, es findet Genexpression während der Mitose statt. Es wurden vorher mRNAs hergestellt, die stabilisiert und in Körperchen verpackt werden (Vorratslager). Können dann abgelesen werden, da manche mRNAs Halbwertszeiten von Stunden bis Tagen haben und noch translatiert werden können.
b) Wenn die DNA stark komprimiert ist wie in der Mitosis, wird zwar kaum die DNA abgelesen, allerdings, wenn eine mRNA eines Gens besteht, kann diese tagelang noch abgelesen werden.
Why do eukaryotic chromosomes have telomeres?
Telomeres contain repeated nucleotide sequences that enable the ends of chromosomes to be replicated. They also cap the end of the chromosome, preventing it from being mistaken by the cell as a broken DNA molecule in need of repair. Da am diskontinuierlichen Strang DNA nur so lange repliziert werden kann, solange die Bindung eines RNA-Primers möglich ist, bleibt minimal ein Stück der Länge des RNA-Primers übrig. Abhängig von der Bindung des letzten RNA-Primers kann dieses Stück deutlich länger sein (bis zu 100 Nukleotide). Somit gehen Sequenzinformationen an den DNA-Enden verloren und die Chromosomenlänge nimmt mit jeder Zellteilung und der damit verbundenen DNA-Replikation ab (siehe Alterung). Telomeres allow the completion of DNA synthesis at the ends of eukaryotic chromosomes. The enzyme telomerase adds a series of repeats of a DNA sequence to the 3’ end of the template strand, which then allows the lagging strand to be completed by DNA-polymerase.
Eukaryotic chromosomes contain many replication origins. Why?
In Prokaryoten ist die DNA zirkulär. Eukaryoten besitzen ein großes Genom. Eukaryotic chromosomes contain many replication origins to ensure that the entire chromosome can be replicated rapidly.
How does the centromere attach the duplicated chromosomes to the mitotic spindle?
???The microtubule eventually attaches to the kinetochore, and this kinetochore microtubules links the chromosome to a spindle pole. Because kinetochores on sister chromatids face in opposite directions, they tend to attach to microtubules from opposite poles of the spindle, so that each replicated chromosomes becomes linked to both spindle poles.
Do you know how DNA compaction could be involved in night sight?
Bei nachtaktiven Säugetieren ist die Anordnung des Chromatins in den Stäbchenkernen umgekehrt: Das dicht gepackte Heterochromatin befindet sich hier im Inneren des Zellkerns, während das weniger dicht gepackte Euchromatin mit den aktiven DNA-Bereichen an der Peripherie liegt.
Die Erklärung für die ungewöhnliche Organisation dieser Zellkerne liegt in der Biologie der Sinneswahrnehmung. Beim Menschen und bei allen anderen Wirbeltieren muss das Licht erst die Netzhaut, die Retina, durchdringen, um auf die lichtempfindlichen äußeren Teile der Fotorezeptoren zu stoßen. Und damit stehen die nachtaktiven Tiere vor einem Dilemma: Sie brauchen besonders viele Stäbchen zur Detektion des schwachen Lichts - aber dadurch wird ihre Retina dicker und verliert mehr Licht durch Streuung, bevor dieses die Außensegmente der Fotorezeptoren erreicht. Zur Lösung dieses Problems machte sich die Evolution offenbar eine physikalische Besonderheit des dicht gepackten Heterochromatins zunutze: Wegen seiner höheren Packungsdichte wirkt Heterochromatin stärker lichtbrechend als Euchromatin. Dieser Effekt kommt nicht zum Tragen, wenn das Heterochromatin in der Peripherie des Zellkerns liegt. Wenn es sich dagegen im Inneren des Zellkerns zusammenballt, wirkt das Heterochromatin wie eine winzige Sammellinse. Weil die Stäbchenkerne in Säulen angeordnet sind, kommen mehrere dieser “Mikrolinsen” übereinander zu liegen. Das an sich wenig intensive Licht wird so fast ohne Streuverluste gebündelt und durch die Retina geleitet wird. Es trifft fokussierter auf die lichtempfindlichen Außensegmente der Fotorezeptoren.
Are histones large or small molecules and are they charged?
All four of the histones that make up the nucleosome core are relatively small proteins with a high proportion of positively charged as (lysine and arginine). The positive charge help the histones bind tightly to the negatively charged sugar-phosphate bachbone of DNA.
What is the function of histone tails?
Each of the core histones also has a long N-terminal as tail, which extends out from the nucleosome core particles. These histone tails are subject to several types of covalent chemical modifications that control many aspects of chromatin structure.
The modifications affect the ability of the histone tails to bind specific proteins and thereby recruit them to particular stretches of chromatin. Different patterns of histone tail modifications attract different proteins, some of which cause further condensation of chromatin, whereas others facilitate access to the DNA by decondensing chromatin.
Das N-terminale Ende eines Histons kann von Enzymen modifiziert werden. Diese Histonmodifikationen können Methylierung, Phosphorylierung, Sumoylierung, Ubiquitinylierungund Acetylierung sowie deren Rückreaktionen umfassen. Hieraus ergibt sich der spezifische Histon-Code einer Zelle. Diese Modifikationen haben Einfluss auf das Chromatingerüst des Zellkerns und somit auf die Genregulation.
Methylierung von Lysin (K) 9 → Heterochromatin bildet sich, Gene silencing
Methylierung von K 4, Acetylierung von K 9 → Genexpression
Phosphorylierung von Serin (S) 10, Acetylierung von K 14 →Genexpression
Why is the eye color not pure white?
The red patches represent cells that express the White gene because the heterochromatin had not spreads across this gene at the time in early development when the founder cell of these patches was formed. The presence of large patches of red and white cells indicates that the particular state of the gene (either active or silenced) is established early in development and is inherited by progeny cells thereafter.
Why is this effect not lost during mitosis when all chromosomes are condensed?
When a chromosome is replicated, its resident histones are distributed more or less randomly to each of the two daughter DNA helices. Thus, each daughter chromosome will inherit about half of its parent’s collection of modified histones. The remaining stretches of DNA receive newly synthesized, not-yet-modified histones. At this point, proteins that recognize a particular modification can bind to the chromatin and catalyze the formation of the same modification on the new histones. This can restore the parental modification pattern and, ultimately, allow the inheritance of the parental chromatin structure. This mechanism appears to apply to some but certainly not all types of histone modifications.
