Thème 1 : La Terre dans l’Univers – A. Expression, stabilité et variation du patrimoine génétique

From DNA... to proteins! a

Introduction: DNA carries the necessary information for cell survival. This information is expressed somehow trough proteins because a change in DNA sequence leads to a structural protein alteration. This 3D modification may induce a functional impairment such as the one in sickle cell anemia with HbA and HbS… But how do cells manage to express their genetic material into proteins? Hypothesis:

♦ The protein synthesis is a one step phenomenon ♦ The protein synthesis needs two or more steps

Historical and experimental data analysis:

Autoradiography is a biochemical method, which enables to locate precisely a radioactive element.

The radiations from the radioactive element change Ag+ ions into Ag metal. This Ag metal can be seen in photography: it appears like black spots on the picture…

To locate protein for example, we use radioactive amino acids for a few minutes in the cells medium and than wash it away (pulse-chase) like in figure 1.

Figure 1: Autoradiography of an animal cell after a few minutes in radioactive leucine (amino acid)

• Draw a diagram of DNA and proteins localisation in the cell in the frame just above

• Which hypothesis can be validated and what biological problem does that raise?

• State an hypothesis to try to answer this problem

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Thème 1 : La Terre dans l’Univers – A. Expression, stabilité et variation du patrimoine génétique

Potential place of DNA in protein synthesis:

Looking at the different molecules into the cytoplasm, one can identify nucleic acid but not DNA. The molecules found are called RNA and differ a little from DNA. Thymine is replaced by uracil and the sugar in the backbone contains an “OH function”, which isn’t in those of DNA nucleotides. RNA does not form double helix!

An autoradiography is done using radioactive uracile. Two different protocols are performed (see figure 2):

♦ The autoradiography directly follows the 30 minutes pulse of radioactive uracile (a)

♦ After 30 minutes of pulse, radioactivity is washed away and cells stay in a non-radioactive bath for one hour before the autoradiography (b)

Figure 2: (a) autoradiography done just after the radioactive uracile pulse, (b)

one hour chase between pulse and autoradiography

• Why do the scientists use uracile? • Where is RNA formed? Where does it go after? • Why does this experiment give credit to an intermediary role of

RNA in proteins synthesis? ………………………………………………………………………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………

Thème 1 : La Terre dans l’Univers – A. Expression, stabilité et variation du patrimoine génétique

LAST & FIRST NAMES: From DNA to RNA:

RNA synthesis needs DNA, RNA nucleotides and an enzyme, the RNA polymerase. In eukaryotic cells, this phenomenon, called transcription, take place into the nucleus. RNA polymerase clips on DNA and the double helix unzips. Than RNA nucleotide are added on the new molecule complementary to those on one strand, “the transcribed strand” (brin transcrit in French). The other strand is called the coding strand because it has the same sequence as the RNA except for the U/T difference (brin non transcrit in French). Use of Anagene software:

⇒ Open directory « thème d’études » and chose « expression de l’information génétique » than chose « Globine α » and finally « Gène et ARNm codant »

⇒ Select sequences by clicking on the left squares in front of each name ⇒ Compare sequences using the ATGC/-C-- button (alphabrin1 is your

reference) ⇒ Using this comparison, demonstrate the RNA synthesis mechanism

and don’t forget to spot the uracile complementary base and to identify the transcribed strand

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Conclusion (graded):

• To validate the practical course, put title and legends on the diagram below

Title:

• What is the phenomenon still missing at the end of this session?

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