TestAS · Medicine
Cells, cell division, genetics, molecular biology and microbes.
testas.de lists this topic for the Medicine module. In the test it comes as testlets: a short text, table or formula, then 4 to 8 questions you answer from it.
Topic list: testas.de
Full exam-length testlets from the Kollego course, generated the same way as our mocks. Answer from the material, then open each answer to check.
The secretory pathway and a pulse-chase experiment
Cells that export large amounts of protein, such as the cells of the pancreas that produce digestive enzymes, contain an extensive system of internal membranes. Proteins destined for secretion are made by ribosomes attached to the rough endoplasmic reticulum (ER). As the growing protein chain emerges from the ribosome, a signal sequence at its beginning directs it through a channel into the interior of the ER. There the protein folds, is often modified by the addition of sugar chains, and is checked for correct folding; misfolded proteins are retained and eventually degraded.
From the ER, proteins travel in small membrane vesicles to the Golgi apparatus, a stack of flattened membrane compartments. The Golgi further modifies the sugar chains, sorts proteins according to their destination and packages them into new vesicles. Some vesicles fuse with the cell membrane immediately and release their contents continuously. Others, in specialised cells, are stored as secretory granules and release their contents only when the cell receives a signal, such as a hormone or a nerve impulse. This regulated secretion allows the pancreas to release large amounts of enzymes after a meal. Because vesicles fuse with the cell membrane, the membrane they carry becomes part of the cell surface, while endocytosis returns membrane to the inside.
The route of secreted proteins was worked out in the 1960s with the pulse-chase technique, for which George Palade shared the Nobel Prize in 1974. Pancreatic cells were exposed for a short time, the pulse, to a radioactive amino acid, which was built into all proteins made during those minutes. Then the cells were given a large excess of the non-radioactive form of the same amino acid, the chase, so that proteins made afterwards were not labelled. By examining the cells at different times after the pulse with autoradiography, the researchers could follow the labelled group of proteins as it moved through the cell.
The same pathway also supplies the lysosomes, compartments filled with digestive enzymes that break down worn-out organelles and material taken up by the cell. Lysosomal enzymes are marked in the Golgi with a special sugar tag that directs them to the lysosomes instead of the cell membrane. If this tag is missing because of a genetic defect, the enzymes are secreted, the lysosomes cannot digest their contents, and undigested material accumulates, causing a severe storage disease.
| Time after pulse | Rough ER | Golgi apparatus | Secretory granules |
|---|---|---|---|
| 3 min | 86% | 3% | 1% |
| 17 min | 43% | 43% | 4% |
| 37 min | 24% | 14% | 49% |
| 117 min | 20% | 4% | 58% |
1. Where is most of the label found 3 minutes after the pulse?
C: In the rough ER
In the table's 3 min row, the rough ER holds 86% of the label, the Golgi 3% and the granules 1%. So most label is in the rough ER, where the proteins were just made.
2. What sequence of compartments do the data show?
C: Rough ER, then Golgi, then secretory granules
Follow the largest share over time: rough ER 86% at 3 min, Golgi rises to 43% at 17 min, granules 49% at 37 min and 58% at 117 min. So the order is rough ER, Golgi, secretory granules.
3. Why is a large excess of non-radioactive amino acid added after the pulse?
A: So that proteins made later are not labelled
The text says the excess non-radioactive amino acid (the chase) is added "so that proteins made afterwards were not labelled". Only the group made during the pulse carries the label and can be followed.
4. What directs a secretory protein into the ER during its synthesis?
A: A signal sequence at the beginning of the protein chain
The text says "a signal sequence at its beginning directs it through a channel into the interior of the ER" as the chain emerges from the ribosome. The sugar tag is added later, in the Golgi.
5. After a meal, the pancreas releases enzymes quickly. Which feature allows this?
B: Stored secretory granules released on a signal
The text says some cells store vesicles as secretory granules that release their contents "only when the cell receives a signal", and this regulated secretion lets the pancreas release large amounts of enzymes after a meal.
6. A genetic defect prevents the sugar tag for lysosomal enzymes. What happens?
A: The enzymes are secreted and material accumulates in lysosomes
The text says without the sugar tag "the enzymes are secreted, the lysosomes cannot digest their contents, and undigested material accumulates". So the enzymes leave the cell and material builds up in the lysosomes.
From gene to protein
DNA consists of two strands wound into a double helix. Each strand is a chain of nucleotides containing one of four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). The strands are held together by pairs of bases, A with T and G with C, and run in opposite directions, labelled 5' and 3'. Before a cell divides, the DNA is replicated semi-conservatively: the strands separate, and each serves as a template for a new partner strand, so each new double helix contains one old and one new strand.
A gene is expressed in two steps. In transcription, which takes place in the nucleus of eukaryotes, the enzyme RNA polymerase reads the template strand of a gene and builds a complementary messenger RNA (mRNA). RNA contains the base uracil (U) instead of thymine, so an A in the template strand becomes a U in the mRNA. The mRNA therefore has the same sequence as the other DNA strand, the coding strand, except that it has U where the coding strand has T.
In translation, ribosomes in the cytoplasm read the mRNA in groups of three bases, called codons. Each codon specifies one amino acid or a stop signal. Transfer RNA molecules bring the matching amino acids, which are joined into a chain. Translation begins at the start codon AUG, which also codes for methionine, and ends at one of three stop codons, UAA, UAG or UGA. Because there are 64 codons but only 20 amino acids, most amino acids are coded by several codons; the code is said to be redundant.
Changes in the DNA sequence, mutations, have different effects. A silent mutation changes a codon into another codon for the same amino acid and does not change the protein. A missense mutation changes one amino acid; in sickle cell disease, a single base change in the gene for haemoglobin replaces glutamic acid with valine. A nonsense mutation turns a codon into a stop codon, so the protein is cut short. Inserting or deleting a single base shifts the reading frame from that point on, a frameshift, and usually changes all following amino acids.
| Codon number | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Codon | AUG | UUU | GGC | UAC | GAA | UAA |
| Amino acid | Methionine | Phenylalanine | Glycine | Tyrosine | Glutamic acid | Stop |
Additional information: the codon GGU also codes for glycine.
1. How many amino acids does this mRNA section code for?
A: 5
The table lists six codons: AUG, UUU, GGC, UAC, GAA, then UAA, which is stop. The stop codon adds no amino acid, so 6 − 1 = 5 amino acids.
2. Which DNA coding-strand sequence corresponds to codon 2?
A: TTT
Codon 2 is UUU. The text says the mRNA has the same sequence as the coding strand, "except that it has U where the coding strand has T". So UUU corresponds to TTT. AAA would be the template strand.
3. A mutation changes codon 3 from GGC to GGU. What type of mutation is this?
A: A silent mutation
The table gives GGC as glycine, and the extra line says GGU "also codes for glycine". The amino acid stays the same, which the text calls a silent mutation.
4. A mutation changes codon 4 from UAC to UAA. What is the consequence?
B: The protein ends after glycine
The text names UAA as a stop codon. If codon 4 becomes UAA, translation stops there, after codon 3, glycine. The protein ends after glycine (a nonsense mutation).
5. A single base is deleted from codon 2. What is most likely?
C: All amino acids after methionine change
The text says deleting a single base "shifts the reading frame from that point on ... and usually changes all following amino acids". Codon 1 (AUG, methionine) is before the deletion, so everything after methionine changes.
6. Which statement about DNA replication is correct?
B: Each new double helix contains one old strand
The first paragraph says replication is semi-conservative: each old strand is a template for a new partner, "so each new double helix contains one old and one new strand".
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