Information processing Flashcards

Question

25. Explain the role of a signal sequence and the process by which a ribosome delivers a polypeptide to the ER

Answer 1

Ribosomes in Eukaryotes can be found in two locations > cytosol > RER - Some proteins are modified - adding sugars to protein to form glycoproteins or adding lipids to form glycolipids - beginning of gene coding for these proteins have signal sequences that get transcribed aswell - this signal sequence gets translated first and pokes out of ribosome translating it - the complex (mRNA, peptide, ribosome) gets bound by a protein that transports it to the membrane of ER ---> complex binds to receptors on ER membrane - Polypeptide coming out of top of large ribosomal unit goes into lumen of ER - signal sequence gets removed and the protein releases from ribosome to be modified and transported to different places - a signal peptide is at the start of the protein sequence for those proteins that are modified, trafficked or secreted - rough ER is named because of the blobs of ribosome attached to the membranes of ER - from the lumen of ER it is sorted through the Golgi apparatus to be trafficked to anywhere in the cell or out of the cell 🧬 Where are ribosomes found in eukaryotes? In the cytosol – make proteins that stay in the cell. On the rough ER – make proteins that are secreted, sent to membranes, or modified. 🏷️ How do proteins go to the rough ER? 1. The mRNA has a signal sequence at the start of the gene (gets translated first). 2. This signal peptide sticks out of the ribosome as the protein starts being made. 3. A special protein binds the ribosome-mRNA complex and guides it to the ER membrane. 3. The complex binds a receptor on the ER. 4. The new protein is fed into the ER as it's being made (into the ER lumen). 5. The signal sequence is cut off. 6. The protein is modified (e.g., sugars/lipids added). 7. The protein is then sent to the Golgi apparatus, where it is sorted and sent to where it needs to go (in the cell or out of it). 📦 Summary: Proteins that are secreted or modified start with a signal. Ribosomes go to the rough ER to make them. Proteins enter the ER, get processed, then go to the Golgi to be shipped to the right location. ✅ Translation begins in the cytosol, and if the protein being made has a signal peptide, then: Translation starts in the cytosol. The signal peptide gets translated and pokes out of the ribosome. The SRP binds the signal peptide → pauses translation. The whole complex (ribosome + mRNA + growing polypeptide) is moved to the ER. Once attached to the ER, translation resumes, and the protein is threaded into the ER lumen. So yes, translation is initiated in the cytosol, then completed on the ER if it's a secreted or membrane protein.

Answer 2

- A third of the human genome is actually coding > 45% is made of transposons (mobile elements) - A transposon is an active piece of DNA that can MOVE from place to place and is usually found in between genes or inside introns - DNA transposons move using DNA intermediate - Retrotransposons move using RNA intermediate *They arent really moving, just are making new copy of themselves in RNA using reverse transcriptase to make DNA copy that inserts elsewhere in genome - predominantly LINE and SINE segments > SINES & LINES = Retrotransposons - Retrotransposons are remnant of retrovirus that no longer makes coat (capsid) or move out of cell - they can only integrate into DNA and can be transcribed/translated - can't leave cell - Central dogma said copies of DNA can be made by DNA replication copies of RNA can be made by transcription for protein translation - But reverse transcriptase does this in reverse - takes RNA and makes DNA copy - LINES = 6kb long and has open reading frame encoding for reverse transcriptase *reverse transcriptase can only make one copy of LINE elements from RNA into DNA which can insert somewhere else (autonomously) - SINE = 350bp long and dont encode reverse transcriptase so cant move autonomously *can only move from one place to the other in a genome is there is a LINE element producing reverse transcriptase that can act on RNA of SINE elements > RETROVIRUSES: - Bind to plasma membrane and inject their RNA genome and reverse-transcriptase into another cell's cytosol - Reverse-transcriptase converts ssRNA into dsDNA - DsDNA can integrated into host genome and another viral genome in RNA can be produced by transcription - mRNA is translated to produce proteins in virus capsids and enzymes like reverse-transcriptase - these proteins get assembled into new virus particles and bursts out of cell to infect other cells - This is a retrovirus not a retrotransposon 🧬 Human Genome: Only ~1/3 of our genome codes for proteins. ~45% is made of transposons – bits of DNA that can copy and insert themselves elsewhere. 🔄 Transposons (Jumping Genes): Two Types: - DNA Transposons – move as DNA. - Retrotransposons – move using RNA → DNA (via reverse transcriptase). 🌀 Retrotransposons: They don't actually "move" — they: Get transcribed into RNA. Use reverse transcriptase to turn that RNA into DNA. That new DNA inserts somewhere else in the genome. 📏 Types of Retrotransposons: 1. LINEs (Long Interspersed Nuclear Elements) ~6,000 bases long. Encode reverse transcriptase → can copy themselves. Move on their own. 2. SINEs (Short Interspersed Nuclear Elements) ~350 bases long. Don’t have reverse transcriptase. Need LINEs to help them move. REVERSE TRANSCRIPTASE USE: | Who uses it? | Why? | **Retroviruses** | To insert their genome into host DNA so they can hijack the cell | | **Retrotransposons** | To make DNA copies of themselves and "jump" to new genome locations | | **Molecular biology** | Scientists use it to make DNA from RNA (e.g., making cDNA from mRNA) |

Answer 3

Nuclear genome - only 1.5% is coding - rest is are introns and non-coding Mitochondrial genome - more densely packed - No introns and genes dont have much space between them - variation in genetic code - 4 of 64 genes are unique - fewer tRNAs used - mitochondria produces own tRNA - circular genome - mitochondria needs proteins encoded by both mitochondrial and nuclear genome

Answer 4

- Mitochondrial diseases affect energy development, vision and cause seizures - Can be caused by mutations in mitochondrial genome or mitochondrial genes in nuclear genome

Answer 5

Molecular Biologists may ask following questions about a gene: - What cell type expresses the gene1? - Under what conditions do they express it? - What can change or control the expression of gene1?

Answer 6

1. The small subunit of the ribosome binds to the Met-tRNA at the P site first and the mRNA. 4. The large subunit of the ribosome binds to the mRNA 3. An amino-acyl tRNA base pairs with the codon in the A site. 5. The ribosome catalyses the formation of a peptide bond. 2. The ribosome moves along so that the tRNA in the A site is moved into the P site.

Information processing Flashcards

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