2.2
Ошибки копирования генома
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Q1: What happens when DNA polymerase makes errors during replication?
DNA polymerase occasionally adds incorrect bases during replication, occurring at a rate of approximately 1 error per 100,000 bases. These mistakes can result in point mutations, such as the GAG to GTG change in beta-hemoglobin that causes sickle cell anemia, or indels where bases are added or deleted, shifting the reading frame and producing defective proteins.
Q2: How does the cell correct DNA replication errors?
Cells employ three levels of error correction. DNA polymerase enzymes use 3'→5' exonuclease activity to proofread and remove incorrect bases during replication. Mismatch repair enzymes fix remaining errors after replication but before mitosis. DNA damage repair pathways operate throughout the cell cycle to fix point mutations and large-scale genome rearrangements, or trigger apoptosis if damage is severe.
Q3: What is cystic fibrosis and how does a copying error cause it?
Cystic fibrosis results from an indel mutation in the CFTR gene that shifts the reading frame, producing a defective protein unable to transport chloride ions out of lung epithelial cells. This causes thick, glue-like mucus accumulation in the lungs, increasing infection risk. A single base addition or deletion is sufficient to disrupt the entire protein's function.
Q4: Are all mutations harmful to organisms?
Most mutations are neutral and have no effect on organism survival. Some mutations confer survival advantages, such as antibiotic resistance in bacteria. When genes coding for DNA gyrase accumulate random base substitutions, the mutated enzyme no longer binds antibiotics like fluoroquinolone, allowing bacteria to survive and grow despite antibiotic presence.
Q5: What are transposons and how do they cause mutations?
Transposons are small DNA repeat elements that jump from one location to another on the DNA strand, disrupting gene functions at their insertion sites. DNA transposons move directly as DNA, while retrotransposons transpose via an RNA intermediate. During insertion, transposons can cause target site duplication, leading to gene duplication and divergence paralogs pseudogenes that may harm cell function.
Q6: Why do copying errors occur despite DNA polymerase's high fidelity?
DNA polymerase must copy over 3 billion base pairs during each human cell division. The sheer volume of information being duplicated makes occasional errors inevitable, even with high-fidelity enzymes. However, the error rate remains low at approximately 1 per 100,000 bases, and most errors are caught by proofreading and repair mechanisms before being passed to daughter cells.
Q7: How can mutations contribute to evolutionary relationships through genome comparisons?
Accumulated mutations over time create genetic variations between species that reflect their evolutionary history. By comparing genomes and identifying patterns of mutations, scientists can trace evolutionary relationships through genome comparisons and understand how species diverged from common ancestors. These genetic differences serve as molecular markers of evolutionary change.