In eukaryotes, the DNA strands are linear, and DNA polymerase can't replicate the very ends of the DNA strands! These ends are "protected" by repeated sequences called "telomeres." Either the chromosome gets shorter with each replication, or else a special enzyme-nucleic acid complex called telomerase adds new telomeres to the ends. A prokaryotic chromosome is circular and thus does not have the problem of having ended.
<span>Eukaryotic DNA is wound around histones, coiled, and supercoiled -- to replicate it, there have to be unwinding mechanisms, and mechanisms to reduce the degree of coiling. In prokaryotes, the winding problem is much less, and there aren't any histones.</span>
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Explanation: Three atmospheric processes modify the solar radiation passing through our atmosphere destined to the Earth's surface. These processes act on the radiation when it interacts with gases and suspended particles found in the atmosphere.
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Cytochrome c is an enzyme found in bacteria, fungi, plants and animals. Here it is given that the human amino acid sequence for cytochrome c differs at 12 locations from the sequence in horses and pigeons. This information might be useful for inferring in preliminary stages that horses and pigeons are closely related to each other since they both show similar amount of difference from humans.
However, this hypothesis is being made with respect to the the comparison between "humans and horses" and "humans and pigeons". To further test this hypothesis pigeons and horses should be directly compared to each other without the use of a third species like humans. Hence, the sequence for cytochrome c in horse and pigeon should be compared with each other rather than comparing to humans.
1. The cell wall has lignin (hard substance): to prevent the xylem vessel from collapsing and withstand high pressure.
2. The absence of protoplasm (nucleus + cytoplasm) and cross wall: to allow water to move through the lumen easily.
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