Meselson and Stahl used density labeling of DNA to show that DNA replication occurs via a semiconservative mechanism. In their e
xperiment, they started with an organism grown in a heavy density label (15N). After two generations of growth in light medium (the more common 14N isotope), if the DNA is isolated and separated by density, how many bands would be observed and how would their density compare with the starting DNA
The organism previously used 15N for replication so all the DNA molecules were of 15N15N type. Then the organism is shifted to a medium where only 14N is available for replication.
According to semi conservative mode of replication, a newly synthesised DNA molecule consists of one new strand and one parental strand. So after the first round of replication, All the 15N strands will synthesise new DNA strands using 14N resulting into intermediate 15N14N DNA molecules. Hence, only one band would be observed (15N14N) above the original 15N15N band since 15N14N has lighter isotope too so it will be lighter than 15N15N molecules and will lie above it.
After second round of replication, 15N strand from 15N14N would synthesise another 14N strand. 14N strand from 15N14N molecules will also synthesise another 14N strand. So now, 50% of the DNA molecules will be of 15N14N intermediate type and 50% of them will be of 14N14N type.Two bands will be observed above the original 15N15N band. One band of 15N14N molecules will be right above it and other band of 14N14N molecules will be even higher because it is the lightest band since it has only the lighter isotope of nitrogen.
incomplete dominance is inheritance in which one allele for a specific trait is not completely expressed over its paired allele. this results in a third phenotype in which the physical trait is a combination of both alleles.
Starch molecules taken into the mouth from food substances are processed to an extent of 30% of its digestion. this is carried out by a specialized protein/ an enzyme that is present in the saliva; called amylase or ptyalin. this enzyme acts on the substrate molecule which in this case is starch molecules and convert it into smaller chains of simple sugars that includes maltose and dextrin which is digested in the small intestine.
Gnetophytes are gymnosperms
, they are woody and the embryo has two cotyledons
. Cycads are woody plants that produce seeds found in Subtropical and warm temperate regions and also they produce motile sperm cells.
