Reproduction in Plants
sult that the mutation rate in mitochondrial replication is substantially higher than it is in replication of nuclear DNA.
Telomeres and Telomerase
hybrid combination of two different types
somatic nonreproductive; not an egg or sperm
Because eukaryotic chromosomes are linear and have ends, another enzyme, called telomerase, is necessary but is not found in the prokaryotes. The problem with chromosome ends, called telomeres, is that the 3 template for the lagging strand cannot be primed at the last nucleotides because there is no further DNA on which to build. By itself, this would reduce the length of the lagging strand and the chromosome would get shorter at each replication. Telomeric DNAs, which are repeats of a specific sequence of six nucleotides, are normally present at the ends of chromosomal DNA and avoid this problem. Telomerase is a hybrid protein-RNA molecule; the RNA sequence is complementary to several repeat lengths of the telomeric DNA. The telomerase uses the RNA sequence to bind to the template end of telomeric DNA and uses the overhang protein portion to add DNA nucleotides to the template, extending it beyond its normal length. With several movements of the enzyme outward and reiterations of this process, the template 3 end is extended sufficiently to allow DNA polymerase to complete synthesis of a normal length lagging strand. In multicellular organisms, somatic cells usually cease mitotic division during development and lack telomerase activity thereafter. Cancer cells abnormally turn their telomerase back on, which enables the cell to divide continually. Telomerase is a target of drug research for the combat of cancer. S E E A L S O Bacterial Cell; Cell Cycle; Chromosome, Eukaryotic; DNA; Nucleotides; RNA; Transcription John Merriam Bibliography Cotterill, Sue, ed. Eukaryotic DNA Replication: A Practical Approach. Oxford: Oxford University Press, 1999. Kornberg, Arthur, and Tania Baker. DNA Replication. New York: W. H. Freeman and Company, 1991.
Reproduction in Plants gamete reproductive cell, such as sperm or egg
chromosome “colored body� in the cell nucleus; made of DNA and protein, and divided functionally into genes and non-gene regions
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Plant reproduction is the process by which plants generate new individuals, or offspring. Reproduction is either sexual or asexual. Sexual reproduction is the formation of offspring by the fusion of gametes. Asexual reproduction is the formation of offspring without the fusion of gametes. Sexual reproduction results in offspring genetically different from the parents. Asexual offspring are genetically identical except for mutation. In higher plants, offspring are packaged in a protective seed, which can be long lived and can disperse the offspring some distance from the parents. In flowering plants (angiosperms), the seed itself is contained inside a fruit, which may protect the developing seeds and aid in their dispersal.
Sexual Reproduction in Angiosperms: Ovule Formation All plants have a life cycle that consists of two distinct forms that differ in size and the number of chromosomes per cell. In flowering plants, the