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12-1 DNA
Griffith had isolated two slightly different strains, or types, of pneumonia bacteria from mice. Both strains grew very well in culture plates in his lab, but only one of the strains caused pneumonia. The disease-causing strain of bacteria grew into smooth colonies on culture plates, whereas the harmless strain produced colonies with rough edges. He then performed the following expiriments on mice. Avery and other scientists discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next. Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.
Using clues from Franklin's pattern, within weeks Watson and Crick had built a structural model that explained the puzzle of how DNA could carry information, and how it could be copied. They published their results in a historic one-page paper in April of 1953. Watson and Crick's model of DNA was a double helix, in which two strands were wound around each other.
Chromosomes and DNA Replication
Prokaryotic cells lack nuclei and many of the organelles found in eukaryotes. Their DNA molecules are located in the cytoplasm. Most prokaryotes have a single circular DNA molecule that contains nearly all of the cell's genetic information. Eukaryotic DNA is a bit more complicated. Many eukaryotes have as much as 1000 times the amount of DNA as prokaryotes. This DNA is not found free in the cytoplasm. Eukaryotic DNA is generally located in the cell nucleus in the form of a number of chromosomes. Eukaryotic chromosomes contain both DNA and protein, tightly packed together to form a substance called chromatin. Chromatin consists of DNA that is tightly coiled around proteins called histones
The structure explained how DNA could be copied, or replicated.The sites where separation and replication occur are called replication forks.During DNA replication, the DNA molecule separates into two strands, then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand. The principal enzyme involved in DNA replication is called DNA polymerase (PAHL-ih-mur-ayz) because it joins individual nucleotides to produce a DNA molecule
RNA and Protein Synthesis
You can think of an RNA molecule as a disposable copy of a segment of DNA. In many cases, an RNA molecule is a working copy of a single gene. The ability to copy a single DNA sequence into RNA makes it possible for a single gene to produce hundreds or even thousands of RNA molecules.
There are three main types of RNA: messenger RNA, ribosomal RNA, and transfer RNA. During transcription, RNA polymerase binds to DNA and separates the DNA strands. RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA. Surprisingly, the DNA of eukaryotic genes contains sequences of nucleotides, called introns, that are not involved in coding for proteins. The DNA sequences that code for proteins are called exons because they are “expressed” in the synthesis of proteins. The genetic code is read three letters at a time, so that each “word” of the coded message is three bases long. Each three-letter “word” in mRNA is known as a codon There are a possible 20 amino acids to be put in order.
The decoding of an mRNA message into a polypeptide chain (protein) is known as translation. Translation takes place on ribosomes. During translation, the cell uses information from messenger RNA to produce proteins.
Similarly, the cell uses the vital DNA “master plan” to prepare RNA “blueprints.”Proteins are microscopic tools, each specifically designed to build or operate a component of a living cell.
images from online textbook and google image search
