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The Molecular Basis of Inheritance (Many proteins work together in DNA…
The Molecular Basis of Inheritance
DNA is the genetic material
Griffith studied Streptococcus pneumoniae, a bacterium that causes pneumonia in mammals
Griffith called this phenomenon transformation, a phenomenon now defined as a change in genotype and phenotype due to the assimilation of external DNA by a cell
Further evidence that DNA was the genetic material came from studies that tracked the infection of bacteria by viruses called bacteriophages (or phages for short)
In 1952, Alfred Hershey and Martha Chase showed that DNA was the genetic material of the phage T2
To determine the source of genetic material in the phage, Hershey and Chase designed an experiment in which they labeled protein and DNA and then tracked which component
entered the E. coli cell during infection
Shortly after the onset of infection, Hershey and Chase spun the cultured infected cells in a blender, shaking loose any parts of the phage that remained outside the bacteria
Watson and his colleague Francis Crick began to work on a model of DNA with two strands, the double helix
In their model, the two sugar-phosphate backbones are antiparallel, with the subunits running in opposite directions
Many proteins work together in DNA replication and repair
During DNA replication, base pairing enables existing DNA strands to serve as templates for new complementary strands
Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each of the daughter molecules has one old strand and one newly made strand
A large team of enzymes and other proteins carries out DNA replication
The replication of a DNA molecule begins at special sites called origins of replication
At the origin sites, the DNA strands separate, forming a replication “bubble” with replication forks at each end, where the parental strands of DNA are being unwound
Helicases untwist the double helix and separate the template DNA strands at the replication fork
Single-strand binding proteins bind to unpaired DNA strands, stabilizing them
This untwisting causes tighter twisting ahead of the replication fork; topoisomerase helps relieve this strain
The initial nucleotide chain is a short stretch of RNA called a primer, synthesized by the enzyme primase
Enzymes called DNA polymerases catalyze the synthesis of new DNA by adding nucleotides to a preexisting chain
The two strands of DNA in a double helix are antiparallel, oriented in opposite directions to each other
Another enzyme, DNA ligase, joins the sugar-phosphate backbones of the Okazaki fragments to form a single DNA strand
An enzyme called telomerase catalyzes the lengthening of telomeres in eukaryotic germ cells, restoring their original length
A chromosome consists of a DNA molecule packed together with proteins
A bacterium has a dense region of DNA called the nucleoid
In the cell, eukaryotic DNA is packaged with protein to form chromatin
Condensed chromatin is called heterochromatin, while more dispersed chromatin is called euchromatin
The chromosome is a dynamic structure that is condensed, loosened, modified, and remodeled as necessary for various cell processes, including mitosis, meiosis, and gene expression
Because of its compaction, heterochromatic DNA is largely inaccessible to the machinery in the cell responsible for expressing the genetic information coded in the DNA
The looped domains of an interphase chromosome are attached to the nuclear lamina, on the inside of the nuclear envelope, and perhaps also to fibers of the nuclear matrix
Although interphase chromatin is generally much less condensed than the chromatin of mitotic chromosomes, it shows several of the same levels of higher-order packing