BSC 310 Lecture Notes - Lecture 21: Endoplasmic Reticulum, Peptide, Repressor
DNA Structure
During the 1950s, a tremendous explosion in biological research occurred, and the
methods of gene expression were elucidated. The knowledge generated during this
period helped explain how genes function in microorganisms and gave rise to the
science of molecular genetics. This science is concerned with the activity of
deoxyribonucleic acid (DNA) and how that activity brings about the production of
proteins in microbial and other cells.
As proposed originally in 1953 by Watson and Crick, deoxyribonucleic acid (DNA)consists of
two long chains of nucleotides. The two nucleotide chains twist around one another to form
a double helix, which resembles a spiral staircase. The two chains of nucleotides are held to
one another by weak hydrogen bonds between bases of the chains.
A nucleotide in the DNA chain consists of three parts: a nitrogenous base, a phosphate
group, and a molecule of deoxyribose. The nitrogenous bases of each nucleotide chain are of
two major types: purines and pyrimidines. Purines have two fused rings of carbon and
nitrogen atoms, while pyrimidines have only one ring. The two purine bases in DNA
are adenine (A) and guanine (G). The pyrimidine bases in DNA arecytosine (C) and thymine
(T). Purine and pyrimidine bases are found in both strands of the double helix.
The phosphate group of DNA is derived from a molecule of phosphoric acid and connects the
deoxyribose molecules to one another in the nucleotide chain.Deoxyribose is a five-carbon
carbohydrate. The purine and pyrimidine bases are attached to the deoxyribose molecules
and stand opposite one another on the two nucleotide chains. Adenine always stands
opposite and binds to thymine. Guanine always stands opposite and binds to cytosine.
Adenine and thymine are said to be complementary, as are guanine and cytosine. This is
known as the principle ofcomplementary base pairing.
DNA replication. Before a cell enters the process of binary fission or mitosis, the DNA
replicates itself to ensure that the daughter cells can function independently. In the process
of DNA replication, specialized enzymes pull apart, or “unzip,” the DNA double helix.
As the two strands separate, the purine and pyrimidine bases on each strand are exposed.
The exposed bases then attract their complementary bases and induce the complementary
bases to stand opposite. Deoxyribose molecules and phosphate groups are brought into the
environment, and the enzyme DNA polymerase unites all the nucleotide components to one
another and forms a long strand of nucleotides. Thus, the old strand of DNA directs the
synthesis of a new strand of DNA through complementary base pairing.
After the synthesis has occurred, one old strand of DNA unites with a new strand to reform
a double helix. This process is called semiconservative replication because one of the old
strands is conserved in the new DNA double helix.
Protein Synthesis
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During the 1950s and 1960s it became apparent that DNA is essential in the synthesis
of proteins. Proteins are used as structural materials in the cells and function as
enzymes. In addition, many specialized proteins function in cellular activities. For
example, in bacteria, flagella and pili are composed of protein.
The genetic code. The key element of a protein molecule is how the amino acids are linked.
The sequences of amino acids, determined by genetic codes in DNA, distinguish one protein
from another. The genetic code consists of the sequence of nitrogenous bases in the DNA.
How the nitrogenous base code is translated to an amino acid sequence in a protein is the
basis for protein synthesis.
In order for protein synthesis to occur, several essential materials must be present. One is a
supply of the 20 amino acids which make up most proteins. Another essential element is a
series of enzymes that will function in the process. DNA and another form of nucleic acid
called ribonucleic acid (RNA) are also essential. RNA carries instructions from the nuclear
DNA into the cytoplasm, where protein is synthesized. RNA is similar to DNA, with three
exceptions. First, the carbohydrate in RNA is ribose rather than deoxyribose. Second, RNA
nucleotides contain the pyrimidine uracil rather than thymine. And third, RNA is usually
single-stranded.
Types of RNA. In the synthesis of protein, three types of RNA are required. The first is
called ribosomal RNA (rRNA) and is used to manufacture ribosomes. Ribosomes are
ultramicroscopic particles of rRNA and protein where amino acids are linked to one another
during the synthesis of proteins. Ribosomes may exist along the membranes of the
endoplasmic reticulum in eukaryotic cells or free in the cytoplasm of prokaryotic cells.
A second important type of RNA is transfer RNA (tRNA), which is used to carry amino acids to
the ribosomes for protein synthesis. Molecules of tRNA exist free in the cytoplasm of cells.
When protein synthesis is taking place, enzymes link tRNA to amino acids in a highly specific
manner.
The third form of RNA is messenger RNA (mRNA), which receives the genetic code from DNA
and carries it into the cytoplasm where protein synthesis takes place. In this way, a genetic
code in the DNA can be used to synthesize a protein at a distant location at the ribosome.
The synthesis of mRNA, tRNA, and rRNA is accomplished by an enzyme called RNA
polymerase.
Transcription. Transcription is one of the first processes in the overall process of protein
synthesis. In transcription, a strand of mRNA is synthesized using the genetic code of DNA.
RNA polymerase binds to an area of a DNA molecule in the double helix (the other strand
remains unused). The enzyme moves along the DNA strand and selects complementary
bases from available nucleotides and positions them in an mRNA molecule according to the
principle of complementary base pairing (Figure 1 ). The chain of mRNA lengthens until a
stop code is received.
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Document Summary
During the 1950s, a tremendous explosion in biological research occurred, and the methods of gene expression were elucidated. The knowledge generated during this period helped explain how genes function in microorganisms and gave rise to the science of molecular genetics. This science is concerned with the activity of deoxyribonucleic acid (dna) and how that activity brings about the production of proteins in microbial and other cells. As proposed originally in 1953 by watson and crick, deoxyribonucleic acid (dna)consists of two long chains of nucleotides. The two nucleotide chains twist around one another to form a double helix, which resembles a spiral staircase. The two chains of nucleotides are held to one another by weak hydrogen bonds between bases of the chains. A nucleotide in the dna chain consists of three parts: a nitrogenous base, a phosphate group, and a molecule of deoxyribose. The nitrogenous bases of each nucleotide chain are of two major types: purines and pyrimidines.
