Transcript Summarize main ideas - Montgomery`s Hedrick Life Science

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RNA
World of Chemistry , 2006, Updated: July 31, DATE:
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RNA is ribonucleic acid, one of two kinds of of nucleic acid (the
other is deoxyribonucleic acid, or DNA).DNA carries a cell's genetic
information, but this information cannot be accessed without RNA. In
some organisms, such as in certain viruses, RNA is the carrier of
genetic information. It has also been discovered that RNA can
sometimes act as a catalyst of chemical reactions.
RNA is almost always a single-stranded molecule constructed as
a long chain of nucleotides. Each nucleotide contains a ribose molecule
(a type of sugar), a phosphate group, and one of four nitrogencontaining cyclic units. The nitrogen-containing units are the bases
adenine, cytosine, guanine, and uracil. The nucleotides of DNA and
RNA differ in two respects. First, an -OH group of the sugar present in
RNA is -H in DNA. Second, one of the bases of DNA is thymine rather
than uracil.
DNA is a double-stranded molecule that contains the blueprint
for constructing proteins necessary for cell structure and function. This
blueprint details which amino acids to use and in what order they
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should appear. However, the information flow from DNA to protein is
RNA allows a cell to access genetic information in DNA, can carry
information, and can catalyze chemical reactions.
RNA is a single strand of nucleotides, each with ribose, phosphate and a
base. Uracil is the RNA base that replaces Thymine.
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DNA is double-stranded with information to construct proteins.
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RNA
World of Chemistry , 2006, Updated: July 31, DATE:
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not direct. One strand of the DNA--the coding strand--serves as a
master copy of the genetic code. The cell uses RNA to construct a
working copy from that strand using a process called transcription.
Transcription begins when an enzyme, RNA polymerase, binds to
the coding strand of DNA just before the start of a gene. A gene is the
stretch of DNA that codes for a particular protein or polypeptide. The
RNA polymerase binds to DNA with the help of other proteins called
transcription factors.
Once RNA polymerase binds to the DNA, RNA construction
begins. The two strands of DNA separate and, for a short while, the
coding strand is paired with RNA as a matching strand. The base
pairing between the DNA strand and the RNA strand follows the same
rules as base pairing between two strands of DNA. The key exception
is that uracil is used in place of thymine. The mRNA (messenger RNA)
detaches from DNA when transcription is complete. It may be modified
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before the next step, release inside the cell.
One DNA strand is the coding / template strand, which is copied to an
RNA strand in transcription. RNA polymerase binds to the coding strand
of a gene, and separates the two DNA strands. Free RNA nucleotides
pair with DNA nucleotides on the coding strand. This messenger RNA
strand releases to be modified and then released inside the cell.
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NAME:
RNA
World of Chemistry , 2006, Updated: July 31, DATE:
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2006
The next phase is translation of the code contained in mRNA.
Translation relies on two other types of RNA: ribosomal RNA (rRNA)
and transfer RNA (tRNA). Although there are three types of RNA, their
differences are based more on their function and their associated
proteins, rather than the nucleic acid itself.
The rRNA is found within a cellular structure called a ribosome.
To start translation, a ribosome attaches itself to a strand of mRNA.
The ribosome holds the mRNA in place and directs the speed of
translation. The transcribed code carried on the mRNA is read one
triplet--three nucleotides--at a time. There are 64 possible mRNA
triplets, or codons. Sixty-one of them code for a specific amino acid;
the remaining three serve as stop signals to end translation. Most amino
acids have more than one associated codon.
As the ribosome gradually moves along the length of the mRNA,
tRNA transports amino acids to the growing protein molecule. The
tRNA contains a nucleotide sequence called the anticodon. The
anticodon determines which amino acid the tRNA carries. When the
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anticodon pairs up with a corresponding mRNA codon, it releases the
Translation of the code in the messenger RNA uses ribosomal RNA and
transfer RNA. A ribosome attaches to the mRNA strand. mRNA is read
one codon at a time. Each tRNA has an anticodon which attaches to the
correct mRNA codon. The tRNA connects its amino acid to the previous
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amino acid to make a protein chain.
NAME:
RNA
World of Chemistry , 2006, Updated: July 31, DATE:
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amino acid to the growing protein chain. Once the mRNA is translated,
it is broken down by an enzyme called ribonuclease. The component
nucleotides can then be recycled by the cell.
In some cases, RNA serves as the carrier of genetic information.
This situation appears with several viruses such as the tobacco mosaic
virus and a class of viruses called retroviruses. Retroviruses include the
human immunodeficiency virus (HIV), which causes acquired
immunodeficiency syndrome (AIDS). In the case of HIV, the virus
contains RNA and an enzyme called reverse transcriptase. When the
virus encounters a particular type of immune system cell, it injects its
contents into it. The reverse transcriptase goes to work to construct a
DNA molecule from the RNA. (This situation is the opposite of the
usual mode of transcription, i.e., from DNA to mRNA.) The newly
constructed DNA then attaches to the rest of the cell's DNA where it
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can hide for months or years before it is used.
mRNA is broken down by the enzyme ribonuclease to be recycled.
RNA can carry genetic information in some cases. Retroviruses,
including HIV, contain RNA and an enzyme that builds DNA copies of the
virus genes in host cells. These copies are hidden along with normal
host DNA until they are used to make viral proteins later.
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NAME:
RNA
World of Chemistry , 2006, Updated: July 31, DATE:
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The discovery that RNA can carry genetic information has led to
other discoveries as well. One such discovery has been that an RNA
molecule can sometimes act as an enzyme by catalyzing the
modification of itself or other RNA molecules. This finding may
provide insight into the origins of life. An outstanding scientific
question regarding the origins of life has been whether enzymes or
genes came first. Discovering that RNA can act as both a carrier of
genetic information and as an enzyme may provide an answer to this
question.
RNA research is also taking a place in the areas of gene therapy
and genetic engineering. Antisense technology makes use of a type of
RNA called antisense RNA. Typically, a cell only contains mRNA that
corresponds to the coding strand of DNA. However, by inserting an
inverted copy of that gene into the genetic material of a cell, it is
possible to make the cell produce mRNA for both the coding strand and
the matching strand. The two mRNA strands are complementary to one
another and form a double-stranded molecule much like DNA. This
pairing prevents the mRNA from being translated and the abnormal
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double-stranded RNA is often destroyed by ribonuclease.
RNA can also be an enzyme, helping modify RNA. This may help us learn
whether enzyme proteins or genes developed first in the history of life.
Antisense technology helps the cell destroy unwanted genes in one 5
form of genetic engieneering.
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RNA
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Gene therapy is also making use of catalytic RNA through the
construction of ribozymes. Ribozymes are catalytic RNA molecules
that destroy RNA that has a specific sequence of nucleotides. A
potential application of ribozymes might be treating certain diseases
that are characterized by the presence of certain proteins. For example,
if the mRNA for a viral protein were destroyed prior to being
translated, new virus particles could not be constructed and the viral
infection would be halted.
Source Citation
"RNA." World of Chemistry. Gale, 2006. Gale Science In Context.
Web. 12 Dec. 2012.
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Gene therapy also includes using ribozymes, RNA that destroys certain
RNA sequences.
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RNA function
World of Biology , 2006Updated: April 3, 2006
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Ribonucleic acid (RNA) has a variety of functions in a cell.
Depending on the type of RNA it can function as a carrier for genetic
information, a catalyst for biochemical reactions, an adapter molecule
in protein synthesis, and a structural molecule in cell organelles.
There are five major types of RNA that are found in the cells of
eukaryotes. These include heterogeneous nuclear RNA (hnRNA),
messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA
(rRNA), and small nuclear RNA (snRNA). Each has a different role in
various cellular processes. In addition to these forms, RNA is a key
component of certain viruses.
One of the primary functions of RNA is to facilitate the
translation of DNA into a useful protein. This process begins in the
nucleus of the cell with a series of enzymatic reactions that copy DNA,
producing heterogeneous nuclear RNA. Since hnRNA is a direct copy
of DNA, it contains exons and introns; coding and noncoding regions
of nucleotides, respectively. hnRNA undergoes splicing which removes
the introns and converts it to mRNA. mRNA is transported out of the
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nucleus into the cytoplasm of the cell.
RNA can be a carrier of genetic info, a catalyst for reactions, a helper in
protein synthesis, and a structural part of cell organelles.
Heterogeneous RNA, messenger RNA, transfer RNA, ribosomal RNA and
small nuclear RNA are the 5 types in most cells, and RNA is also in some
viruses. hnRNA is also called the primary RNA transcript, which is 7
modified to become the messenger RNA
RNA function
World of Biology , 2006Updated: April 3, 2006
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Ribosomes interact with the mRNA and construct a protein based
on the nucleotide sequence. Part of this process involves tRNA. tRNA
is an adapter molecule which functions as a bridge between the codons
on the mRNA and the amino acids that are used to construct the
protein. During protein synthesis, one end of the tRNA interacts with
the triplet of nucleotides on the mRNA (this arrangement of
nucleotides is called a codon). The other end of the tRNA carries an
amino acid. This amino acid is then transferred and chemically bonded
to a series of amino acids to produce a protein. Another type of RNA
that is involved in protein synthesis is rRNA. rRNA has two functions
in a ribosome. First, it provides the structure and shape for the catalytic
areas of the ribosome. Second, it helps speed up, or catalyze, the action
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of the tRNA.
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RNA function
World of Biology , 2006Updated: April 3, 2006
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While DNA and RNA are very similar in their composition, RNA
has a much more versatile role. This is because it not only conveys
genetic information, but also catalyzes reactions. The latter RNA
molecules are known as ribozymes. As previously mentioned, rRNA
functions as a ribozyme during protein synthesis. Another example of a
ribozyme is form of RNA that acts as a ribozyme is snRNA. During the
process of RNA splicing snRNA catalyzes reactions in the spliceosome,
a group of biomolecules that splice hnRNA. snRNA also plays a
structural role in this process.
Source Citation
"RNA function." World of Biology. Gale, 2006. Gale Science In
Context. Web. 12 Dec. 2012.
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Transcription
World of Genetics , 2007
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The genetic information that is passed on from parent to
offspring is carried by the DNA of a cell. The genes on the DNA code
for specific proteins that control all aspects of life, including
appearance, different facets of personality, health etc. In order for the
genes to produce the proteins, they must first be transferred from DNA
to RNA in a process known as transcription. Thus, transcription is
defined as the transfer of genetic information from the DNA to the
RNA.
The process of transcription occurs in the nucleus of the cell.
There are three different phases involved: initiation, elongation, and
termination.
To initiate the process of information transfer, one of the strands
of the double stranded DNA serves as a template for the synthesis of a
single strand of RNA that is complementary to the DNA strand. The
enzyme RNA polymerase binds to a particular region of the DNA that
is termed as the "promoter." The promoter is a particular unidirectional
sequence that appears at the beginning of the genes, and tells the
enzyme where to start the synthesis and which strand to synthesize.
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Transcription
World of Genetics , 2007
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Once the enzyme is bound to the promoter, it unwinds the DNA
and starts to make a strand of RNA with a base sequence
complementary to the DNA template that is downstream of the RNA
polymerase binding site. The strand from which it copies is known as
the template or the antisense strand, while the other strand to which it
is identical is called the sense or the coding strand.
After initiation, is the process of elongation. The substrates for
RNA polymerase are nucleoside triphosphates. The RNA polymerase
matches a base on the DNA to an RNA nucleotide (by complementary
base pair binding) and then adds that nucleotide to the elongating RNA
strand. As a new ribonucleotide triphosphate forms a bond with the 3'hydroxyl end of the growing strand, a pyrophosphate is given off. The
energy that is needed for synthesizing RNA is derived from splitting up
of the triphosphate into a monophosphate and releasing the other two
inorganic phosphates.
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Transcription
World of Genetics , 2007
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The next phase is called termination. Termination occurs when
the RNA polymerase reaches a signal on the DNA template strand that
tells it to stop. Once this termination signal is recognized by the RNA
polymerase, it releases the DNA and transcription ceases. The newly
synthesized RNA strand now undergoes "post-transcriptional
processing."
Eukaryotic genes are not continuous. A typical gene consists of
both coding sequences (exons) and non-coding sequences (introns).
The primary transcript that is formed at the end of the transcription is
actually known as hnRNA and is an exact copy of the gene with both
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introns and exons. A process called RNA splicing occurs and the
introns are removed. The remaining exons are joined together to form
the final mRNA product that codes for a single protein. This posttranscriptional RNA processing takes place in the nucleus.
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Transcription
World of Genetics , 2007
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Besides splicing, the hnRNA strand also has to be capped and
poly-adenylated before being transported to the cytoplasm for
translation into proteins. The 5' capping of the hnRNA occurs soon
after the beginning of transcription. A methylated guanosine (G)
nucleotide that is believed to play an important role in the initiation of
protein synthesis is added to the 5' end. For the addition of a poly A
tail, one hundred to two hundred residues of adenylic acid are added by
an enzyme known as poly-A polymerase. This tail helps to guard the
RNA transcript against degradation and enables the transcript to exit
from the nucleus to the cytoplasm where it can be translated into
proteins.
Source Citation
"Transcription." World of Genetics. Gale, 2007. Gale Science In
Context. Web. 12 Dec. 2012.
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Translation
World of Genetics , 2007
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Translation is the process in which genetic information, carried
by messenger RNA (mRNA), directs the synthesis of proteins from
amino acids, whereby the primary structure of the protein is determined
by the nucleotide sequence in the mRNA.
A molecule known as the ribosome is the site of the protein
synthesis. The ribosome is protein bound to a second species of RNA
known as ribosomal RNA (rRNA). Several ribosomes may attach to a
single mRNA molecule, so that many polypeptide chains are
synthesized from the same mRNA. The ribosome binds to a very
specific region of the mRNA called the promoter region. The promoter
is upstream of the sequence that will be translated into protein.
The nucleotide sequence on the mRNA is translated into the
amino acid sequence of a protein by adaptor molecules composed of a
third type of RNA known as transfer RNAs (tRNAs). There are many
different kinds of tRNAs, with each type binding a particular type of
amino acid. In protein synthesis, the nucleotide sequence on the mRNA
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does not specify an amino acid directly, rather, it specifies a particular
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Translation
World of Genetics , 2007
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species of tRNA. Complementary tRNAs match up on the strand of
mRNA every three bases and add an amino acid onto the lengthening
protein chain. A three base sequence on the mRNA is known as a
"codon," while the complementary sequence on the tRNA is the "anticodon."
The ribosomal RNA has two subunits, a large subunit and a small
subunit. When the small subunit encounters the mRNA, the process of
translation to protein begins. There are two sites in the large subunit, an
"A" site, and a "P" site. The start signal for translation is the codon
ATG that codes for methionine. A tRNA charged with methionine binds
to the translation start signal. After the first tRNA bearing the amino
acid appears in the "A" site, the ribosome shifts so that the tRNA is
now in the "P" site. A new tRNA molecule corresponding to the codon
of the mRNA enters the "A" site. A peptide bond is formed between the
amino acid brought in by the second tRNA and the amino acid carried
by the first tRNA. The first tRNA is now released and the ribosome
again shifts. The second tRNA bearing two amino acids is now in the
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"P" site, and a third tRNA can now bind to the "A" site. The process of
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Translation
World of Genetics , 2007
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the tRNA binding to the mRNA aligns the amino acids in a specific
order. This long chain of amino acids constitutes a protein. Therefore,
the sequence of nucleotides on the mRNA molecule directs the order of
the amino acids in a given protein. The process of adding amino acids
to the growing chain occurs along the length of the mRNA until the
ribosome comes to a sequence of bases that is known as a "stop
codon." When that happens, no tRNA binds to the empty "A" site. This
is the signal for the ribosome to release the polypeptide chain and the
mRNA.
After being released from the tRNA, some proteins may undergo
post-translational modifications. They may be cleaved by a proteolytic
(protein cutting) enzyme at a specific site. Alternatively, they may have
some of their amino acids biochemically modified. After such
modifications, the polypeptide forms into its native shape and starts
acting as a functional protein in the cell.
There are four different nucleotides, A, U, G and T. If they are
taken three at a time (to specify a codon, and thus, indirectly specify an
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amino acid), 64 codons could be specified. However, there are only 20
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Translation
World of Genetics , 2007
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different amino acids. Therefore, several triplets code for the same
amino acid; for example UAU and UAC both code for the amino acid
tyrosine. In addition, some codons do not code for amino acids, but
code for polypeptide chain initiation and termination. The genetic code
is non-overlapping, i.e., the nucleotide in one codon is never part of the
adjacent codon. The code also seems to be universal in all living
organisms.
Source Citation
"Translation." World of Genetics. Gale, 2007. Gale Science In Context.
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Web. 12 Dec. 2012.
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Ribosome
World of Biology , March 8, 2010
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Ribosomes are structures that play a key role in the manufacture
of proteins in both prokaryotes (cells that lack a defined nuclear
membrane, and whose genetic material is distributed throughout the
cell) and eukaryotes (in which the genetic material is enclosed within
the nuclear membrane). They are the sites of protein synthesis.
Found throughout the cell, ribosomes are composed of ribosomal
ribonucleic acid (rRNA) and proteins. They are organized in two
functional subunits. One is a small subunit that is reminiscent in shape
to a squashed ball, while another is a large subunit that is more
spherical. The large subunit is about twice as big as the small unit. The
subunits usually exist separately, but join when they are attached to a
messenger RNA (mRNA). This initiates protein synthesis.
Production of a protein begins with initiation. In this step, the
ribosomal small subunit binds to the mRNA along with the first
transfer RNA (tRNA). The next step is elongation, where the ribosome
moves along the mRNA and strings together the amino acids one by
one. Finally, the ribosome encounters a stop sequence and the two
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subunits release the mRNA, the polypeptide chain, and the tRNA.
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Ribosome
World of Biology , March 8, 2010
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Protein synthesis occurs at specific sites within the ribosome. The
P site of a ribosome contains the growing protein chain. The A site
holds the tRNA that has the next amino acid. The two sites are held
close together and a chemical reaction occurs. When the stop signal is
present on the mRNA, protein synthesis halts. The polypeptide chain is
released and the ribosome subunits are returned to the pool of ribosome
units in the cytoplasm.
Ribosomes are found in two locations in the eukaryotic cell. Free
ribosomes are dispersed throughout the cytoplasm. Bound ribosomes
are attached to a membranous structure called the endoplasmic
reticulum. Most cell proteins are made by the free ribosomes. Bound
ribosomes are instrumental in producing proteins that function within
or across the cell membrane. Depending on the cell type, there can be
as many as a few million ribosomes in a single cell.
Since most cells contain a large number of ribosomes, rRNA is
the most abundant type of RNA. rRNA plays an active role in ribosome
function. It interacts with both the mRNA and tRNA and helps
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maintain the necessary structure. Transfer RNA is the molecule that
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Ribosome
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World of Biology , March 8, 2010
Gale Science In Context. Web. 12 Dec. 2012. PERIOD:
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interacts with the mRNA during protein synthesis and is able to read a
three amino acid sequence. On the opposite end of the tRNAs, amino
acids are bonded on a growing polypeptide chain. Generally, it takes
about a minute for a single ribosome to make an average sized protein.
However, several ribosomes can work on a single mRNA at the same
time. This allows the cell to make many copies of a single protein
rapidly. Sometimes these multiple ribosomes, or polysomes, can
become so large that they can be seen with a light microscope.
The ribosomes in eukaryotes and prokaryotes are slightly
different. Eukaryotic ribosomes are generally larger and are made up of
more proteins. Since many diseases are caused by prokaryotes, these
slight differences have important medical implications. Drugs have
been developed that can inhibit the function of a prokaryotic ribosome
but leave the eukaryotic ribosome unaffected. One example is the
antibiotic tetracycline. In October 2007, Israeli biochemist Ada Yonath
and American chemists Venkatraman Ramakrishnan and Thomas Steitz
shared the 2007 Nobel Prize for Chemistry for their articulation of the
molecular structure of ribosomes. The trio's work offered insight into
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how antibiotics such a tetracycline interact with ribosomes.
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