12 Flashcards
(216 cards)
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Transcription and Translation
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➢ Genes provide information for building proteins. They don’t however directly
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create proteins. The production of proteins is completed through two processes:
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transcription and translation.
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➢ Transcription and translation take the information in DNA and use it to produce
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proteins. Transcription uses a strand of DNA as a template to build a molecule
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called RNA.
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➢ The RNA molecule is the link between DNA and the production of
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proteins. During translation, the RNA molecule created in the transcription
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process delivers information from the DNA to the protein-building machines.
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➢ Each strand of the DNA double helix contains a sequence of nucleotides that is
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exactly complementary to the nucleotide sequence of its partner strand. Each strand
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can therefore act as a template, or mold, for the synthesis of a new complementary
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strand (Figure 6–2). In other words, if we designate the two DNA strands as S and
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S, strand S can serve as a template for making a new strand S, while strand S` can
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serve as a template for making a new strand S (Figure 6–3).
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➢ Thus, the genetic information in DNA can be accurately copied by the beautifully
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simple process in which strand S separates from strand S`, and each separated strand
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then serves as a template for the production of a new complementary partner strand
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that is identical to its former partner.
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➢ DNA replication produces two complete double helices from the original DNA
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molecule, each new DNA helix identical (except for rare copying errors) in nucleotide
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sequence to the parental DNA double helix.
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Figure 6–3 DNA acts as a template for its own duplication. Because
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Figure 6–2 A DNA strand can serve as a the nucleotide A will successfully pair only with T, and G with C, each
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template. Preferential binding occurs between strand of DNA in the double helix—labeled here as the S strand and its
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pairs of nucleotides (A with T, and G with C) that complementary S’ strand—can serve as a template to specify the
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can form base pairs. This enables each strand to sequence of nucleotides in its complementary strand. In this way,
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act as a template for forming its complementary double-helical DNA can be copied precisely. Keep in mind that although
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strand. they are colored differently here, the template strands (orange) and the
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new strands (red) are chemically identical.
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➢ The DNA double helix is normally very stable: the two DNA strands are locked
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together firmly by the hydrogen bonds formed between the bases on each strand. To
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begin DNA replication, the double helix must first be opened up and the two strands
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separated to expose unpaired bases. As we shall see, the process of DNA replication is
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begun by special initiator proteins that bind to double-stranded DNA and pry the two
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strands apart, breaking the hydrogen bonds between the bases.
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➢ The positions at which the DNA helix is first opened are called replication origins. In
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simple cells like those of bacteria or yeast, origins are specified by DNA sequences
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several hundred nucleotide pairs in length. This DNA contains both short sequences
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that attract initiator proteins and stretches of DNA that are especially easy to open.
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There is an A-T base pair held together by fewer hydrogen bonds than is a G-C base
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pair. Therefore, DNA rich in A-T base pairs is relatively easy to pull apart, and
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regions of DNA enriched in A-T base pairs are typically found at replication origins.
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Stages of DNA replication
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DNA replication can be thought of in three stages: initiation, elongation and termination
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1) Initiation
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➢ DNA synthesis is initiated at particular points within the DNA strand known as ‘origins’,
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which have specific coding regions. These origins are targeted by initiator proteins, which go
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on to recruit more proteins that help aid the replication process, forming a replication complex
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around the DNA origin. Multiple origin sites exist within the DNA’s structure; when replication
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of DNA begins, these sites are referred to as replication forks.
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➢ Within the replication complex is the DNA helicase. This enzyme unwinds the double helix
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and exposes each of the two strands so that they can be used as a template for replication. It
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does this by hydrolyzing the ATP used to form the bonds between the nucleobases, thereby
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breaking the bond holding the two strands together.
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➢ DNA primase is another enzyme that is important in DNA replication. It synthesizes a
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small RNA primer, which acts as a ‘kick-starter’ for DNA polymerase. This enzyme is
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ultimately responsible for the creation and expansion of new strands of DNA.
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Stages of DNA replication
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2) Elongation
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➢ Once DNA Polymerase has attached to the two unzipped strands of DNA (i.e.
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the template strands), it is able to start synthesizing new strands of DNA to match the
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templates. DNA polymerase is only able to extend the primer by adding free nucleotides to
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the 3’ end.
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➢ One of the template strands is read in a 3’ to 5’ direction, therefore the new strand will be
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formed in a 5’ to 3’ direction. This newly formed strand is referred to as the leading
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strand. Along the leading strand, DNA primase only needs to synthesize an RNA primer once,
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at the beginning, to initiate DNA polymerase. This is because DNA polymerase is able to
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extend the new DNA strand by reading the template 3′ to 5′, synthesizing in a 5′ to 3′ direction
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as noted above.
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➢ However, the other template strand (the lagging strand) is antiparallel and is therefore read in
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a 5’ to 3’ direction. Continuous DNA synthesis, as in the leading strand, would need to be in
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the 3′ to 5′ direction, which is impossible as DNA polymerase cannot add bases to the 5′ end.
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Instead, as the helix unwinds, RNA primers are added to the newly exposed bases on
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the lagging strand and DNA synthesis occurs in fragments, but still in the 5′ to 3′ direction as
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before.
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1/16/2025 These fragments are known as Okazaki fragments. 12
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Stages of DNA replication
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3) Termination
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➢ The process of expanding the new DNA strands continues until there is either no more DNA
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template strand left to replicate (i.e. at the end of the chromosome) or two replication forks
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meet and subsequently terminate. The meeting of two replication forks is not regulated and
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happens randomly along the course of the chromosome.
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➢ Once DNA synthesis has finished, the newly synthesized strands are bound and stabilized. For
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the lagging strand, two enzymes are needed to achieve this stabilization: RNAase H removes
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the RNA primer at the beginning of each Okazaki fragment, and DNA ligase joins these
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fragments together to create one complete strand.
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RNA polymerase is the enzyme responsible for making mRNA copies of genes
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Figure 7–6 Transcription produces
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an RNA complementary to one
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strand of DNA. The nontemplate
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strand of the DNA (the top strand in
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this example) is sometimes called the
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coding strand because its sequence is
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equivalent to the RNA product
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Translation is the process by which the genetic code contained within a messenger RNA (mRNA)
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molecule is decoded to produce a specific sequence of amino acids in a polypeptide chain.
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➢ It occurs in the cytoplasm following DNA transcription and, like transcription, has three stages:
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initiation, elongation, and termination. In this article, we will discuss the components and stages of
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DNA translation.
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Components of Translation
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➢ The key components required for translation are mRNA, ribosomes, and transfer RNA (tRNA).
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➢ During translation, mRNA nucleotide bases are read as codons of three bases. Each codon codes for
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a particular amino acid.
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➢ Every tRNA molecule possesses an anticodon that is complementary to the mRNA codon, and at the
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opposite end lies the attached amino acid. tRNA molecules are therefore responsible for bringing
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amino acids to the ribosome in the correct order, ready for polypeptide assembly. Page 247
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➢ A single amino acid may be coded for by more than one codon. There are also specific codons that
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signal the start and the end of translation.
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➢ Aminoacyl-tRNA synthetases are enzymes that link amino acids to their corresponding tRNA
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molecules. The resulting complex is charged and is referred to as an aminoacyl-tRNA.
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1) Initiation
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➢ For translation to begin, the start codon (5’AUG) must be recognized. This codon is
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specific to the amino acid methionine, which is nearly always the first amino acid in a
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polypeptide chain.
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➢ At the 5’ cap of mRNA, the small 40s subunit of the ribosome binds. Subsequently, the
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larger 60s subunit binds to complete the initiation complex. The next step (elongation) can
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now commence.
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2) Elongation
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➢ The ribosome has two tRNA binding sites; the P site which holds the peptide chain and the
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A site which accepts the tRNA.
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➢ While Methionine-tRNA occupies the P site, the aminoacyl-tRNA that is complementary to
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the next codon binds to the A site, using energy yielded from the hydrolysis of GTP.
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➢ Methionine moves from the P site to the A site to bond to a new amino acid there, starting
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the growth of the peptide. The tRNA molecule in the P site no longer has an attached amino
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acid, so leaves the ribosome.
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➢ The ribosome then translocates along the mRNA molecule to the next codon, again using
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energy yielded from the hydrolysis of GTP. Now, the growing peptide lies at the P site and
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the A site is open for the binding of the next aminoacyl-tRNA, and the cycle continues.
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➢ The polypeptide chain is built up in the direction from the N terminal (methionine) to the C
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terminal (the final amino acid).
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3) Termination
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➢ One of the three stop codons enters the A site. No tRNA molecules bind to these codons, so
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the peptide and tRNA in the P site become hydrolyzed, therefore releasing the polypeptide
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into the cytoplasm. The small and large subunits of the ribosome then dissociate, ready for
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the next round of translation.
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Figure 7–24 The nucleotide sequence of an mRNA is translated into the amino acid sequence of a protein via the
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genetic code. All the three-nucleotide codons that specify a given amino acid are listed above that amino acid, which is given
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in both its three-letter and one-letter abbreviations (see Panel 2–5, pp. 72–73, for the full name of each amino acid and its
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structure). By convention, codons are always written with the 5`-terminal nucleotide to the left. Note that most amino acids
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are represented by more than one codon, and that there are some regularities in the set of codons that specify each amino acid.
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Codons for the same amino acid tend to contain the same nucleotides at the first and second positions and to vary at the third
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position. There are three codons that do not specify any amino acid but act as termination sites (stop codons), signaling the
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end of the protein-coding sequence. One codon—AUG—acts both as an initiation codon, signaling the start of a protein-
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coding message, and as the codon that specifies methionine.
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Figure 7–23 Procaryotes and eucaryotes
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handle their RNA transcripts differently. (A)
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In eucaryotic cells, the initial RNA molecule
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produced by transcription contains both intron
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and exon sequences. Its two ends are modified,
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and the introns are removed by an enzymatically
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catalyzed RNA splicing reaction. The resulting
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mRNA is then transported from the nucleus to
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the cytoplasm, where it is translated into protein.
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Although these steps are depicted as occurring
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one at a time, in a sequence, in reality they occur
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simultaneously. For example, the RNA cap is
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usually added and splicing often begins before
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the transcript has been completed. Because of
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this coupling, transcripts of the entire gene
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(including all introns and exons) do not typically
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exist in the cell.
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Main Difference between Codon and Anticodon
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➢ Codon and anticodon are nucleotide triplets which specify a particular amino acid in a polypeptide.
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A specific rule set exists for the storage of genetic information as a nucleotide sequence either
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on DNA or mRNA molecules in order to synthesize proteins. That specific rule set is referred to as the
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genetic code. Codon is a group of three nucleotides, especially on the mRNA. Anticodon is present on
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tRNA molecules.
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➢ The main difference between codon and anticodon is that codon is the language which represents an
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amino acid on mRNA molecules whereas anticodon is the complement nucleotide sequence of the
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codon on tRNA molecules.
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Codon is a sequence of three nucleotides which specifies one amino acid in the polypeptide chain. Every
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gene that encodes a specific protein consists of a sequence of nucleotides, which represent the amino acid
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sequence of that particular protein. Genes utilize a universal language, the genetic code, in order to store
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the amino acid sequences of proteins. Genetic code consists of nucleotide triplets which are called codons.
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For example, the codon TCT represents the amino acid serine. Sixty-one codons can be identified in order
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to specify the twenty essential amino acids required by the translation.
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Anticodon
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➢ The three-nucleotide sequence on the tRNA, which is complementary to the codon
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sequence on the mRNA is referred to as the anticodon.
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➢ During translation, anticodon is complementary base paired with the codon via
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hydrogen bonding.
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➢ Therefore, each codon contains a matching anticodon on distinct tRNA molecules.
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The complementary base pairing of anticodon with its codon is shown in figure 4.
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Codon Anticodon
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Location Codon is located on the mRNA molecule Anticodon is located on the tRNA molecule
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Complementary Codon is complementary to the nucleotide Anticodon is complementary to the codon
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Nature triplet in the DNA
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Continuity Codon is sequentially present on the Anticodon is individually present on tRNAs
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mRNA
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Function Codon determines the position of the Anticodon brings the specified amino acid by
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amino acid the codon
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