replication, transcription, & translation
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Replication, Transcription, & Translation. Replication, Transcription, & Translation. DNA Replication. DNA Replication. Before a cell can divide by mitosis or meiosis , it must first make a copy of its chromosomes . - PowerPoint PPT PresentationTRANSCRIPT
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Replication,Transcription, & Translation
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DNA Replication
• Before a cell can divide by mitosis or meiosis, it must first make a copy of its chromosomes.
• The DNA in the chromosomes is copied in a process called DNA replication.
• Without DNA replication, new cells would have only half the DNA of their parents.
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DNA Replication
• The blue strands represent the original DNA strands.
• The red strands are the new, synthesized strands.
DNA
Replication
Replication
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DNA Replication
• Click the image to view a video clip.
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Copying DNA
• DNA is copied during interphase prior to mitosis and meiosis.
• It is important that the new copies are exactly like the original molecules.
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Original DNA
Original DNA
Strand
Original DNA
Strand
Free Nucleotides New DNA
moleculeNew DNA
Strand
New DNA molecule
Copying DNA
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Genes and Proteins
• The sequence of nucleotides in DNA contain information that is put to work through the production of proteins.
• Proteins fold into complex, three-dimensional shapes to become key cell structures and regulators of cell functions.
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Genes and Proteins
• Some proteins become important structures, such as the filaments in muscle tissue.
• Other proteins, such as enzymes, control chemical reactions that perform key life functions—breaking down glucose molecules in cellular respiration, digesting food, or making spindle fibers during mitosis.
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Genes and Proteins
• In fact, enzymes control all the chemical reactions of an organism.
• Thus, by encoding the instructions for making proteins, DNA controls cells.
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Genes and Proteins
• Remember, proteins are polymers of amino acids.
• The sequence of nucleotides in each gene contains information for assembling the string of amino acids that make up a single protein.
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RNA
• RNA like DNA, is a nucleic acid. RNA structure differs from DNA structure in three ways.
• First, RNA is single stranded—it looks like one-half of a zipper—whereas DNA is double stranded.
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RNA
• The sugar in RNA is ribose; DNA’s sugar is deoxyribose.
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RNA
• Both DNA and RNA contain four nitrogenous bases, but rather than thymine, RNA contains a similar base called uracil (U).
• Uracil forms a base pair with adenine in RNA, just as thymine does in DNA.
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RNA
• DNA provides workers with the instructions for making the proteins, and workers build the proteins.
• The workers for protein synthesis are RNA molecules.
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RNA
• RNA takes the instructions from DNA (telling how the protein should be assembled), then—amino acid by amino acid—RNA assembles the protein.
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RNA
• There are three types of RNA that help build proteins.
• Messenger RNA (mRNA), brings instructions from DNA in the nucleus to the cell’s factory floor, the cytoplasm.
• On the factory floor, mRNA moves to the assembly line, a ribosome.
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RNA
• The ribosome, made of ribosomal RNA (rRNA), binds to the mRNA and uses the instructions to assemble the amino acids in the correct order.
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RNA
• Transfer RNA (tRNA) is the supplier. tRNA delivers amino acids to the ribosome to be assembled into a protein.
Click image to view movie
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Transcription
• In the nucleus, enzymes make an RNA copy of a portion of a DNA strand in a process called transcription.
Click image to view movie
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Transcription
DNA strand
RNA strand
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Transcription
• The main difference between transcription and DNA replication is that transcription results in the formation of one single-stranded RNA molecule rather than a double-stranded DNA molecule.
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The Genetic Code
• The nucleotide sequence transcribed from DNA to a strand of mRNA acts as a genetic message, the complete information for the building of a protein.
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The Genetic Code
• Biochemists began to crack the genetic code when they discovered that a group of three nitrogenous bases in mRNA code for one amino acid. Each group is known as a codon.
• 64 combinations are possible when a sequence of three bases is used; thus, 64 different mRNA codons are in the genetic code.
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The Genetic Code
• Some codons do not code for amino acids; they provide instructions for making the protein.
• More than one codon can code for the same amino acid.
• However, for any one codon, there can be only one amino acid.
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The Genetic Code
• All organisms use the same genetic code.
• This provides evidence that all life on Earth evolved from a common origin.
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Translation
• The process of converting the information in a sequence of nitrogenous bases in mRNA into a sequence of amino acids in protein is known as translation.
• Translation takes place at the ribosomes in the cytoplasm.
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Translation
• In prokaryotic cells, which have no nucleus, the mRNA is made in the cytoplasm.
• In eukaryotic cells, mRNA is made in the nucleus and travels to the cytoplasm.
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Translation
• In cytoplasm, a ribosome attaches to the strand of mRNA like a clothespin clamped onto a clothesline.
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The Role of tRNA
• For proteins to be built, the 20 different amino acids dissolved in the cytoplasm must be brought to the ribosomes.
• This is the role of transfer RNA (tRNA).
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The Role of tRNA
• Each tRNA molecule attaches to only one type of amino acid.
Amino acid
Chain of RNA
nucleotides
Transfer RNA
molecule
Anticodon
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The Role of tRNA
• As translation begins, a ribosome attaches to the starting end of the mRNA strand. Then, tRNA molecules, each carrying a specific amino acid, approach the ribosome.
• When a tRNA anticodon pairs with the first mRNA codon, the two molecules temporarily join together.
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The Role of tRNA
Ribosome
mRNA codon
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The Role of tRNA
• Usually, the first codon on mRNA is AUG, which codes for the amino acid methionine.
• AUG signals the start of protein synthesis.
• When this signal is given, the ribosome slides along the mRNA to the next codon.
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The Role of tRNA
tRNA anticodon
Methionine
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The Role of tRNA
Alanine
• A new tRNA molecule carrying an amino acid pairs with the second mRNA codon.
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The Role of tRNA
• The amino acids are joined when a peptide bond is formed between them.
AlanineMethionine
Peptide bond
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The Role of tRNA
• A chain of amino acids is formed until the stop codon is reached on the mRNA strand.
Stop codon
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Summary
• Replication = copying DNA
• Transcription = using DNA to make RNA
• Translation = using RNA to make protein