Biology 10.1 How Proteins are Made: How Proteins are Made
Decoding the Information of DNA: • Traits, such as eye color, are determined by proteins that are built according to instructions coded in DNA. • Proteins are not built directly from DNA however. Ribonucleic acidis also involved. • Like DNA, ribonucleic acid is a molecule made of three nucleotides linked together. RNA
How Proteins are Made: • RNAa Blueprint Copy • When an organism needs to use the data stored in the genome, e.g. to build components of a new cell, a copy of the required DNA part is made. • This copy is called RNA and is almost identical to DNA. Just like DNA, RNA is an abbreviated form of a chemical name which in the case of RNA is ribonucleic acid.
How Proteins are Made: • RNA : 3 differences between DNA and RNA • Unlike the double stranded DNA, RNA is only made up of a single strand. • Furthermore, the base T, thymine, is replaced by U, uracil in RNA. • RNA nucleotides also contain the five-carbon sugar ribose rather than the sugar deoxyribose, which is found in DNA nucleotides.
How Proteins are Made: • RNA • This RNA string is used by the organism as a template when it builds protein molecules, sometimes called the building blocks of the body. • For example, your muscles and hair are mostly made up of proteins.
How Proteins are Made: • RNA: Transcription • A gene’s instructions for making a protein are coded in the sequence of nucleotides in the gene. • The instructions for making a protein are transferred from a gene to an RNA molecule in a process calledtranscription. • Cells than use two different types of RNA to read the instructions on the RNA molecule and put together the amino acids in a process called translation.
How Proteins are Made: • RNA : Transcription • The entire process by which proteins are made based on the information encoded in DNA is called gene expression or protein synthesis.
Transfer of Information from DNA to RNA RNA:Transcription • The first step in the making of a protein, transcription, takes the information found in a gene in the DNA and transfers it to a molecule of RNA. • RNA polymerase , an enzyme that adds and links complementary RNA nucleotides during transcription, is required for this process.
Transfer of Information from DNA to RNA RNA:Transcription Steps • Step 1: • Transcription begins when RNA polymerase binds to the gene’s promoter; a specific sequence of DNA that acts as a “start” signal for transcription. • Step 2: • RNA polymerase than unwinds and separates the two strands of the double helix, exposing the DNA nucleotides on each strand.
Transfer of Information from DNA to RNA RNA:Transcription Steps • Step 3: • RNA polymerase adds and than links complementary RNA nucleotides as it “reads” the gene. • RNA polymerase moves along the nucleotides of the DNA strand that has the gene, like a train moves along a track. • Transcription follows the base-pairing rules for DNA replication except that in RNA, uracil, rather than thymine, pairs with adenine.
Transfer of Information from DNA to RNA RNA:Transcription Steps • As transcription proceeds, the RNA polymerase eventually reaches a “stop signal” in the DNA. • The stop signal is a sequence of bases that marks the end of each gene in eukaryotes, or the end of a set of genes in prokaryotes.
Transfer of Information from DNA to RNA RNA:Transcription • When the RNA nucleotides are added during transcription, they are linked together with covalent bonds. • As RNA polymerase moves down the strand, a single strand of RNA grows. • Behind RNA polymerase, the two strands of DNA close up forming hydrogen bonds between complementary bases, reforming the DNA double-helix.
Transfer of Information from DNA to RNA RNA:Transcription • Like DNA replication, transcription uses DNA nucleotides as a template for making a new molecule. • In DNA replication, the new molecule made is DNA. • In RNA transcription, the new molecule made is RNA instead. • In DNA replication, both strands of DNA serve as templates. • In transcription, only part of one of the strands of DNA (gene) serves as a template.
Transfer of Information from DNA to RNA RNA: Transcription: • Transcription in prokaryote cells occurs in the cytoplasm. • Transcription in eukaryote cells occurs in the nucleus, where the DNA is located. • During transcription, many identical RNA molecules are made simultaneously from a single gene.
The Genetic Code RNA: The Genetic Code: Three-Nucleotide “Words” • Different types of RNA are made during transcription, depending on the gene being expressed. When a cell needs a particular protein, it is messenger RNA that is made. • Messenger RNA (mRNA) is a form of RNA that carries the instructions for making a protein from a gene and delivers it to the site of translation. • The information is translated from the language of RNA, nucleotides, to the language of proteins, amino acids.
The Genetic Code RNA: The Genetic Code: Three-Nucleotide “Words” • The RNA instructions are written as a series of three-nucleotide sequences on the mRNA called codons. • Each codon along the mRNA strand corresponds to an amino acid or signifies a start or stop signal for translation.
The Genetic Code RNA: The Genetic Code: Three-Nucleotide “Words” • In 1961, an American biochemist Marshall Nirenberg, deciphered the first codon by making artificial mRNA that contained only the base uracil. • The mRNA was translated into a protein made up entirely of phenylalanine amino-acids subunits.
The Genetic Code RNA: The Genetic Code: Three-Nucleotide “Words” • Nirenberg concluded that the codon UUU is the instruction for the amino acid phenylalanine. Later, scientists deciphered the other codons.
RNAs Role in Translation: RNA: The Genetic Code: Three-Nucleotide “Words”: • The complete list of codons, in their groups of threes, makes up the genetic code deciphered by scientists that provides the instructions for all the amino acids and the “start” and “stop” signals that are coded by each of the 64 mRNA possible combinations.
RNAs Role in Translation: • Translation takes place in the cytoplasm. Here transfer RNA molecules and ribosomes help in the synthesis of proteins. • Transfer RNA (tRNA) molecules are single strands of RNA that temporarily carry a specific amino acid on one end. • Each tRNA is folded into a compact shape and has an anticodon . An anticodonis a three-nucleotide sequence on a tRNA that is complementary to an mRNA codon.
RNAs Role in Translation: • Ribosomes are composed of both proteins and ribosomal RNA (rRNA). Ribosomal RNA molecules are RNA molecules that are part of the structure of ribosomes. • A cell’s cytoplasm contains thousands of ribosomes. Each ribosome temporarily holds one mRNA and two tRNA molecules.
RNAs Role in Translation: • Translation is the process of synthesis of a protein by ribosomes, using mRNA as a template. • The genetic message inmRNA is 'read' by organelles called ribosomesin order to make a particular protein. tRNA is also required for this process. • tRNAs are specific for one particular amino acid and each tRNA carries required amino acids to the ribosome in order to synthesize the polypeptide chain.
RNAs Role in Translation: • The ribosome 'reads' the mRNA language in the 5' to 3' direction. • Each codon (sets of three nucleotide bases) specifies one amino acid from which proteins are made.
RNAs Role in Translation: • So, the mRNA language indicates the sequence of amino acids for the synthesis of a protein. • The mRNA language begins with the codon AUG (initiation codon, which starts making a protein chain) and ends with UAA, UAG or UGA (stop codons also called terminators of a protein chain).
As each codon is 'read', the amino acids are carried to the site of formation of the polypeptide chain by the particular tRNA. • Each tRNA has an anticodon that are opposite to the particular codon on the mRNA e.g. if the mRNA codon is AGG then the matching tRNA anticodon is UCC.
Once a amino acid is bound to the forming polypeptide chain the next codon is read by the ribosome. • The sequence of reading the mRNA and adding an amino acid continues until the 'stop' sequence (codon) is recognized.
As the mRNA moves across the ribosome, another ribosome can find the AUG codon on the same mRNA and begin making a second copy of the same protein. • In this way many copies of the same protein are made from a single mRNA molecule. • With few exceptions, the genetic code is the same in all organisms. For this reason, the genetic code is often described as being nearly universal.