Protein Synthesis

Replication A single DNA strand can serve as a template, or pattern, for a new strand. This is the principle of semi-conservation

Replication copies genetic information. Suppose everyone took off their shoes and placed their left shoe in a line, and the right was tossed into a pile. You could easily match the right shoe with the left. Similarly, a new strand of DNA can be synthesized when the other strand is a template to guide the process. The order of the bases is preserved, and DNA can be accurately replicated over and over again.

Proteins carry out the process of replication. Enzymes and other proteins do the actual work of replication. An enzyme unzips the DNA An enzyme holds the DNA apart during replication DNA polymerase bonds new nucleotides to the “opened” DNA strand.

The Replication Process 1. Enzymes begin to unzip the double helix at numerous places along the chromosome. 2. Free-floating nucleotides pair, one by one, with the bases on the template strand as they are exposed. DNA polymerase bonds the nucleotides together. 3. Two identical molecules of DNA result Each new molecule has one strand from the original molecule and one new strand. Semi-conservative

Replication is fast and accurate. About 50 nucleotides are added every second to a new strand of DNA. The process takes just a few hours. DNA polymerase can detect errors in the strand and correct them. Errors are limited to about one error per 1 billion nucleotides.

Central Dogma Central Dogma Information flows from DNA to RNA to proteins Replication copies DNA Transcription converts a DNA message into an intermediate molecule called RNA Translation interprets an RNA message into a string of amino acids, called a polypeptide. A single polypeptide or many polypeptides working together make a protein.

Transcription When DNA converts into an intermediate molecule called RNA. RNA is necessary to make proteins.

Transcription Replication and transcription occur in the nucleus. Translation will occur in the cytoplasm Transcription is the process of copying a sequence of DNA to produce a complimentary strand of RNA It is catalyzed by RNA polymerase. Bonds nucleotides together in a chain to make a new RNA molecule.

Transcription 1. RNA polymerases recognize the transcription “start” site of a gene. RNA polymerase begins to unwind a segment of DNA. 2. RNA polymerase, using one strand of DNA as a template, strings together a complimentary strand of RNA nucleotides. The growing RNA strand hangs freely as it is transcribed, and the DNA strand zips back together. 3. Once the entire gene has been transcribed, the RNA strand detaches completely from the DNA.

RNA Carries DNA’s Instructions RNA acts as an intermediate link between DNA in the nucleus and protein synthesis in the cytoplasm. RNA is like a temporary copy of DNA that is used and then destroyed

Transcription makes three types of RNA Messenger RNA (mRNA) Intermediate message that is translated into a protein. Ribosomal RNA (rRNA) Forms part of ribosomes, a cell’s protein factory. Transfer RNA (tRNA) Brings amino acids from the cytoplasm to a ribosome to help make a growing protein.

The transcription process is cool! Transcription enables a cell to adjust to changing demands. Say you are trapped by an ANGRY bear! Your body needs adrenaline (a hormone/protein) to run FAST! Transcription allows your body to make a lot of adrenaline very quickly 

Translation Translation is the process that converts, or translates, an mRNA message into a polypeptide. One or more polypeptides make a protein. RNA uses adenine, uracil, guanine, cytosine to code for 20 amino acids

Triplet Code In the genetic code, all “words” are made up of three letters (nucleotides). A codon is a three-nucleotide sequence that codes for an amino acid.

Amino Acids Amino acids are bonded by peptide bonds to create polypeptides and then proteins. Many amino acids are coded for by more than one codon. For example: Leucine is represented by six codons: CUU, CUC, CUA, CUG, UUA, and UUG In most cases, codons that represent the same amino acid share the same first two nucleotides. Having many codons represent a single amino acid makes DNA more tolerant of point mutations.

Start and Stop Codons Start Codon Stop Codon Signals the start of translation Is the amino acid methionine (AUG) Translation always begins with AUG… Stop Codon Signals the end of the amino acid chain Coded for by UAA, UAG, or UGA

The Reading Frame For the mRNA code to be translated correctly, codons must be read in the right order. This order is called the reading frame Changing the reading frame completely changes the resulting protein. Therefore, it is very important for the mRNA to have a clear START and STOP codon.

Common Language The genetic code is shared by almost all organisms (even viruses) It is considered to be a universal code That means that the codon UUU codes for phenylalanine in an armadillo, a cactus, a yeast, or a human. This suggests that all organisms arose from a common ancestor. This also means that scientists can insert a gene from one organism into another organism to make a functional protein

Ribosomes To translate codons into a physical amino acid, a ribosome and tRNA are used. Ribosomes are a combination of rRNA and proteins. They catalyze the reaction that forms bonds between amino acids. They are the site of protein synthesis.

Ribosomes Ribosomes have a large and small subunit that fit together and pull the mRNA strand through. The small subunit holds onto the mRNA strand. The large subunit holds onto the growing protein.

Transfer RNA (tRNA) tRNA acts as a sort of adaptor between mRNA and amino acids. It carries free floating amino acids from the cytoplasm to the ribosome. The tRNA is shaped like a plus sign One end of the plus sign is attached to a specific amino acid. The bottom has an anticodon that recognizes a specific codon on the mRNA strand Ex. The anticodon CCC pairs with the mRNA codon GGG.

Amino Acids are linked to become a protein 1. The ribosome attaches to an mRNA molecule and exposes one codon. 2. The exposed codon attracts a complimentary tRNA molecule bearing an amino acid. The tRNA anticodon pairs with the mRNA codon.

Amino Acids are linked to become a protein 3. The ribosome helps form a peptide bond between the two amino acids. The ribosome then breaks the bond between the tRNA molecule and its amino acid. 4. The ribosome pulls the mRNA strand the length of one codon. The tRNA disengages from the mRNA (leaving its amino acid bonded with the mRNA) to pick up a new amino acid. 5. The ribosome continues to move down the mRNA strand, attaching new amino acids to the growing protein, until it reaches a stop codon.

How to make a protein… P Site A Site Ribosomes A U G C Large subunit mRNA A U G C Small subunit

How to make a protein… G aa2 A U U A C aa1 A U G C U A C U U C G A 2-tRNA G aa2 A U 1-tRNA U A C aa1 anticodon A U G C U A C U U C G A hydrogen bonds codon mRNA

How to make a protein… G A aa3 peptide bond aa1 aa2 U A C G A U A U G 3-tRNA G A aa3 peptide bond aa1 aa2 1-tRNA 2-tRNA anticodon U A C G A U A U G C U A C U U C G A hydrogen bonds codon mRNA

Ribosomes move over one codon aa1 peptide bond 3-tRNA G A aa3 aa2 1-tRNA U A C (leaves) 2-tRNA G A U A U G C U A C U U C G A mRNA Ribosomes move over one codon

peptide bonds G C U aa4 aa1 aa2 aa3 G A U G A A A U G C U A C U U C G 4-tRNA G C U aa4 aa1 aa2 aa3 2-tRNA 3-tRNA G A U G A A A U G C U A C U U C G A A C U mRNA

Ribosomes move over one codon peptide bonds 4-tRNA G C U aa4 aa1 aa2 aa3 2-tRNA G A U (leaves) 3-tRNA G A A A U G C U A C U U C G A A C U mRNA Ribosomes move over one codon

peptide bonds U G A aa5 aa1 aa2 aa4 aa3 G A A G C U G C U A C U U C G 5-tRNA aa5 aa1 aa2 aa4 aa3 3-tRNA 4-tRNA G A A G C U G C U A C U U C G A A C U mRNA

Ribosomes move over one codon peptide bonds U G A 5-tRNA aa5 aa1 aa2 aa3 aa4 3-tRNA G A A 4-tRNA G C U G C U A C U U C G A A C U mRNA Ribosomes move over one codon

Termination aa5 aa4 aa3 primary structure of a protein aa2 aa1 A C U C terminator or stop codon 200-tRNA A C U C A U G U U U A G mRNA

Transcription to Translation

End Product –The Protein! The end product of protein synthesis is a primary structure of a protein A sequence of amino acids bonded together by peptide bonds aa1 aa2 aa3 aa4 aa5 aa200 aa199

Gene Expression and Regulation Depending on an organisms needs, a gene can make a lot of protein, a little protein, or none at all.