Before a cell can divide, the DNA in the nucleus of the cell must be duplicated. Since the DNA molecule consists of two complimentary stands, if those two strands separate and the right conditions are present, two new stands that are the compliments of the originals will be produced. Each new DNA molecule will consist of one old stand, and a new complimentary strand. Replication(not part of transcription/translation) • The gray strands in the figure are new strands in the process of being assembled.
Assembling the New Bases • The term semiconservative replication means that in the new DNA molecule there is one old and one new strand. • This is seen in the figure below.
DNA Replication Since the DNA molecule is very large, there must be a way to copy it faster than just unwinding from one end to the other! This happens when the DNA molecule separates at many sites, forming thousands of replication bubbles. This allows parts of the DNA message to be replicated simultaneously in many locations. DNA polymerase adds new nucleotides , while DNA ligase joins the DNA segments together.
Why the fuss about DNA replication?? • The process of DNA replication involves a number of enzymes and proteins, and it a bit more complicated than seen in the previous slide. • The important idea is that an exact duplication of the DNA message is required, so that each new cell in the body has the same set of genetic instructions as the cells that preceded it. • This also insures that every new generation of individuals has the same genetic information as his/her parents.
TRANSCRIPTION & TRANSLATION(Protein Synthesis)DNA carries information that can be used to construct the proteins which form structures and regulate the body’s activities. • Protein synthesis involves two processes: transcription and translation. • In transcription the DNA message is converted into an RNA molecule. • In translation the RNA message is used to assemble amino acids into a protein chain.
The genetic code • The genetic code is written in the sequence of the 4 bases of DNA: A, T, C, and G. • Three bases read in sequence specify one of the 20 amino acids found in protein molecules. • A codon is the 3-base sequence for an amino acid. • The message in the DNA is transcribed into an RNA molecule, and then translated into a polypeptide
There are 64 (4X4X4) possible triplet codes, but only 20 amino acids. As seen in the table, more than 1 triplet may code for the same amino acid. This is no problem, as long as no triplet can code for more than one amino acid. Note that several codons can also act as start (AUG) or stop (UAA) signals. The Genetic Code II
Why do we need RNA too? • There are three types of RNA produced in the nucleus: mRNA, tRNA, rRNA. • Messenger RNA (mRNA) copies the DNA that codes for a polypeptide. - transcription • The mRNA continues until it reaches a termination (STOP) signal (a sequence of bases in the DNA strand). At this time the RNA molecule is released from the DNA, allowing the DNA strands to reunite. • After production the RNA molecules leave the nucleus and enter the cytoplasm. (with the code or codons)
You must know your base pairs!! • There must be a different tRNA molecule for each of the possible triplets. This means 64 anticodons. The anticodons of the tRNAs each have a complimentary codon in the mRNA. For example the codon AUG would be the compliment of the anticodon UAC.
A second type of RNA is transfer RNA (tRNA), whose function is to attach to a specific amino acid and bring that amino acids to the site where polypeptides are being constructed. One end of the molecule attached to a specific amino acid. The other end has an exposed sequence of 3-bases. These are called the anticodon. How many kinds of tRNA must there be? TRANSLATION
The third type of RNA is ribosomal RNA (rRNA). Ribosomes are the ‘decoding’ units of the cell. Each ribosome consists of two subunits Ribosomes have binding sites for both tRNA and mRNA molecules. The role of Ribosomes
Building Site Foreman (ribosome)
mRNA molecule attaches to a ribosome. As the ribosome moves along the mRNA, 3-base codons are exposed one at a time. A tRNA with an anticodon that is complimentary to the codon of the mRNA temporarily bonds with the mRNA. As the ribosome continues its journey along the mRNA additional tRNAs bring their a.a. to the site of peptide synthesis. Reading the Message
TRANSFER RNA The Anti-Codons
As new amino acids are brought to the ribosome, the growing peptide chain is attached to the new amino acid by a peptide bond. The chain continues until a stop codon is encountered. Once a tRNA gives up its amino acid it can return to the cytoplasm and attach to another of its specified amino acid. Elongation of the chain
Information is stored in the triplet codes (codons) of DNA nucleotides. This information is transcribed into 3 types of RNA. mRNA carries the information to assemble a polypeptide. In the nucleus, introns are removed and the remaining exons spliced together to make a functional mRNA strand. tRNA molecules attach to specific amino acids. rRNA and proteins form ribosomes. mRNA attaches to a ribosome and the message is decoded when the anticodon of a tRNA is bonded to a mRNA codon. Subsequent amino acids are attached to the growing peptide chain until a stop codon is reach and the chain is terminated. A summary of these events can be seen in the next slide. A Summary of the flow of Genetic Information in a Cell
Mutation: When the Code is Miscopied • A mutation occurs when the code doesn’t copy correctly, and a protein is formed that doesn’t function. • If a base is substituted or deleted, the triplet(s) are different and so is the protein formed. • Mutations can also involved inversion or deletion of larger sections of the message. • Substances that trigger mutations are called mutagens and can be physical or chemical in nature.