Genetics classwork Watch and summarize the following animations: Make sure to mention the key steps, enzymes or ideas per animation: 1. DNA replication overview Summary DNA polymerase adds nucleic acids and does not start replication from scratch. In DNA replication, cells use an enzyme called primase to synthesize a short piece of RNA called and RNA primer. The primer provides a 3’ end on which DNA polymerase can act. DNA polymerase catalyzes the addition of bases to the 3’ end of the growing DNA molecule. This reaction is powered by the high-energy bonds of the nucleotide triphosphates. Hydrogen bonding between the parental strand and nucleotide triphosphates dictate which base is added. Adenine – Thymine, and Guanine – Cytosine. The enzyme RNAse removes RNA primers, leaving a gap in the DNA which can be filled by DNA polymerase. DNA ligase seals the gaps between the DNA fragments created during synthesis of the lagging strand. 2. Transcription: the process Summary Occurs in three stages: initiation, elongation, and termination. Initiation: During this stage, RNA polymerase binds to a DNA and recognizes a site known as the promotor. When RNA binds to a promoter, it breaks the hydrogen bond holding the strands of the DNA together at the site of the promoter and transcription begins. RNA polymerase does not bind to all promoters with same affinity which helps control gene expression. Elongation: During elongation, RNA polymerase moves along the DNA opening a bubble in the DNA. As it moves, RNA polymerase adds bases to the 3’ end of the growing RNA transcript. Termination: Elongation continues until RNA polymerase reaches a site known as the terminator. At this site, RNA polymerase and the newly synthesized RNA transcript are released from the DNA. Termination can occur due to self-termination or enzyme-dependent termination. In self-termination, the RNA sequence transcribed at the terminator causes the RNA to hydrogen bond with itself, forming a stem-loop structure which pulls the RNA off the DNA. In enzymedependent termination, a termination protein binds to the terminator and pushes the RNA off the DNA. 3. Translation: the process Summary Like transcription, translation occurs in three stages: initiation, elongation, and termination. Initiation: During initiation, the two ribosomal subunits, a tRNA with an anticodon complementary to start codon, and several protein factors assemble on the mRNA at the start codon. This results in an initiation complex ready for translation. The codon AUG is the start codon, which tells the ribosome where to start reading the message contained in the messenger RNA. AUG also codes for methionine, and for formyl methionine at the start of the bacterial transcription. The ribosome contains three binding sites for tRNA molecule: Amino acid site(A), Polypeptide site(P), and the Exit site(E). Transfer RNA with attached amino acids are called charged tRNA molecules which carry amino acids to the ribosome. The anticodon on each tRNA is complementary to the corresponding codon on the mRNA, which allows for translation from genetic code to amino acid. Elongation: During this stage, the tRNA molecule with an anticodon corresponding to the next codon enters the A site. The ribosome catalyzes the transfer of the amino acid or polypeptide on the tRNA in the P site to the amino acid on the tRNA on the A site forming a peptide bond. After the formation of the peptide bond, the ribosome moves along the mRNA to the next codon. This shifts the tRNA containing the amino acid chain to the P site, which was vacated by the initiator tRNA, which has moved into the E site. Termination: During this process, the ribosome encounters a stop codon, and a protein called the release factor binds on the A site halting elongation. The ribosome separates into its two subunits, and the mRNA and newly synthesized polypeptide are released. 4. Operons: induction Summary The genes of inducing operons are usually not transcribed. The operon is induced to produce proteins when lactose is available to produce. It acts by turning off the repressor not by actively stimulating the gene to be transcribed. 5. Operons: repression Summary A repressible operon is one that is usually on, but which can be repressed in the presence of a repressor molecule. The repressor binds to the operator in such a way that the movement or binding of RNA polymerase is blocked, and transcription cannot proceed. An inducible operon is one that is usually off. 6. Transposons: overview Summary Transposons are sequence of DNA that can be transposed from one location in DNA to another. They carry a gene that codes foe an enzyme called transposase There are two types of transposons: simple and complex. Simple transposons which only contain essential elements needed for transposition. Complex transposons carry in a other elements (e.g. gene coding for antibiotic resistance) in addition to elements essential for transposition. 7. Mutation: Types Summary Mutations are errors that occur in the DNA replication process. Mutagens are chemicals that increase the chances of errors in DNA replication. There are two classes of mutations: Base-substitution mutations and frameshift mutations. 1. Base Substitution mutations can be divided into: silent mutation, missense mutation, and nonsense mutation. In silent Mutation, changes in the DNA sequence does not lead to a change in the Amino Acid sequence of the proteins. This affects the genotype and not the phenotype. In Missense mutation, the base substitution sequence causes a change in a single amino acid sequence to change in the protein. It can be harmful, neural, or beneficial depending on the sequence of amino acid for proteins. In nonsense mutation, a base sequence change leads to a coding from an amino acid to a stop codon. Harmful in general. 2. Frameshift Mutations can de divided into frameshift insertion or frameshift deletion. In frameshift insertion, occurs when nucleotides are inserted into a DNA sequence. In frameshift deletion, a nucleotide is removed from a DNA sequence. Frameshift mutations generally lead to nonfunctional proteins.
