Lecture 5 Biology: DNA Replication PDF

LECTURE 5 BIOLOGY CODE: FM 106 FIRST YEAR DR WESAM SALAMA FIRST SEMESTAR 2024-2025 REPLICATION Make your own comment  DNA (DeoxyriboNucleic Acid) The genetic material Double helix Composed from a chain of nucleotides DNA is suga...

LECTURE 5 BIOLOGY CODE: FM 106 FIRST YEAR DR WESAM SALAMA FIRST SEMESTAR 2024-2025 REPLICATION Make your own comment  DNA (DeoxyriboNucleic Acid) The genetic material Double helix Composed from a chain of nucleotides DNA is sugar-phosphate backbone DEOXYRIBOSE SUGAR  DNA bases pair via hydrogen bonds Complementary bases pair:  nDNA and mtDNA  Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA)  DNA REPLICATION Replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules. Four main steps to DNA replication: 1-Initiation, 2-Primer binding 3- Elongation, 4- Termination  Memorize those enzymes 1 2 Helicase 3 Ligase Exonucleasase Primase Gyrase proffreaders Polymerase  1- INITIATION HELICASE The double helix structure of the DNA molecules has to be ‘unzipped.’ Breaking the hydrogen bonds between base pairs. Produce replication fork  Leading strand One of the strands is oriented in the 3’ to 5’ direction (replicated towards the replication fork)l Lagging strand The other strand is oriented in the 5’ to 3’ direction (replicated away from the replication fork)  2- primer binding Primer Binding The leading strand is the simplest to replicate. Once the DNA strands have been separated, a short piece of DNA called a primer binds to the 3' end of the strand. The primer always binds as the starting point for replication. Primers are generated by the enzyme DNA primase  3- Elongation Enzymes known as DNA polymerases are responsible creating the new strand by a process called elongation. DNA polymerase III binds to the strand at the site of the primer and begins adding new base pairs complementary to the strand during replication.  DNA polymerase DNA polymerase, only functions in the 5′ to 3′ direction one adding nucleotides one by one in the direction of the replication fork. The other able to add nucleotides only in chunks (Okazaki fragments). The first strand, which replicates nucleotides one by one is the leading strand; the other strand, which replicates in chunks, is the lagging strand.  4- Termination  An enzyme called EXONUCLEASES removes all primers from the original strands.  Another exonuclease “PROOFREADERS” the newly formed DNA to check, remove and replace any errors.  Another enzyme called DNA LIGASE joins Okazaki fragments together. The ends of the parent strands consist of repeated DNA sequences called telomeres. Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing.  A special type of DNA polymerase enzyme called TELOMERASE catalyzes the synthesis of telomere sequences at the ends of the DNA. Once completed, the parent strand and its complementary DNA strand coils into the familiar double helix shape.  DNA helicase - unwinds and separates double stranded DNA as it moves along the DNA. It forms the replication fork by breaking hydrogen bonds between nucleotide Important pairs in DNA. DNA primase - a type of DNA polymerase that ENZYMES** generates primers. DNA polymerases - synthesize new DNA molecules by adding nucleotides. Topoisomerase or DNA Gyrase - unwinds and rewinds DNA strands to prevent the DNA from becoming tangled or supercoiled. Exonucleases - group of enzymes that remove nucleotide bases from the end of a DNA chain. DNA ligase - joins DNA fragments together by forming phosphodiester bonds between nucleotides. How does the cell convert DNA into working proteins?  Before we answer we should differentiate between DNA & RNA  Replication Transcription processing translation DNA encodes all of the information necessary for cellular functions cells can make exact copies of DNA RNA is made from a DNA template and functions in protein synthesis proteins are translated from messenger RNA and carry out cellular functions STEP 1  The information in DNA is transferred to a messenger RNA (mRNA) molecule.  During transcription, the DNA of a gene serves as a template for complementary base-pairing, and an enzyme called RNA polymerase II catalyzes the formation of a pre-mRNA molecule, which is then processed to form mature mRNA. STEP 2  The resulting mRNA is a single-stranded copy of the gene, which next must be translated into a protein molecule.  Each group of three bases in mRNA constitutes a codon, and each codon specifies a particular amino acid (hence, it is a triplet code) WHERE DO ALL THE PREOCESS OCCUR?  TYPES OF RNA Messenger RNA (mRNA)----------Carries genetic material from DNA in nucleus to cytoplasm. Transfer RNA (tRNA)-------Transport amino acids to the site of protein synthesis Ribosomal RNA (rRNA)------Binds to protein to form ribosomes BRAIN STORMING  Mention the enzyme responsible for: 1- Unzipped the helical DNA strand 2- Unwinds and rewinds DNA strands to prevent the DNA from becoming tangled or supercoiled. 3- Joins DNA fragments (okazaki fragments) together by forming phosphodiester bonds between nucleotides 4- Check, remove and replace any errors from newly formed DNA strand 5- Remove extra primers from DNA strand 6- Catalyzes the synthesis of telomere sequences at the ends of the DNA