DNA and RNA Structure and Replication PDF

# Today's Recap - DNA and RNA are nucleotides. - Both are composed of a nitrogenous base, a sugar (ribose for RNA & deoxyribose for DNA), and a phosphate group. - There are four nitrogenous bases in DNA: - Adenine (A) - Guanine (G) - Cytosine (C) - Thymine (T) - These bases pair u...

# Today's Recap - DNA and RNA are nucleotides. - Both are composed of a nitrogenous base, a sugar (ribose for RNA & deoxyribose for DNA), and a phosphate group. - There are four nitrogenous bases in DNA: - Adenine (A) - Guanine (G) - Cytosine (C) - Thymine (T) - These bases pair up to form the rungs of the DNA ladder: - A pairs with T - G pairs with C - A-T base pairs are held together by 2 hydrogen bonds. - G-C base pairs are held together by 3 hydrogen bonds. - In RNA, thymine (T) is replaced by uracil (U), so A pairs with U in RNA instead of T. ## DNA Structure - DNA consists of two strands that wind into a double helix. - These strands are antiparallel, meaning one runs in the 3' to 5' direction, and the other in the 5' to 3' direction. - The strands are held together by the complementary base pairs. ## RNA Structure - RNA is typically single-stranded and does not form a double helix like DNA. ## DNA Replication (Before Cell Division) - Before a cell divides, it needs to replicate its DNA to pass an identical copy to its daughter cells. This process is key to heredity. - Steps in DNA replication: 1. Helicase unwinds the double helix to expose the bases. 2. Primase synthesizes a short RNA primer to provide a starting point for DNA polymerase. 3. DNA polymerase then creates a complementary strand by matching the bases (A with T, and G with C). - This process forms two identical copies of the DNA, each with one original and one new strand (semiconservative replication). ## Why do Okazaki fragments form? - DNA replication is bidirectional, meaning that both strands are copied simultaneously. - However, the DNA strands are antiparallel (one strand runs 5' to 3', and the other runs 3' to 5'). - DNA polymerase, the enzyme that adds nucleotides to form the new DNA strand, can only add nucleotides in the 5' to 3' direction. - This creates a problem for the strand that runs in the 3' to 5' direction, known as the lagging strand. - To solve this, the lagging strand is synthesized in small discontinuous sections, each initiated by a short RNA primer. - These sections are called Okazaki fragments, after the scientist Reiji Okazaki, who discovered them. - The process: 1. Helicase unwinds the DNA double helix. 2. Primase synthesizes a short RNA primer on the lagging strand. 3. DNA polymerase adds nucleotides to the RNA primer, extending the Okazaki fragment in the 5' to 3' direction. 4. Once an okazaki fragment is complete, the process repeats with a new RNA primer further down the lagging strand 5. DNA Ligase eventually joins the Okazaki fragments together to form a continuous strand. - In contrast, the leading strand is synthesized continuously in the 5' to 3' direction. ## Transcription (DNA to RNA) - This is the process by which messenger RNA (mRNA) is synthesized from a DNA template. - Steps in Transcription: 1. RNA polymerase binds to a region of DNA called the promoter to initiate transcription. 2. RNA polymerase separates the DNA strands and uses one strand (the template strand) to synthesize a complementary RNA strand. - The RNA strand is antiparallel to the DNA template and uses uracil (U) in place of thymine (T). - This newly formed strand is called pre-mRNA 3. Splicing occurs: non-coding regions of RNA called introns are removed by a complex called the spliceosome, leaving only the coding regions (exons). The result is mature mRNA. ## Translation (mRNA to Protein) - Once the mRNA exits the nucleus, it moves to the ribosome, where it serves as a template for protein synthesis. - Steps in Translation: 1. The ribosome reads the mRNA in groups of three bases called codons. Each codon corresponds to a specific amino acid. 2. Transfer RNA (tRNA) molecules bring the appropriate amino acids to the ribosome, matching their anticodon to the mRNA codon. 3. The amino acids are linked together in a chain, forming a polypeptide, which folds into a functional protein. 4. Stop codons (UAA, UGA, UAG) signal the end of translation, and the newly formed protein is released. ## Important Points - Replication ensures genetic material is accurately copied. - Transcription transfers information from DNA to RNA. - Translation converts the mRNA sequence into a protein.