DNA Replication and Protein Synthesis

DNA Replication

  • Definition and Importance of DNA Replication

    • DNA replication occurs during interphase, a phase in the cell cycle where the cell grows, develops, and prepares for division.

    • It is essential to produce new DNA for new cells, replace or repair cells, and ensure genetic continuity by providing identical DNA in each daughter cell.

    • Mutations and Mistakes: Changes in DNA during replication may lead to mutations, which can be harmful. Accurate replication is critical to pass on traits necessary for survival and function.

  • Structure of DNA

    • DNA is structured as a double helix, resembling a twisted ladder.

    • Comprised of two strands of nucleotides, where each nucleotide has:

    • A deoxyribose sugar

    • A phosphate group

    • A nitrogen base

    • The four nitrogen bases are:

    • Adenine (A)

    • Thymine (T)

    • Cytosine (C)

    • Guanine (G)

    • Base Pairing Rules:

    • Adenine pairs specifically with Thymine (A-T)

    • Cytosine pairs specifically with Guanine (C-G)

    • These pairings hold the genetic code together.

  • Process of DNA Replication

    1. Separation of Strands:

    • Enzyme helicase breaks the hydrogen bonds between the nitrogen bases, separating the two strands.

    1. Addition of Complementary Bases:

    • The enzyme DNA polymerase attaches new nucleotides to each separated strand, forming complementary base pairs.

    1. Creation of Identical Strands:

    • After the addition of nucleotides, two identical DNA strands are formed.

    1. Proofreading:

    • DNA polymerase also proofreads the new DNA strands to minimize mistakes, ensuring accuracy.

    1. Final Result:

    • Two identical double-stranded DNA molecules are produced, each containing one original strand and one new strand.

      • Semi-Conservative Replication: This mechanism is termed semi-conservative replication because each new DNA molecule conserves one strand from the original.

Protein Synthesis - Transcription

  • Definition and Importance of Transcription

    • Transcription is the process of producing a copy of a specific DNA gene into a readable form known as messenger RNA (mRNA).

    • This step is crucial in protein synthesis, functioning like a recipe where DNA contains the instructions and transcription transcribes those into mRNA.

  • Location

    • Transcription occurs in the nucleus of the cell.

  • Process of Transcription

    1. Initiation:

    • The enzyme RNA polymerase attaches to the DNA at the beginning of the gene to be transcribed.

    1. Separation of DNA Strands:

    • The enzyme unzips the DNA at the section of the gene being copied, separating the two strands.

    1. mRNA Formation:

    • Using one of the DNA strands as a template, RNA polymerase pairs complementary RNA bases to the DNA template to form an mRNA strand.

    • In RNA, Uracil (U) replaces Thymine (T); thus, the pairing rules are as follows:

      • Adenine (A) pairs with Uracil (U) (A-U)

      • Thymine (T) pairs with Adenine (A) (T-A)

      • Cytosine (C) pairs with Guanine (G) (C-G)

      • Guanine (G) pairs with Cytosine (C) (G-C)

    1. Detachment and Exit:

    • Once the mRNA strand is synthesized, it detaches from the DNA and exits the nucleus through a nuclear pore into the cytoplasm.

    1. Completion:

    • The DNA strands rewind into their double helix structure after transcription is complete.

      • Function of mRNA: The mRNA strand carries the genetic code from the DNA, providing essential instructions for protein synthesis.

Protein Synthesis - Translation

  • Definition and Importance of Translation

    • Translation is the process where the mRNA code is used to assemble a polypeptide chain, ultimately forming a protein.

    • This process occurs in the cytoplasm at the ribosome.

  • Components Required for Translation

    • Three main elements are necessary for the initiation of translation:

    1. The mRNA strand, which conveys the genetic code from the nucleus.

    2. Transfer RNA (tRNA), which is responsible for bringing specific amino acids to the ribosome.

    3. The ribosome, which interprets the mRNA code and facilitates the connection of amino acids.

  • Process of Translation

    1. tRNA Functionality:

    • Each tRNA molecule carries a specific amino acid and possesses an anticodon that matches with a complementary codon (three-base sequence) on the mRNA.

    1. Codon Recognition:

    • As the ribosome reads the mRNA codons, the matching tRNA binds, ensuring the correct amino acid is brought to the ribosome.

    1. Polypeptide Chain Formation:

    • Amino acids are linked together by peptide bonds, forming a polypeptide chain.

    1. Termination of Translation:

    • This process continues until the ribosome encounters a stop codon.

    • Upon reaching a stop codon, the completed polypeptide is released.

    1. Folding into Protein:

    • The polypeptide chain subsequently folds into its final three-dimensional shape, resulting in a functional protein.