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Cell Cycle and Genetic Information Flow: Study Notes for Anatomy & Physiology

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Cells: The Living Units

Introduction

Cells are the fundamental units of life in all living organisms. Understanding their structure, function, and life cycle is essential for the study of Anatomy & Physiology. This section covers the cell cycle, DNA replication, and the flow of genetic information from DNA to proteins.

The Cell Cycle

Main Periods of the Cell Cycle

The cell cycle consists of two major periods: Interphase and Cell Division (Mitotic Phase).

  • Interphase: The cell grows and carries out its usual activities.

  • Cell Division (Mitotic Phase): The cell divides into two daughter cells.

Interphase Subphases

Interphase is subdivided into three distinct subphases:

  • G1 (Gap 1): Period of vigorous growth and metabolism. Cells that permanently cease dividing are said to be in the G0 phase.

  • S (Synthetic): DNA replication occurs during this phase.

  • G2 (Gap 2): Preparation for cell division.

DNA Replication

Process of DNA Replication

Prior to cell division, the cell must duplicate its DNA to ensure each daughter cell receives a complete set of genetic instructions.

  • Unwinding and Unzipping: Double-stranded DNA helices unwind and unzip at the replication fork, the point where the strands separate.

  • Semiconservative Replication: Each new double-stranded DNA molecule consists of one old strand and one newly synthesized strand.

End Result: Two identical "daughter" DNA molecules are formed from the original DNA.

Key Terms

  • Replication Fork: The region where the DNA strands separate and replication occurs.

  • Semiconservative Replication: The mechanism by which DNA is copied, preserving one original strand in each new molecule.

Stages of Cell Division

Mitosis

Mitosis is the division of the nucleus, ensuring that duplicated DNA is distributed equally to daughter cells. It consists of four stages:

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

Each stage ensures that each daughter cell receives a full copy of the replicated DNA.

Cytokinesis

Cytokinesis is the division of the cytoplasm, which begins during late anaphase and continues through mitosis. A ring of actin microfilaments forms a cleavage furrow, pinching the cell into two separate daughter cells.

Genetic Information Flow: From DNA to Protein

DNA as the Blueprint

DNA contains the instructions for building proteins, which are essential for cell structure and function. The sequence of nitrogen bases (Adenine, Guanine, Thymine, Cytosine) determines the genetic code.

  • Gene: A segment of DNA that holds the code for one polypeptide.

  • Triplet Code: Each set of three sequential bases (triplet) codes for a specific amino acid.

  • Exons: Coding regions of a gene.

  • Introns: Noncoding regions interspersed among exons.

Transcription and Translation

The process of protein synthesis involves two main steps: transcription and translation.

  • Transcription: The process by which messenger RNA (mRNA) is synthesized from a DNA template in the nucleus.

  • Translation: The process by which the mRNA sequence is decoded to build a polypeptide (protein) in the cytoplasm.

Role of RNA in Protein Synthesis

  • Messenger RNA (mRNA): Carries instructions from DNA to the ribosome for protein synthesis.

  • Ribosomal RNA (rRNA): Structural component of ribosomes, the site of protein synthesis.

  • Transfer RNA (tRNA): Transfers specific amino acids to the ribosome, matching its anticodon to the mRNA codon.

Transcription Phases

  • Initiation: RNA polymerase binds to DNA and separates the strands.

  • Elongation: RNA polymerase adds complementary nucleotides to the growing mRNA strand.

  • Termination: Transcription ends when RNA polymerase reaches a terminator sequence.

Processing of mRNA

Newly formed mRNA (pre-mRNA) is edited before translation. Introns are removed by spliceosomes, leaving only coding exons.

Genetic Code and Codons

  • Each three-base sequence on DNA (triplet) is represented by a complementary three-base sequence on mRNA called a codon.

  • There are 64 possible codons (), but only 20 amino acids, so some amino acids are coded by more than one codon (redundancy).

  • Three codons are "stop" codons, signaling the end of translation.

Translation Phases

  • Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA (methionine).

  • Elongation: Involves codon recognition, peptide bond formation, and translocation of the ribosome along the mRNA.

  • Termination: Occurs when a stop codon enters the ribosome, releasing the polypeptide and disassembling the ribosome.

Summary Table: Cell Cycle Phases

Phase

Main Events

G1 (Gap 1)

Cell growth and metabolism

S (Synthetic)

DNA replication

G2 (Gap 2)

Preparation for division

M (Mitotic Phase)

Mitosis and cytokinesis

Summary Table: Types of RNA

Type of RNA

Function

mRNA (Messenger RNA)

Carries genetic code from DNA to ribosome

rRNA (Ribosomal RNA)

Forms the core of ribosome structure and catalyzes protein synthesis

tRNA (Transfer RNA)

Brings amino acids to ribosome and matches them to mRNA codons

Key Equations

  • Number of possible codons:

Example: Codon-Anticodon Pairing

If the mRNA codon is AUG, the tRNA anticodon will be UAC, and the amino acid added will be methionine.

Additional info: The cell cycle and genetic information flow are foundational concepts in cell biology and are directly relevant to Anatomy & Physiology, as they explain how cells grow, divide, and produce proteins necessary for tissue function and repair.

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