Cell Cycle, Mitosis, and Protein Synthesis: Study Notes for Anatomy & Physiology

Cell Cycle

Overview of the Cell Cycle

The cell cycle is a series of changes a cell undergoes from the time it is formed until it reproduces. It consists of two major phases: Interphase and the Mitotic (M) phase.

• Interphase: The period when the cell grows and carries on its usual cellular activities.

• Mitotic phase: The period when the cell divides to become two cells.

Major Periods of the Cell Cycle

• Interphase: Includes cell growth, normal metabolism, and preparation for cell division. It is subdivided into three parts:

  1. G1 phase: Vigorous growth and metabolism of the cell.

  2. S phase: DNA replication occurs.

  3. G2 phase: Preparation for division continues.

  Cells that permanently cease dividing are said to be in G0 phase (e.g., muscle and nervous tissue).

• Mitotic (M) phase: Includes mitosis and cytokinesis, resulting in two daughter cells.

DNA Replication

Process of DNA Replication

DNA replication occurs prior to cell division and ensures each new cell receives an exact copy of all nuclear DNA.

• Double-stranded DNA helices unwind and "unzip".

• Each DNA strand acts as a template for a new complementary strand.

• DNA polymerase is the enzyme necessary to begin adding DNA nucleotides to complementary bases.

• Complementary bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

• Base pairing: A-T, G-C.

Chromosomes and Chromatids

Structure and Terminology

• Replicated strands of DNA are connected by a centromere.

• Duplicated DNA is called a sister chromatid.

• During mitotic cell division, one complete copy of DNA is given to each new cell.

Cell Division

Purpose and Types

• Most cells replicate continuously for growth and repair.

• Skeletal muscle cells, cardiac muscle cells, and nerve cells do not divide efficiently.

• Damaged cells are replaced with scar tissue.

Mitotic (M) Phase of Cell Cycle

• Consists of two distinct events:

  1. Mitosis: Division of the nucleus and distribution of duplicated DNA to daughter cells.

  2. Cytokinesis: Division of the cytoplasm.

Mitosis

Stages of Mitosis

Mitosis ensures each cell receives a full copy of the replicated DNA. The four stages are:

• Prophase:

  • Chromatin condenses into chromosomes.

  • Each chromosome duplicates and is held together by a centromere.

  • Centrioles begin synthesizing microtubules (spindle fibers).

  • In late prophase, the nuclear envelope breaks up, and microtubules attach to centromeres.

• Metaphase:

  • Centromeres of chromosomes align at the cell's equator (metaphase plate).

• Anaphase:

  • Chromatids are pulled apart to opposite poles.

  • Each sister chromatid becomes a separate chromosome.

  • Cytokinesis begins during anaphase.

• Telophase:

  • Chromosome movement stops.

  • New nuclear membranes form, nucleoli reappear, and spindle fibers disappear.

  • Cytokinesis continues, forming two daughter cells.

Protein Synthesis

Overview

Proteins are composed of polypeptide chains made up of amino acids. DNA is the master blueprint that holds the code for protein synthesis and directs the order of amino acids in a polypeptide.

Definitions

• Gene:

  • A segment of DNA that holds the code for the synthesis of one polypeptide (protein).

  • A segment of DNA that holds the order of sequence of amino acids for the synthesis of a protein.

  • A sequence of DNA nucleotides that code for the synthesis of a polypeptide.

Genetic Code

• The genetic code is the specific order of three nitrogen bases of mRNA that represents an amino acid.

• Three nitrogen bases of mRNA = codon.

• Three nucleotides on DNA = triplet.

• Three nucleotides on mRNA = codon.

• Three nucleotides on tRNA = anticodon.

Examples

• DNA triplet: GGC (codes for amino acid proline)

• mRNA codon: CCG (codes for amino acid proline)

• tRNA anticodon: GGC (codes for amino acid proline)

Genes: Exons and Introns

• Exons: Coding segments that are expressed as mRNA.

• Introns: Noncoding segments interspersed among exons; remain inside the nucleus.

RNA and Its Types

Types of RNA

• Messenger RNA (mRNA): Copies genetic code from DNA and carries it to ribosomes.

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

• Transfer RNA (tRNA): Carrier RNA for specific amino acids; contains anticodon that matches mRNA codon.

Transcription and Translation

Transcription

Transcription is the process of transferring DNA gene base sequence (triplets) to complementary base sequence of mRNA (codons).

• Occurs in the nucleus.

• RNA polymerase synthesizes mRNA.

• Phases:

  1. Initiation: RNA polymerase separates DNA strands.

  2. Elongation: RNA polymerase adds complementary nucleotides to growing mRNA.

  3. Termination: Transcription stops when RNA polymerase reaches a termination signal.

• Newly formed mRNA is called pre-mRNA and contains both introns and exons.

• Introns are removed from pre-mRNA, leaving only coding regions.

Translation

Translation is the process where the language of nucleic acids is translated into the language of amino acids to form proteins. It occurs at the ribosome.

• tRNA anticodon pairs with mRNA codon at the ribosome.

• tRNA drops off its specific amino acid, forming a growing polypeptide chain.

• Ribosomes have three binding sites for tRNA: A (aminoacyl), P (peptidyl), E (exit).

• Sequence of events:

  1. Initiation: Ribosome scans mRNA for start codon (AUG).

  2. Elongation: Amino acids are added one at a time to the growing chain.

  3. Termination: Stop codon (UAA, UAG, UGA) signals end of translation.

Genetic Code Table

Term	Definition	Example
Triplet	3 DNA nucleotides	GGC
Codon	3 mRNA nucleotides	CCG
Anticodon	3 tRNA nucleotides	GGC

Endoplasmic Reticulum (ER) in Protein Synthesis

Role of Rough ER

• Short amino acid segment present on a growing polypeptide chain signals the ribosome to dock on the rough ER surface.

• Once docked, the growing polypeptide enters the RER.

• Protein is enclosed in a vesicle for transport to the Golgi apparatus.

Cell Death and Destruction

Apoptosis and Autophagy

• Apoptosis: Programmed cell death; disposes of unneeded or damaged cells.

• Cells shrink and are phagocytized by macrophages.

• Autophagy: Self-eating; disposal of nonfunctional organelles and cytoplasmic bits by lysosomes.

Cell Destruction and Modified Rates of Cell Division

• Cell division is needed for growth and repair.

• Hyperplasia: Accelerated growth that increases cell numbers when needed.

• Atrophy: Decrease in size of organ or tissue due to loss of stimulation or disease.

Key Equations and Terms

• Base pairing: $A-T$, $G-C$

• Central Dogma: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Summary Table: Stages of Mitosis

Stage	Main Events
Prophase	Chromatin condenses, spindle forms, nuclear envelope breaks down
Metaphase	Chromosomes align at cell equator
Anaphase	Chromatids separate and move to opposite poles
Telophase	Nuclear envelope reforms, chromosomes decondense, cytokinesis completes

Additional info:

• Some amino acids are represented by more than one codon, providing redundancy and protection against transcription errors.

• There is one start codon (AUG) and three stop codons (UAA, UAG, UGA) in the genetic code.

• Polyribosomes are multiple ribosomes attached to one mRNA, producing multiple copies of the same protein.