Genes and Cellular Function Study Notes

DNA Structure and Function

• Deoxyribonucleic acid (DNA) serves as the primary genetic material in human cells.

• Human cells typically contain $46$ DNA molecules (chromosomes) located within the cell nucleus.

• Each nucleotide of DNA is constructed from three distinct components:

  • A sugar molecule: Deoxyribose.

  • A phosphate group.

  • A nitrogenous base.

• DNA features four nitrogenous bases:

  • Adenine (A)

  • Thymine (T)

  • Cytosine (C)

  • Guanine (G)

• Structural Morphology: DNA exists as a double helix shape, which resembles a spiral staircase (illustrated in Figure 4.2).

• DNA Base Pairing Rules:

  • Adenine (A) pairs strictly with Thymine (T) via $2$ hydrogen bonds.

  • Cytosine (C) pairs strictly with Guanine (G) via $3$ hydrogen bonds.

• Gene: A defined segment of DNA that codes for the synthesis of a specific protein.

• Genome: The entire complement of genes belonging to one individual person.

Chromatin and Chromosomes

• Chromatin: Fine, filamentous DNA material complexed with packaging proteins (illustrated in Figure 4.4).

• Histones:

  • Specialized proteins associated directly with DNA.

  • Function to package, fold, and organize DNA strands into structured chromosomes.

RNA Structure and Function

• Ribonucleic Acids (RNAs):

  • Contain the nitrogenous base Uracil (U) in place of Thymine (T).

  • Consist of a single nucleotide chain rather than a double-stranded double helix.

• Three major types of RNA are essential for cellular protein synthesis:

  • Messenger RNA (mRNA)

  • Transfer RNA (tRNA)

  • Ribosomal RNA (rRNA)

Genes and the Genetic Code

• Gene Definition: A segment of DNA coding for the production of an RNA molecule that ultimately synthesizes one or more proteins.

• Chromosomal Organization: The $46$ human chromosomes exist in two distinct sets of $23$ chromosomes each.

• Coding Capacity: A single gene has the capacity to code for many different proteins.

• Genetic Similarity: All human beings share at least $99.99\%$ genetic identity with one another.

• Diversity of Proteins: The human body is capable of manufacturing millions of different proteins.

• Base Triplet: A sequence of $3$ DNA nucleotides that stands for one specific amino acid.

• Codon: A sequence of $3$ consecutive bases on an mRNA strand that stands for one specific amino acid.

Protein Synthesis: Transcription

• Cellular Gene Distribution: All cells in the human body—with the exception of sex cells and select immune cells—contain an identical set of genes.

• Direction of Information Flow: Protein synthesis proceeds via the sequence DNA $\rightarrow$ mRNA $\rightarrow$ Protein.

• Transcription Overview: The enzymatic process of copying genetic code instructions directly from DNA into an mRNA molecule.

• Initiation Signals: Specific base sequences in DNA act as signals to initiate RNA polymerase activity.

• Actions of RNA Polymerase:

  • Binds directly to DNA and opens up the double helix.

  • Reads nitrogenous bases from one template strand of DNA to assemble a complementary strand of mRNA.

  • Rewinds the DNA double helix behind itself as it moves along the sequence.

Protein Synthesis: Translation

• Translation Overview: The process of converting the code carried by mRNA into a sequence of amino acids to form a protein.

• Primary Structural Components:

  • Messenger RNA (mRNA): Transports the genetic code out of the nucleus and into the cytoplasm. Functions as the initial binding site for the small ribosomal subunit.

  • Transfer RNA (tRNA):

  • Small RNA molecule exhibiting an angular L shape.

  • Contains an anticodon at one end consisting of a series of $3$ nucleotides complementary to a specific mRNA codon.

  • Features an amino acid binding site at its opposing end specific to one single amino acid.

  • Binds and picks up free amino acids present in the cytosol for delivery to the ribosome.

  • Ribosomal RNA (rRNA):

  • Located free within the cytosol, attached to the rough endoplasmic reticulum (ER), and along the nuclear envelope.

• Ribosomal Assembly & Reading:

  • The large ribosomal subunit attaches to the small subunit after mRNA binding.

  • The assembled ribosome pulls the mRNA molecule through its structure like a ribbon, reading the bases sequentially.

  • Protein synthesis is officially initiated when the ribosome reaches the start codon (AUG).

• Detailed Steps of Translation:

  1. Initiation:

  • The leader sequence of mRNA binds to the small ribosomal subunit.

  • An initiator tRNA carrying the amino acid methionine pairs with the start codon (AUG).

  • The large ribosomal subunit joins to complete the functional ribosome complex.

  1. Elongation:

  • The next incoming tRNA (bearing its corresponding amino acid) binds to the ribosome as its anticodon pairs with the complementary next mRNA codon.

  • A peptide bond is formed between adjacent amino acids.

  • The ribosome slides along to read the next codon and releases the now-empty initiator tRNA.

  • The next tRNA bearing an appropriate complementary anticodon delivers its amino acid to the ribosome.

  • Another peptide bond is formed.

  • This cycle repeats continuously, extending the peptide chain into a complete protein.

  1. Termination:

  • When the ribosome encounters a stop codon, a release factor protein binds to the complex.

  • The completed protein detaches and breaks away from the ribosome.

  • The ribosome dissociates back into its two separate subunits.

Protein Processing, Secretion, and Gene Regulation

• Protein Folding and Transport (Figure 4.10 and Table 4.3):

  • To become biologically active, a newly synthesized protein must fold into precise secondary and tertiary structural shapes.

  • Proteins destined for use inside the cytosol are synthesized on free ribosomes.

  • Proteins destined for packaging into lysosomes or secretion out of the cell are assembled on the rough ER and transferred to the Golgi complex for packaging.

• Mechanisms of Gene Regulation:

  • Genes can be actively turned on or off.

  • Permanent Inactivation: Specific genes are permanently turned off depending on cell type (e.g., liver cells permanently turn off hemoglobin genes).

  • Inducible Expression: Cells turn on genes as needed; gene expression rates vary dynamically from day to day or hour to hour and are regulated by hormones.

  • Example: Mammary gland cells turn on the gene for casein protein exclusively when breast milk is being produced.

  • Regulatory Steps: Gene expression is controlled through a $7$-step sequence (illustrated in Figure 4.11).

Synthesis of Non-Protein Molecules

• Cellular non-protein molecules include glycogen, fats, steroids, phospholipids, pigments, and other compounds.

• Non-protein products are produced through enzymatic reactions.

• Because enzymes are proteins encoded directly by genes, gene activity indirectly controls non-protein synthesis.

• Example - Testosterone Production:

  • The cell internalizes cholesterol.

  • An enzyme converts cholesterol directly into the steroid hormone testosterone.

DNA Replication

• General Requirements (Figure 4.13):

  • DNA must be fully replicated prior to cell division.

  • Replication must occur with exact precision to preserve normal cellular function.

• Sequential Steps of Replication:

  1. Unwinding: DNA unwinds from its organizing histones. The point of DNA opening is called the replication fork.

  2. Unzipping: The enzyme DNA helicase unzips a segment of the double helix, exposing its nitrogenous bases.

  3. Strand Synthesis: DNA polymerase builds new DNA strands.

  • Polymerase reads exposed template bases and pairs them with complementary free nucleotides.

  • Separate DNA polymerase molecules work on each template strand simultaneously, moving in opposite directions.

  • Generates two identical daughter DNA molecules from the original parental DNA template.

  1. Repackaging: Each newly synthesized DNA double helix wraps around histones to form new nucleosomes.

DNA Replication Errors and Mutations

• Mutation Definition: Any structural change in DNA caused by replication errors or environmental mutagens (such as radiation, viruses, or chemical exposure).

• Outcomes of Mutations:

  • Some mutations produce no noticeable ill effects.

  • Other mutations can kill the cell, transform it into a cancerous cell, or cause inheritable genetic defects in future generations.

The Cell Cycle and Interphase

• Cell Cycle Overview (Figure 4.14): Composed of Interphase and Mitosis.

• Phases of Interphase:

  1. G1 Phase (First Gap Phase):

  • The interval between cell birth (resulting from division) and DNA replication.

  • The cell performs normal physiological tasks and accumulates materials necessary for the next phase.

  1. S Phase (Synthesis Phase):

  • The cell replicates its entire complement of nuclear DNA and duplicates its centrioles.

  1. G2 Phase (Second Gap Phase):

  • The interval between completed DNA replication and cell division.

  • The cell repairs DNA replication errors, grows, and synthesizes enzymes required to control cell division.

Mitosis and Cytokinesis

• Biological Functions of Mitosis (Figure 4.15):

  • Drives the development of an individual from a single fertilized egg into approximately $50\text{ trillion}$ cells.

  • Enables growth of all tissues and organs after birth.

  • Replaces dead or worn-out cells.

  • Repairs damaged tissues.

• Phases of Mitosis:

  1. Prophase:

  • Genetic material condenses into compact, visible chromosomes.

  • The nuclear envelope disintegrates.

  • Centrioles sprout spindle fibers (long microtubules).

  1. Metaphase:

  • Chromosomes align along the cell equator.

  • Spindle fibers complete the lemon-shaped mitotic spindle.

  • Shorter microtubules radiating from centrioles form an aster, anchoring to the inner cell membrane.

  1. Anaphase:

  • An enzyme cleaves sister chromatids apart at their centromere.

  • Single-stranded daughter chromosomes migrate to opposite poles of the cell.

  1. Telophase:

  • Daughter chromosomes cluster at each pole of the cell.

  • Rough ER constructs a new nuclear envelope around each cluster.

  • Chromosomes uncoil into fine chromatin.

  • The mitotic spindle disintegrates.

  • Each new nucleus forms nucleoli.

• Cytokinesis:

  • Physical division of the cytoplasm into two individual cells.

  • Telophase represents the end of nuclear division, but chronologically overlaps with cytokinesis.

Regulation of Cell Division and Cell Cycle Checkpoints

• Conditions Stimulating Cell Division:

  • Cells possess sufficient cytoplasm for two functional daughter cells.

  • Nuclear DNA has been completely replicated.

  • An adequate supply of nutrients is available.

  • Cells receive stimulation from chemical growth factors.

  • Neighboring cells die, opening up physical space.

• Conditions Inhibiting Cell Division:

  • Cells snugly contact neighboring cells (contact inhibition).

  • Nutrients or chemical growth factors are withdrawn.

• Cell Cycle Control Checkpoints:

  1. G1 Checkpoint:

  • Determines whether the cell proceeds toward the S phase or enters the non-cycling G0 resting phase.

  1. G2 Checkpoint:

  • Occurs late in the G2 phase to determine whether the cell is ready to enter mitosis.

  1. Metaphase-to-Anaphase Checkpoint:

  • Occurs at the transition between metaphase and anaphase to determine whether sister chromatids can safely separate.

• Associated Topic: Deeper Insight 4.3 Cancer.