Cell Nucleus, Protein Synthesis, and Membrane Transport: Study Notes for Anatomy & Physiology

3-3 Cell Nucleus

Information Storage in the Nucleus

The cell nucleus serves as the control center for cellular activities, primarily by storing genetic information in the form of DNA. This information is essential for the regulation of cell structure and function.

• Genetic Code: The genetic code is the chemical language of DNA instructions, composed of sequences of four nucleotide bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

• Triplet Code: DNA bases are read in groups of three, called triplets. Each triplet (codon) corresponds to one amino acid in a protein.

• Gene: A gene is a segment of DNA that contains instructions for synthesizing one specific protein. Genes are the functional units of heredity.

Example: The gene for hemoglobin contains the DNA sequence that determines the amino acid sequence of the hemoglobin protein.

3-4 Protein Synthesis

Overview of Protein Synthesis

Protein synthesis is the process by which cells assemble functional polypeptides (proteins) in the cytoplasm, following instructions encoded in DNA. This process involves several key steps: gene activation, transcription, RNA processing, and translation.

• Gene Activation: DNA is uncoiled and histones are temporarily removed to allow access to the genetic code.

• Transcription: The synthesis of RNA from a DNA template. All types of RNA, including messenger RNA (mRNA), are produced by transcription.

DNA Strands in Protein Synthesis

• Coding Strand: Specifies the sequence of amino acids in polypeptides.

• Template Strand: Used for mRNA production during transcription.

Process of Transcription

Transcription is the first step in protein synthesis, where the genetic information in DNA is copied into mRNA.

1. RNA Polymerase Binding: The enzyme RNA polymerase binds to the promoter region of DNA.

2. Nucleotide Linking: RNA polymerase reads the DNA code and links nucleotides to form mRNA in three-base sequences called codons.

3. Detachment of mRNA: The enzyme and mRNA strand detach from DNA at a "stop" signal.

RNA Processing

Before mRNA leaves the nucleus, it undergoes processing:

• Noncoding sequences (introns) are removed.

• Coding segments (exons) are spliced together to form mature mRNA.

Translation

Translation is the process by which the sequence of codons in mRNA is used to assemble amino acids into a polypeptide chain.

• After leaving the nucleus, mRNA binds to ribosomal subunits in the cytoplasm.

• Each mRNA codon translates to one amino acid.

• Amino acids are delivered by transfer RNA (tRNA).

• A tRNA anticodon binds to a complementary mRNA codon.

• Enzymes join amino acids with peptide bonds.

• At the stop codon, the components separate, releasing the completed polypeptide.

Table: Examples of the Genetic Code

The genetic code is universal and specifies which codons correspond to which amino acids.

DNA Triplet (Template Strand)	Coding Strand	mRNA Codon	tRNA Anticodon	Amino Acid
AAA	TTT	UUU	AAA	Phenylalanine
AAT	TTA	UUA	AAU	Leucine
ACA	TGT	UGU	ACA	Cysteine
CAA	GTT	GUU	CAA	Valine
ATG	TAC	AUG	UAC	Methionine
TCC	AGG	AGG	UCC	Serine
CGG	GCC	GCC	CGG	Proline
GCG	CGC	CGC	GCG	Alanine

DNA Control of Cell Structure and Function

DNA directs the synthesis of specific proteins, which determine cell structure and function. Changes in the extracellular environment can influence intracellular activities, often through chemical signaling pathways. Substances that cross the plasma membrane may enter the nucleus and interact with DNA to regulate gene expression.

3-5 Diffusion and Osmosis

Plasma Membrane Permeability

The plasma (cell) membrane acts as a selective barrier, controlling the movement of substances into and out of the cell. Permeability is a key property that determines what can cross the membrane.

• Impermeable: Lets nothing in or out.

• Freely Permeable: Lets anything pass.

• Selectively Permeable: Restricts movement based on specific criteria.

Selective Permeability Factors

• Size of the molecule

• Electrical charge

• Molecular shape

• Lipid solubility

Transport Mechanisms Across the Plasma Membrane

Transport can be passive (no energy required) or active (requires energy).

• Passive Processes: Diffusion and osmosis

• Carrier-mediated Transport: Can be passive or active

• Vesicular Transport: Active process

Diffusion

Diffusion is the net movement of a substance from an area of higher concentration to an area of lower concentration, driven by random molecular motion.

• Ions and molecules are constantly in motion.

• Random motion causes mixing of substances.

Concentration Gradient

The concentration gradient is the difference between high and low concentrations of a substance. Diffusion occurs down the concentration gradient.

Example: Oxygen diffuses from the alveoli (high concentration) into the blood (low concentration) in the lungs.

Equation:

$J = -D \frac{dC}{dx}$

Where $J$ is the diffusion flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Additional info: The notes cover foundational concepts in cell biology relevant to Anatomy & Physiology, including the molecular basis of heredity, protein synthesis, and membrane transport mechanisms. These processes are essential for understanding cellular function and tissue physiology.