Cell Nucleus, Protein Synthesis, and Membrane Transport: Key Concepts in 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 bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

  • The triplet code refers to the fact that three consecutive DNA bases (a triplet) specify one amino acid in a protein.

• Gene:

  • A gene is a segment of DNA that contains instructions for making one specific protein.

  • Genes are considered the functional units of heredity, as they determine inherited traits.

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:

  • Involves uncoiling DNA and temporarily removing histones to expose the gene for transcription.

• Transcription:

  • Transcription is the synthesis of RNA from a DNA template.

  • All types of RNA, including messenger RNA (mRNA), are formed through transcription.

• DNA Strands:

  • The coding strand of DNA specifies the sequence of amino acids in polypeptides.

  • The template strand is used for mRNA production during transcription.

• Process of Transcription:

  1. RNA polymerase binding: The enzyme RNA polymerase binds to the DNA at the promoter region.

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

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

• RNA Processing:

  • Before leaving the nucleus, mRNA is "edited" to remove noncoding sequences (introns).

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

Translation

Translation is the process by which the information in mRNA is used to assemble amino acids into a polypeptide chain, forming a protein. This occurs in the cytoplasm at ribosomes.

• 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.

Examples of the Genetic Code

The genetic code is universal and specifies which mRNA codons correspond to which amino acids. The following table summarizes examples:

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
TAC	ATG	AUG	UAC	Methionine
TCT	AGA	AGA	UCU	Serine
CGG	GCC	CGC	GCG	Proline
GCG	CGC	GCG	CGC	Alanine

Summary of Protein Synthesis Steps

1. Transcription: DNA is transcribed into mRNA in the nucleus.

2. RNA Processing: Introns are removed and exons are spliced together.

3. Translation: mRNA is translated into a polypeptide at the ribosome in the cytoplasm.

Regulation of Cell Structure and Function

DNA controls cell structure and function by directing the synthesis of specific proteins. Changes in the extracellular environment can alter the intracellular environment, potentially initiating chemical signaling pathways. Substances that cross the plasma membrane may enter the nucleus and bind to receptors or promoters on DNA, influencing gene expression.

3-5 Diffusion and Osmosis

Plasma Membrane Permeability

The plasma (cell) membrane acts as a barrier, regulating the movement of nutrients, products, and wastes. Its permeability determines what substances can enter or exit the cell.

• Impermeable: Lets nothing in or out.

• Freely permeable: Lets anything pass.

• Selectively permeable: Restricts movement based on specific criteria.

Selectively Permeable Membrane

The plasma membrane is selectively permeable, allowing some materials to move freely while restricting others based on:

• Size

• Electrical charge

• Molecular shape

• Lipid solubility

Transport Mechanisms Across the Plasma Membrane

• Passive processes: Do not require energy (e.g., diffusion, osmosis).

• Active processes: Require energy (e.g., active transport, vesicular transport).

• Carrier-mediated transport: Can be passive or active, involving specific carrier proteins.

• Vesicular transport: Active process involving movement of substances in vesicles.

Diffusion

Diffusion is a passive process involving the net movement of a substance from an area of higher concentration to an area of lower concentration.

• Ions and molecules are constantly in motion, moving randomly in solution.

• 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.

• Equation for Diffusion Rate: $J = -D \frac{dC}{dx}$ where $J$ is the diffusion flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Example: Oxygen Diffusion

Oxygen diffuses from areas of high concentration (such as alveoli in the lungs) to areas of low concentration (such as blood capillaries), enabling efficient gas exchange.

Additional info: Osmosis, a specific type of diffusion involving water, is also a key process in maintaining cellular homeostasis, though not detailed in these slides.