Cells & Genetics: Structure and Function of the Nucleus, Chromatin, Chromosomes, Nucleolus, and Genetic Code

Cells & Genetics: Chapter 3 (Sections 3.6–3.7)

Learning Outcomes

• Describe the structure and function of the nucleus, chromatin, chromosomes, and nucleolus.

• Recognize the structure of DNA and RNA nucleotides and how they combine with phosphodiester and hydrogen bonds.

• Describe and be able to use the genetic code in the context of replication, transcription, and translation.

Nucleus, Chromatin, Chromosomes, and Nucleolus

Structure and Function of the Nucleus

The nucleus is the largest organelle in most eukaryotic cells and serves as the control center for cellular activities, especially gene expression and DNA replication.

• Number of Nuclei: Most cells have one nucleus. Mature red blood cells have none, while some muscle and liver cells may have multiple nuclei.

• Function: Contains all the genes that determine which proteins a cell will produce by selectively expressing specific sets of genes.

Nuclear Envelope

The nuclear envelope is a double membrane structure that separates the nucleus from the cytoplasm.

• Double Membrane: Composed of two lipid bilayers (total of four phospholipid layers).

• Outer Membrane: Continuous with the endoplasmic reticulum.

• Ribosomes: Embedded on the cytoplasmic side of the outer membrane.

• Inner Membrane: Supported by a network of intermediate filaments called the nuclear lamina.

• Nuclear Pores: Large protein complexes that facilitate active transport of RNA (out), proteins, and chemical messengers (in and out).

Nucleolus

The nucleolus is a dense region within the nucleus responsible for ribosome production.

• Function: Site of ribosomal RNA (rRNA) synthesis and assembly of ribosomal subunits.

• Composition: Contains DNA, RNA, and proteins.

Chromatin and Chromosomes

Chromatin is the complex of DNA and proteins (mainly histones) found in the nucleus. During cell division, chromatin condenses to form chromosomes.

• Chromatin: 30% DNA, 60% histone proteins, 10% RNA.

• Chromosomes: Highly condensed chromatin visible during cell division. Each chromosome consists of two identical sister chromatids joined at a centromere.

• Centromere: Region where chromatids are attached; contains the kinetochore, a protein structure for spindle fiber attachment during mitosis.

• Human Karyotype: Humans have 23 pairs of chromosomes (46 total).

Structure of DNA and RNA Nucleotides

Nucleotide Structure

Nucleotides are the building blocks of nucleic acids (DNA and RNA). Each nucleotide consists of three components:

• Phosphate group

• Pentose sugar: Deoxyribose in DNA, ribose in RNA

• Nitrogenous base: Purines (adenine, guanine), Pyrimidines (cytosine, thymine in DNA, uracil in RNA)

Bonding in Nucleic Acids

• Phosphodiester Bonds: Link the 5' phosphate group of one nucleotide to the 3' hydroxyl group of the next, forming the sugar-phosphate backbone.

• Hydrogen Bonds: Form between complementary nitrogenous bases (A=T, G≡C in DNA; A=U, G≡C in RNA).

Example: DNA sequence: 3'-AGCTTTCG-5' pairs with 5'-TCGAAAGC-3'

DNA vs. RNA

• DNA: Double-stranded helix, stable hereditary material, contains thymine.

• RNA: Usually single-stranded, contains uracil instead of thymine, can fold into complex structures.

The Genetic Code and Central Dogma

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a cell:

• Replication: DNA is copied to produce identical DNA molecules.

• Transcription: DNA is used as a template to synthesize RNA.

• Translation: RNA directs the synthesis of proteins.

Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Types of RNA

• Messenger RNA (mRNA): Carries genetic information from the nucleus to ribosomes.

• Ribosomal RNA (rRNA): Structural and enzymatic component of ribosomes.

• Transfer RNA (tRNA): Transfers specific amino acids to mRNA during translation.

• MicroRNAs: Regulate gene expression and defense mechanisms.

Genes and the Genome

• Gene: Sequence of DNA that encodes for a sequence of RNA, which may be translated into protein or have regulatory functions.

• Genome: All the DNA in a cell; human genome has ~20,000–25,000 genes across 23 pairs of chromosomes.

• Gene Length: Typical gene is ~3,000 base pairs (bp), but can range up to 2.4 million bp.

• Gene Content: Only about 2% of the genome codes for genes; the rest is structural, regulatory, or non-coding ('junk DNA').

The Genetic Code

The genetic code is a set of rules by which information encoded in DNA or RNA sequences is translated into proteins by living cells.

• Codons: Triplets of nucleotides in mRNA that specify amino acids.

• Start Codon: AUG (codes for methionine, initiates translation).

• Stop Codons: UAA, UAG, UGA (signal termination of translation).

Example:

• mRNA sequence: 5'-AUGCCAUAGCUG-3' translates to Met-Pro-Ile-Ala

• mRNA sequence: 5'-AUGCUGGUCAAAUGA-3' translates to Met-Leu-Val-Lys-Stop

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double	Single (usually)
Bases	A, T, G, C	A, U, G, C
Function	Genetic storage	Protein synthesis, regulation

Additional info: The notes above expand on brief points and images, providing academic context and definitions for all major terms and processes relevant to college-level Anatomy & Physiology.