Cells & Genetics: Chapter 3.6-3.7 Study Notes

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.

Nucleus, Chromatin, Chromosomes, and Nucleolus

Structure and Function of the Nucleus

The nucleus is a membrane-bound organelle found in eukaryotic cells. It serves as the control center of the cell, housing genetic material and regulating gene expression.

• Most cells have one nucleus.

• Red blood cells have no nucleus (anucleate).

• Muscle and liver cells may have multiple nuclei (multinucleate).

• The nucleus contains all the genes necessary for cellular function.

• It determines which proteins a cell will produce by selectively expressing sets of genes.

Structure of the Nuclear Envelope

The nuclear envelope is a double membrane that surrounds the nucleus, separating it from the cytoplasm.

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

• The outer membrane is continuous with the endoplasmic reticulum (ER).

• Ribosomes are embedded in the cytoplasmic side of the outer membrane.

• The inner membrane rests on an intermediate filament scaffold called the nuclear lamina.

• Nuclear pores facilitate active transport of RNA, proteins, and chemical messengers in and out of the nucleus.

Nucleoplasm and Nucleolus

The nucleoplasm is the semi-fluid matrix inside the nucleus, containing nutrients, nucleotides, and enzymes.

• The nucleolus is a dense region within the nucleus, functioning as a ribosome factory.

• It synthesizes ribosomal RNA (rRNA) and assembles ribosomal subunits.

Chromatin and Chromosomes

Chromatin is the complex of DNA, histone proteins, and RNA found in the nucleus. During cell division, chromatin condenses to form chromosomes.

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

• Each chromosome consists of two identical DNA sequences called sister chromatids, joined at a region called the centromere.

• The kinetochore is a protein structure at the centromere, serving as the attachment point for microtubules during mitosis.

• The human karyotype consists of 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:

• Pentose sugar (deoxyribose in DNA, ribose in RNA)

• Phosphate group

• Nitrogen-containing base

Phosphodiester and Hydrogen Bonds

• Phosphodiester bonds link the 5' carbon of one sugar to the 3' carbon of the next, forming the backbone of DNA and RNA.

• Hydrogen bonds form between complementary nitrogenous bases, stabilizing the double helix structure of DNA.

Nitrogenous Bases

• Pyrimidines: Cytosine (C), Thymine (T) (DNA only), Uracil (U) (RNA only)

• Purines: Adenine (A), Guanine (G)

DNA and RNA Structure

• DNA is typically a double helix, with two antiparallel strands held together by hydrogen bonds.

• RNA is usually single-stranded, but can form secondary structures by folding back on itself.

• Base pairing rules: A-T (DNA), A-U (RNA), G-C (both)

Example: DNA Sequence

5'-AGCTTTCG-3' 3'-TCGAAAGC-5'

Example: RNA Sequence

5'-UCGAA-3'

The Genetic Code and Central Dogma

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information in cells:

• 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.

Central Dogma Equation:

• DNA → RNA → Protein

Types of RNA

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

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

• Transfer RNA (tRNA): Transfers specific amino acids to the growing polypeptide chain during translation.

• MicroRNAs: Regulate gene expression and defend against viral RNA.

Genes and the Human Genome

• A gene is a sequence of DNA that encodes for a functional RNA or protein product.

• The human genome consists of all the DNA in a typical cell, spread across 23 pairs of chromosomes.

• Humans have approximately 20,000-25,000 genes, but only about 2% of the genome codes for proteins; the rest is regulatory or structural DNA.

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

The Genetic Code

The genetic code is a set of rules by which the sequence of nucleotides in mRNA is translated into the sequence of amino acids in a protein.

• Each group of three nucleotides (a codon) specifies one amino acid.

• There are 64 possible codons, coding for 20 amino acids and stop signals.

Example: mRNA Translation

Base Pairing and Codon Table

Codon tables are used to determine which amino acid corresponds to each mRNA codon during translation.

Summary Table: DNA vs. RNA

Additional info: The notes have been expanded with academic context and examples for clarity and completeness.