Biology Chapter: Cell Nucleus and Genetic Information

Learning Outcomes (1)

• Students will be able to explain the necessity for safe and reliable storage and transmission of cellular information.
• Describe the key structural features of the nucleus leading to the DNA double helix.
• Define the fundamental terms used in the lecture (e.g., gene, genome, etc.).
• Explain the chemical nature of polynucleotides.

Learning Outcomes (2)

• Briefly explain crucial experiments that led to identifying DNA as the genetic material (e.g., Griffith, Hershey-Chase).
• Explain the significance of the Watson-Crick DNA model from a functional perspective.
• Identify diverse DNA structures and their key characteristics.
• Define the enzyme type responsible for supercoiling and explain its importance in the process.

Introduction

• Secure and reliable storage and transfer of biological information are essential for cell growth and division, and species survival.
• Without effective information storage and transfer, the correct structural configuration of the macromolecule and the cell's energy production process do not provide a viable organism.

Flow of Information in Cells

• Information flow between cell generations occurs through DNA replication and cell division
• Information flow within a cell involves transcription (RNA synthesis), translation (protein synthesis), and the production of proteins.

Nucleus

• The nucleus houses the genetic material of eukaryotic cells.
• Chromosomes, chromatin, nucleosomes, and genes are structural components of the nucleus.

Basic Terminologies

• Nucleus: the organelle containing the eukaryotic cell's genetic material.
• Ribosomes: sites of protein synthesis.
• Mitochondria: sites of energy production.
• Golgi complex: modification and packaging of proteins.
• Lysosomes: involved in digestion and waste removal.
• Smooth endoplasmic reticulum: lipid synthesis and detoxification.
• Microfilaments: structural proteins, forming part of the cytoskeleton.
• Plasma membrane: outer boundary of the cell.
• Cytosol: cytoplasm, excluding the organelles.
• Nuclear envelope: boundary between the nucleus and cytoplasm, composed of two membranes.
• Nucleoplasm: fluid in the nucleus where solutes are dissolved.
• Nucleolus: site of rRNA synthesis
• Nuclear Pore Complex: involved in the transport of RNA in and out of the nucleus.
• Nuclear lamina: a network of proteins lining the inner surface of the nuclear envelope.

Chromosomes

• Thread-like structures composed of nuclear DNA.
• Carriers of genetic information visible during mitosis.
• Composed of chromatin fibres, DNA, and associated proteins.
• Individual chromosomes consist of single continuous DNA.
• Structural proteins called histones and non-histone chromosomal proteins compose them.

Nucleosomes

• Basic structural unit of chromosome organization.
• Consists of DNA wrapped around a core complex of histone proteins.
• These histone proteins (H2A, H2B, H3, and H4) form an octamer structure.
• Histone H1 is located exterior to the core particle and functions in stabilizing the nucleosomal structure.

DNA: The Transforming Material

• The use of chemical substances (and their nature) to define the transforming substance.
• In 1944, researchers excluded both protein and RNA as agents of bacterial transformation.
• Their findings confirmed DNA to be the transforming agent in bacterial transformation.

DNA Confirmation

• Geneticists initially doubted DNA's significance due to its repetitive base sequences
• By 1953, Watson and Crick proposed a double helix DNA model while Chargaff determined that the DNA bases are not equally distributed.
• Hershey and Chase's experiments further demonstrated that DNA is the genetic material.

Procedure for the Hershey-Chase Experiments

• Bacteriophages with radioactive DNA or protein were created.

The Chemical Nature of Polynucleotides

• Biochemists identified DNA components in the 1940s
• Components included nitrogenous bases (adenine, cytosine, guanine, and thymine), phosphoric acid, and deoxyribose sugar.

Nucleotides and Nucleosides

• RNA components comparable to DNA with uracil replacing thymine
• Carbons in sugars identified through primed numbers.
• Nucleotides contain phosphoric acid.
• Nucleosides lack phosphoric acid.

Purines and Pyrimidines

• Adenine and guanine are structurally related to purine
• Cytosine, thymine, and uracil resemble pyrimidine

DNA Linkage

• Nucleotides linked through phosphodiester bonds.
• Nucleotides can have one, two, or three phosphate groups.

A Trinucleotide

• Top of molecule has a free 5' phosphate group.
• Bottom has a 3' hydroxyl group.

Summary

• DNA and RNA's are chain-like molecules comprising nucleotides.
• Nucleotides contain a base attached to a sugar's 1' position along with a phosphate group.
• Phosphate forms phosphodiester bonds connecting the 5' and 3' hydroxyl groups in DNA or RNA chains.

X-ray Diffraction

• X-ray diffraction pattern of DNA fibers provided crucial information that suggests the double helix nature of DNA

Importance of the Watson-Crick Model

• DNA stores genetic information.
• Watson-Crick model plays a dominant role in storing genetic information, hence a gene encodes a protein containing specific instructions.

Replication and Duplication

• DNA contains instructions (information) for synthesis of new DNA strands.

Expression of the Genetic Message

• DNA information is used to determine the order of amino acids in protein synthesis.

Genes Made of RNA

• Viruses are gene packages without metabolic activity.
• Replication processes occur when viruses infect host cells.
• some viruses contain RNA genes instead of DNA genes.

DNA Sizes

• DNA size can be measured in base pairs, molecular weight, or length
• electron microscopy and gel electrophoresis methods can be used to measure DNA sizes

DNAs of Various Sizes and Shapes

• Phage DNA is generally circular
• Supercoiled DNA wraps around itself like a twisted band.

Supercoiled DNA

• Supercoiling a crucial process for compactness in both circular and linear DNAs (from prokaryotes and eukaryotes) in fitting into cells.
• Topoisomerases catalyze the interconversion of relaxed and supercoiled DNA forms

Relationship between DNA Size and Genetic Capacity

• Gene quantity cannot be determined by DNA size alone.
• Factors to consider include DNA arrangement within genes and the space between genes.

DNA Size and Genetic Capacity

• Gene calculation involves considering average protein size and assuming 3 DNAS per amino acid; then the number of nucleotides in a gene is determined

DNA Genetic Capacity

• Example sizes of various DNA molecules and chromosome segments

Summary

• Natural DNAs' sizes range from kilobases to megabases.
• Electron microscopy estimates small DNAs' sizes.
• Visualizing DNA form, whether linear or circular, can be determined by using electron microscopy.

Flow of Information in a Eukaryotic Cell

• DNA in chromosomes stores all genetic information.
• The DNA sections are transcribed, resulting in pre-mRNAs.
• pre-mRNA is then processed into messenger RNAs (mRNAs).

Test your knowledge on the function of the nucleus in eukaryotic cells, the relationship between genes and polypeptides, and the structure and function of DNA. This quiz covers essential concepts in genetics and molecular biology.