Chapter 9: The Cell Cycle – DNA Packaging and Cell Division

Chapter 9: The Cell Cycle

Learning Outcomes

This chapter focuses on the fundamental processes of the cell cycle, including DNA packaging, cell division in prokaryotes and eukaryotes, and the molecular basis of genetic inheritance. Students should be able to:

• Describe the chemical basis of life as it relates to the cell cycle

• Explain the structure and function of cells

• Understand the principles of genetics and evolution as they pertain to cell division

DNA Packaging

Genome and Chromosomes

The genome is defined as all of the DNA in an organism, including nuclear and organelle DNA (such as cpDNA for chloroplasts and mtDNA for mitochondria). In eukaryotes, DNA is organized into linear structures called chromosomes.

• Chromosome: A single, long DNA molecule associated with proteins, carrying genetic information.

• Gene: A segment of DNA that codes for a specific protein or functional RNA.

• Locus: The specific physical location of a gene on a chromosome.

Example: The human genome consists of 46 chromosomes (23 pairs), while the rice plant (Oryza sativa) has a circular chloroplast genome of 134,525 base pairs.

Levels of DNA Packaging

DNA must be efficiently packaged to fit within the nucleus and to facilitate proper segregation during cell division.

• Chromatin: The complex of DNA and proteins (mainly histones) that forms chromosomes within the nucleus.

• Nucleosome: The fundamental unit of chromatin, consisting of DNA wrapped around a core of eight histone proteins.

• Chromosome: Highly condensed chromatin, visible during cell division.

Folding proteins help maintain chromosome structure and ensure accurate DNA segregation.

Homologous Chromosomes

In diploid organisms, chromosomes exist in pairs called homologous chromosomes. Each pair contains one chromosome from the mother (maternal) and one from the father (paternal), carrying genes for the same traits at corresponding loci.

• Humans have 23 pairs of homologous chromosomes (46 total).

• The number of chromosomes varies among species and does not indicate organismal complexity.

Prokaryotic Cell Division

Binary Fission

Prokaryotes (such as bacteria) reproduce by a process called binary fission, a simple form of cell division.

• The circular DNA molecule is replicated.

• The two DNA molecules attach to different parts of the cell membrane.

• The cell elongates and the membrane pinches inward, dividing the cytoplasm.

• Two genetically identical daughter cells are produced.

Example: Escherichia coli divides by binary fission approximately every 20 minutes under optimal conditions.

Eukaryotic Cell Cycle

Overview of the Cell Cycle

The cell cycle is a regular, repeating sequence of events that includes cell growth, DNA replication, and cell division. It ensures that genetic material is accurately passed to daughter cells.

• Interphase: The period of cell growth and DNA replication (includes G1, S, and G2 phases).

• M phase (Mitosis and Cytokinesis): The period of nuclear and cytoplasmic division.

Phases of the Eukaryotic Cell Cycle

• G1 phase (Gap 1): Cell grows and performs normal metabolic functions.

• S phase (Synthesis): DNA is replicated; each chromosome is duplicated to form sister chromatids.

• G2 phase (Gap 2): Final preparations for mitosis; cell continues to grow and produce proteins.

• M phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus to produce two genetically identical daughter nuclei. It is followed by cytokinesis, which divides the cytoplasm.

Stages of Mitosis

1. Prophase: Chromatin condenses into visible chromosomes, each consisting of two sister chromatids joined at a centromere. The mitotic spindle begins to form.

2. Metaphase: The nuclear envelope breaks down, and chromosomes align at the metaphase plate (midplane) of the cell. Spindle fibers attach to kinetochores at the centromeres.

3. Anaphase: Sister chromatids separate at the centromere and are pulled to opposite poles of the cell by spindle fibers. Once separated, each chromatid is considered an individual chromosome.

4. Telophase: Nuclear envelopes reform around the two sets of chromosomes, which decondense back into chromatin. The spindle apparatus disassembles.

Cytokinesis

• Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells.

• In animal cells, a cleavage furrow forms to pinch the cell in two.

• In plant cells, a cell plate forms to separate the two new cells.

Chromosome Structure During Mitosis

• After DNA replication (S phase), each chromosome consists of two identical sister chromatids joined at a centromere.

• During anaphase, chromatids separate and are considered individual chromosomes.

Karyotype

A karyotype is an organized profile of an individual's chromosomes, typically captured during metaphase when chromosomes are most condensed and visible. Karyotyping is used to detect chromosomal abnormalities.

Summary Table: Key Differences in Cell Division

Feature	Prokaryotic Cell Division	Eukaryotic Cell Division
Process	Binary Fission	Mitosis (plus Cytokinesis)
DNA Structure	Circular DNA	Linear Chromosomes
Number of Chromosomes	Usually one	Multiple (species-specific)
Spindle Apparatus	Absent	Present
Outcome	Two identical cells	Two identical cells

Key Terms and Concepts

• Genome: All genetic material in an organism

• Chromosome: DNA-protein complex carrying genetic information

• Gene: DNA segment coding for a protein

• Homologous Chromosomes: Chromosome pairs with genes for the same traits

• Binary Fission: Prokaryotic cell division

• Mitosis: Eukaryotic nuclear division

• Cytokinesis: Division of cytoplasm

• Interphase: Cell growth and DNA replication phase

• Karyotype: Chromosome inventory

Additional info: The cell cycle is tightly regulated by checkpoints to ensure accurate DNA replication and division. Errors in these processes can lead to genetic disorders or cancer.