Genetics Chapter 3: The Cell Cycle

Ferhat Abbas University, Setif 1

• Faculty of Natural and Life Sciences
• Department of Basic Studies
• 2nd Year LMD
• Module: Genetics
• Academic Year 2024/2025
• Module Coordinators: Dr Bouzid and Dr Ouaret-Guidoum Mona ([email protected])
• Dates: 27/10 - 07/11/2024

Chapter 3: The Cell Cycle in Eukaryotes

• Transmission of Genetic Traits During Mitosis and Meiosis
• Key concepts included Interphase and Mitosis

Concepts of Chromatin and Chromosome

• During cell division (mitosis or meiosis), chromatin coils and condenses into chromosomes, visible under a microscope.
• Outside of cell division, chromosomes unwind and decondense to form chromatin, dispersed in the nucleus.

Chromosome Morphology

• Each chromosome consists of two chromatids joined at a condensed region called the centromere.
• Chromosome arms extend from either side of the centromere.
• The shorter arm is called the "p arm," and the longer arm is called the "q arm."
• Telomeres are the natural ends of the chromosome.

Types of Chromosomes Based on Centromere Location

• During cell division, chromosomes appear in various sizes and shapes.
• Based on the centromere location, chromosomes are classified into metacentric, submetacentric, acrocentric, and telocentric.

Organization of DNA into Chromosomes

• DNA in eukaryotic cells is always associated with proteins, forming a structure called chromatin.
• There are different levels of DNA compaction.
• The first level is the nucleosome, a bead-like structure formed by DNA wrapped around histone proteins(2 H2A, 2 H2B, 2 H3, and 2 H4).

Second Level of Organization: Chromatin

• A 30 nm fiber, corresponding to the natural state of DNA in the nucleus.
• Nucleosomes wrap around themselves to form a solenoid, with the help of a histone protein H1.
• Highly condensed chromatin is called heterochromatin; less condensed is called euchromatin.

Third Level of Organization: The Chromatid

• The maximum supercoiling of DNA, occurring when the 30 nm chromatin fiber condenses to form a chromatid.
• Chromatin is organized into loops and helices around a central protein structure.

The Cell Cycle in Eukaryotes

• Unicellular and multicellular eukaryotes possess organelles and a cytoskeleton.
• DNA is compacted into chromosomes during cell division.
• Eukaryotes exhibit true sexuality.

Eukaryotic Cell Division Processes

• Multicellular organisms develop from a single fertilized egg.
• Chromosomes are present in a diploid number (2n) in somatic cells and a haploid number (n) in sexual cells.
• This indicates two nuclear division processes: Mitosis and Meiosis.
• Mitosis maintains the number of chromosomes and meiosis divides their number by half.

The Cell Cycle

• Eukaryotic cells go through repeating stages in the cell cycle.
• Each cell cycle includes:
  • A DNA synthesis phase (S phase, ~8 hrs). followed by.
  • A gap phase (G2 phase, ~4 hrs).
  • Mitosis (M phase, ~1 hr), followed by.
  • A gap phase (G1 phase, various duration).

Control of the Cell Cycle

• Every cell must maintain a memory of its phase in the cell cycle (G1 or G2).
• Cell cycle transitions (G1→S, G2→M) are controlled by specific cell cycle proteins.
• These proteins are encoded by numerous genes called cdc (cell division cycle).
• The G0 phase is a non-dividing state, which is a part of the interphase.

Interphase

• Interphase is the time between cell divisions.
• It is characterized by an increase in cell volume to accommodate more organelles for offspring.
• RNA transcription and enzyme secretion occur during the G1 phase. -DNA replication and chromosome duplication occur during the S phase.
• Further growth and preparation for mitosis occur during the G2 phase.

Mitosis (and Meiosis) Stages

• In eukaryotes most cells take 18-24 hours to complete a full cycle.
• The duration of each phase varies among cells, with mitosis always being the shortest.
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase (and cytokinesis).

Prophase(and Meiosis stages)

• Condensation of chromatin into chromosomes.
• Breakdown and fragmentation of the nuclear membrane.
• Migration of centrioles to opposite ends of the cell
• Formation of the spindle fibers.

Prometaphase

• Fragmented nuclear envelope
• Connection of kinetochores to microtubules.
• Chromosome movement resulting from tension

Metaphase

• Chromosomes align at the cell's equator (metaphase plate).
• Chromosomes are aligned according to their centromeres.

Anaphase

• Sister chromatids are "pulled apart" to opposite poles.
• The centrioles will migrate along their respective poles.

Telophase

• Chromosomes decondense into chromatin.
• New nuclear membranes are formed around the separated sets of chromosomes.
• The microtubules disappear.
• Cytokinesis occurs, separating the cytoplasm and organelles.

Meiosis

• Meiosis is a reductive cell division that produces gametes (sex cells) for sexual reproduction.
• Meiosis produces haploid cells from diploid cells (2n → n).
• Meiosis allows for genetic recombination to generate genetically distinct gametes.
• Meiosis involves two successive divisions (Meiosis I and Meiosis II).

Meiosis I Stages

• Prophase I: Homologous chromosomes pair up and align along their entire length, forming tetrads. -Zygotene, Pachytene, Diplotene, and Diakinesis are distinct stages of this phase.
• Metaphase I: Homologous chromosome pairs align along the metaphase plate.
• Anaphase I: Homologous chromosomes separate and move to opposite poles.
• Telophase I: Chromosomes arrive at opposite poles, and cytokinesis follows.

Meiosis II Stages

• Prophase II: Chromosomes condense, and the nuclear envelope breaks down if not already broken.
• Metaphase II: Individual chromosomes align along the metaphase plate.
• Anaphase II: Sister chromatids separate and move to opposite poles.
• Telophase II: Chromosomes decondense; nuclear envelope reforms, resulting in 4 genetically different haploid cells.

Relationship between chromatids and chromosomes

• Depending on the stage of division and the type of division, we can have different combinations of chromosomes and chromatids.

Two Types of Cells

• Somatic cells and germ cells (2n), and sex cells (gametes n).
• Examples provided comparing autosomal and sex chromosomes to show 2n and n.