Cell Cycle and Chromosome Number

The Cell Cycle

• The cell cycle describes the stages of a single cell's life.
• Interphase is the longest phase of the cell cycle.
• The G1 stage is when the cell grows.
• The S stage is when DNA replicates, doubling the genetic material in a cell.
• The G2 stage is when the cell prepares to divide.
• Mitosis is the nuclear division that produces an exact replica of the nucleus and all of its chromosomes.
• Cytokinesis is the final stage of the cell cycle where the cytoplasm divides into two.

Chromosome Number

• Every species has a characteristic chromosome number, which is the number of chromosomes in its body cells, known as the diploid number.
• To create two identical copies of itself, a cell must first duplicate its nucleus.
• A parent cell with 46 chromosomes will produce two daughter cells, each with 46 chromosomes.
• DNA replication occurs during the S phase of interphase, and is required for a cell to make a copy of itself.
• Each chromosome consists of two DNA "arms" with a centromere in the center.
• Replication produces an identical copy of each "arm," which are still joined at the centromere and are called sister chromatids, which remain joined until mitosis.

DNA Replication

• DNA replication is carried out by enzymes.
• DNA polymerase helps assemble a DNA polymer.
• DNA helicase binds to a site on the double-stranded DNA and begins to separate the two strands.
• DNA polymerase moves along each strand, pairing free nucleotides to the nucleotides on the strand.
• Complementary base pairing rules ensure new strands are identical to old strands (Adenine (A) with Thymine (T), Cytosine (C) with Guanine (G)).
• Original DNA serves as a template for the new strand.
• Semiconservative replication means each "new" double strand consists of one original strand and one newly assembled strand.

Mitosis

• DNA is replicated during the S phase.
• Proteins are synthesized during the G2 phase.
• Mitosis is the process of duplicating the nucleus, and consists of four phases: prophase, metaphase, anaphase, and telophase.

Prophase

• The nucleus disappears.
• Chromosomes condense from chromatin.
• Centrioles begin to move toward the poles.

Metaphase

• Centrioles produce spindle fibers.
• Spindle fibers attach to the centromeres on the sister chromatids.
• Chromatids line up in the center of the cell.

Anaphase

• Spindles shorten, pulling sister chromatids apart.
• Chromatids from each pair move toward opposite poles.

Telophase

• New nuclear membranes form around the newly separated chromosomes.
• Chromosomes unwind, becoming longer and thinner.

Cytokinesis

• Cytokinesis is the process in which the cytoplasm, organelles, and plasma membrane of a cell divide to form two daughter cells.
• Plant cells must form a cell plate between the two nuclei during cytokinesis.

How Mitosis Ensures Correct Chromosome Number

• Each chromosome has replicated its DNA at the beginning of mitosis.
• Two identical sister chromatids are joined at the centromere.
• During metaphase, spindle fibers attach to the centromeres.
• The fibers are anchored to centrioles at opposite ends of the cell.
• When chromatids separate, each one is joined to one centromere.
• The chromatids are pulled to opposite sides of the cell, and each one becomes part of one new nucleus.
• This ensures each nucleus contains one copy of each chromosome.

Explore the essential stages of the cell cycle and understand the importance of chromosome number in cellular reproduction. This quiz covers interphase, mitosis, cytokinesis, and DNA replication, providing insights into how cells duplicate and divide. Test your knowledge on how these processes are fundamental to life.