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Biology - Mitosis and Cell Reproduction

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    Study Guide Biology – Mitosis and Cell Reproduction 1. The Cell Cycle: How Cells Grow and Divide Living cells are constantly growing, working, and reproducing. Most cells in the body repeat this pattern over and over in a process called the cell cycle . With a few exceptions — such as red blood cells, which do not have a nucleus — almost all cells go through this cycle. The cell cycle has two main stages : 1. Interphase – when the cell grows and carries out its normal functions 2. Mitosis – when the cell divides to form two new cells 1.1 Interphase: The Cell’s Busy Life Interphase is the longest part of the cell cycle. During this time, the cell is not resting — it is very active. Interphase is divided into three phases: G1, S, and G2 . 1.1.1 G1 Phase: Growth and Normal Work The G1 phase happens right after mitosis. During this phase, the cell grows and produces the proteins and enzymes it needs to do its specific job. For example: • A pancreas cell produces insulin • A muscle cell contracts to allow movement • A salivary gland cell produces digestive enzymes At this stage, each chromosome is made of one DNA molecule wrapped around histone proteins. • Normal human body cells have 46 chromosomes • Sex cells have 23 chromosomes • Red blood cells have no chromosomes because they lack a nucleus

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    Study Guide 1.1.2 S Phase: DNA Is Copied The S (synthesis) phase is when DNA replication occurs. During this phase: • Each chromosome is copied exactly • Every DNA molecule is duplicated By the end of the S phase, the cell contains twice the amount of DNA . In human cells, this means there are 92 chromosomes present, although they are still arranged in pairs. 1.1.3 G2 Phase: Preparing for Division In the G2 phase , the cell gets ready for mitosis. Special proteins assemble to form the spindle , a structure that helps move chromosomes during cell division. • The spindle is made of microtubules • It is built for each mitosis and taken apart afterward • The amino acids are reused by the cell Once preparation is complete, the cell enters mitosis. 1.2 Mitosis: Dividing the Nucleus The word mitosis comes from a Latin term meaning “threads,” because chromosomes look like thin threads when they become visible under a microscope. During mitosis: • Chromosomes become visible • They line up in the center of the cell • They separate evenly into two new cells Although mitosis is continuous, scientists divide it into four stages to make it easier to study: prophase, metaphase, anaphase, and telophase .

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    Study Guide 1.2.1 Prophase: Chromosomes Appear Mitosis begins with prophase . During this stage: • Chromatin condenses into visible chromosomes • Each chromosome has two identical copies , called sister chromatids • The chromatids are joined at a region called the centromere As prophase continues: • The nucleoli disappear • The nuclear envelope breaks down • Spindle fibers begin to form In animal cells: • Centrioles move to opposite ends of the cell • They form star - shaped structures called asters • Centrioles and asters are usually absent in plant and fungal cells Spindle fibers attach to the chromosomes at the kinetochore , a protein structure located on the centromere. Eventually, all chromosomes move toward the middle of the cell, known as the equatorial plate . 1.2.2 Metaphase: Chromosomes Line Up During metaphase , all chromosome pairs line up neatly along the equatorial plate (also called the metaphase plate ). In a human cell: • 92 chromosomes (46 pairs) are aligned • Each pair is still connected at the centromere • Spindle fibers remain attached at the kinetochore This careful alignment ensures each new cell will receive the correct number of chromosomes.

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