Genetics, Agriculture, and Biotechnology (Suza and Lee)

Introduction

Multicellular organisms such as plants and animals are composed of millions to trillions (1,000,000,000) of cells that work together. The cells that make up different tissues have different shapes and perform different functions for the plant or animal. Even though they have diverse functions, each somatic cell in the organism normally has the same chromosomes and therefore the same genetic makeup. Furthermore, the millions of cells that makeup a mature organism originated from a single cell formed when the male and female gametes from the parents of the organism fused. This single cell established the life of the organism. Understanding multicellular organisms requires an understanding of the lifecycle of the cells that make up the organism.

The Cell Cycle

Let us think about the cell cycle from a personal point of view. Your age plus about nine months ago you were a zygote, a single cell formed when the sperm and egg from your biological parents fused in a fallopian tube (or possibly in a test tube if in vitro fertilization factored into your birth). You have come a long way since then, progressing one cell cycle at a time. The cell cycle is the life cycle of a single cell. We depict the cell cycle in a circular diagram although the cells in your body do not actually go around in circles. The main idea is that when new cells are made from existing cells, the new cells start their lifecycle, and the old cells end theirs.

Figure 1. The cell cycle depicts the stages in the life of a cell. Image by NIH-NHGRI.

The first part of the cell cycle is the G1 phase. Cells can go through growth and development in this phase. Some cells differentiate into specialized cells and then never leave this phase. However, a zygote does not grow in size, but instead continues the cell cycle so it can quickly give rise to more cells.

The second part of the cell cycle is the S phase (synthesis phase). Here, the cell replicates its chromosomes so that it has a copy of each chromosome to pass on to daughter cells. The third part of the cell cycle is the G2 phase where the cell prepares for division. The G1, S and G2 phases together are called interphase. The M phase completes the cell cycle. ’M’ could be mitosis or meiosis depending on the type of cell. For the zygote, the goal is to make more somatic cells. Therefore, it goes through mitosis and gives rise to two daughter cells. This completes the life cycle of the zygote and starts the lifecycle of the new cells. Rounds of the cell  Learning Objectives 1.1.2 https://bio.libretexts.org/@go/page/73666 cycle continued over and over to form the body you have today. As long as you live, some of your cells must be able to complete the cell cycle.

From a cytogenetics point of view, you have two types of cells in your body. You have cells with 46 chromosomes (somatic or body cells) and cells with 23 chromosomes (gamete or sex cells). Since you started as a single cell with 46 chromosomes, there must be two types of cell division taking place in your body to accommodate both somatic and gamete cells. Mitosis and meiosis are the two types of cell division.