Introduction

Before discussing the steps a cell must undertake to replicate and divide its DNA, a deeper understanding of the structure and function of a cell’s genetic information is necessary. A cell’s DNA, packaged as a double-stranded DNA molecule, is called its genome. In eukaryotes the genetic material is packaged the nucleus. In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle located in the cell’s cytoplasm.

Learning

Prokaryotic Cells

In prokaryotic cells the region in the cell containing this genetic material is called a nucleoid. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth. Bacteria can exchange these plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA. Antibiotic resistance is one trait that often spreads through a bacterial colony through plasmid exchange from resistant donors to recipient cells.

Human Chromosomes and the Life Cycle

As mentioned previously, humans possess 23 pairs of chromosomes (46 total). 1 set of chromosomes is received from each individual’s mother (maternal chromosomes) and the other set is received from the father (paternal chromosomes). We will explore the inheritance of genes and chromosomes in the in more detail in another lesson.

Meiotic cell division leads to the formation of haploid (n) gametes (sperm or egg) which are fused together in the process of fertilization. A fertilized egg cell contains a complete set of maternal and paternal chromosomes and is therefore diploid (2n).  The fertilized egg is referred to as a zygote.

Mitotic cell division commences allowing the zygote to grow into an embryo, then a fetus, and subsequently into a new individual (baby). After birth, growth continues through continual mitotic cell division, each cell in the body receiving a full diploid set of chromosomes.

In adults, a specialized set of cells (ovaries in females, testes in males) are responsible for undergoing meiotic cell division to produce new gametes, preparing to start the life cycle all over again.

 Summary

• The principle that “every cell comes from another cell” is demonstrated in the cellular life cycle of any living organism.
• In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle called a nucleoid.
• In eukaryotes, humans for example, here are two types of cells; sex cells (the egg and the sperm) and somatic cells (everything else).
• Somatic cells divide by mitosis and sex cells divide by meiosis.
• The natural result of mitosis is the production of two genetically identical (somatic) cells.
• Mitosis is important for growth, development, replacement, and repair of most of the cells in our bodies.
• Meiosis is the process of gamete (sperm and egg) formation in which reproductive cells divide to yield four genetically different haploid cells (half the normal amount of genetic material).
• This process is important in providing genetic diversity and the proper amount of genetic material in offspring.
• When an egg fuses with a sperm, a zygote is created.
• This single cell then begins to divide by mitosis to create an embryo. The embryo continues to develop and once born will continue to develop until reproductive age when the organism’s life cycle can start again.

Sources:

“Genomic DNA.” By OpenStax Biology 2e. Retrieved from: https://openstax.org/books/biology-2e/pages/10-1-cell-division/ Licensed under: CC-BY: Attribution

Genomic DNA and Chromosomes. (2020, August 15). Retrieved May 20, 2021, from https://bio.libretexts.org/@go/page/13232