Deoxyribonucleic acid (DNA) is the blueprint for life. Inside the nucleus of a microscopic eukaryotic cell, chromosomal DNA stores all the instructions needed for making a full-fledged adult organism.
Nuclear DNA is organized into chromosomes; humans have 46 total per cell. Haploid vs. diploid refers to the number of chromosomes and chromosome sets present in the cell.
How Does DNA Work?
DNA is comprised of four chemical bases: adenine (A), guanine (G), cytosine (C) and thymine (T). Adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G). The bases attach to a sugar and phosphate molecule, forming nucleotides arranged in a double-stranded helix molecule of DNA. The sequence of nucleotides tells the cells what to do.
Each strand of DNA copies itself during cell division. The nucleus won’t give the signal to divide until genetic material in stringy chromatin has finished replicating. Sister chromatids condense and line up in the middle of the cell. Spindle fibers pull apart the chromosomes, and two daughter cells result from the process of mitosis.
What Are Homologous Chromosomes?
Homologous chromosomes are chromosome pairs that are alike in size and shape; one set is inherited from the mother, and the other set from the father.
These homologues have the same genes in the same location, although gene alleles on the chromosomes differ slightly. Gene swapping occurs in meiosis, which is how siblings may have different eye and hair color.
Understanding Chromosome Sets
Learning word definitions in introductory cell biology provides a solid foundation for comprehending more advanced genetics. Terminology seems a little confusing at first, but it makes more sense when you see how it all fits together. Unusual words like “ploidy” are a good starting point when diving into the study of a cell’s DNA and life cycle.
Ploidy refers to the number of chromosomes present in the cell. Simple organisms like bacteria only have a ringlet of DNA instead of linear chromosomes. Multicellular life forms have sets of homologous chromosomes that replicate in the nucleus, pair up and divide during mitosis and meiosis.
Haploid cells, symbolized by the variable n, possess one set of chromosomes. Gametes, or sex cells, are haploid. Bacteria can be haploid organisms. Chromosomes in haploid cells contain one gene allele (copy) for a particular trait.
Diploid cells, symbolized by 2n, possess two sets of chromosomes. Somatic (body) cells are diploid. Chromosomes contain two gene alleles (copies) for inheritable traits. Two haploid gametes result in a diploid zygote.
You'll also read about polyploid cells, which are other ploidys such as triploid (3n) and hexaploid (6n) in plants and animals. For instance, certain species of cultivated wheat have three sets of chromosomes (3n) or even six sets of chromosomes (6n). Extra copies of chromosomes are beneficial to some organisms but potentially fatal to others, depending on how regulatory genes are affected.
What Do Haploid and Diploid Mean?
The life stages of a cell include interphase, cell division, cytokinesis and death. As part of the life cycle, the cell may divide asexually by mitosis or sexually through meiosis. The simpler type of cell division is mitosis, which doesn’t involve gene recombination.
Diploid cells have two sets of chromosomes (2n). That means there are two homologous chromosomes present in each cell. Most somatic cells in the body are diploid. Differentiated somatic cells (2n) grow and divide by mitosis into daughter cells (2n).
Haploid cells have one set of chromosomes (n), meaning there are no homologous chromosomes. Just one set is present. Reproductive cells are haploid and carry half the number of chromosomes as somatic diploid cells. When two haploid gametes get together, they form a diploid cell that can grow by mitosis.
Why Are Diploid Cells Important?
Most cells in the body are diploid. In humans that means they have two sets of 23 chromosomes in the nucleus of the cell. Non-reproductive cells, also called somatic cells, contain all your chromosomal genetic information – not just half of it. Diploid cells carry out most of the body’s functions.
Diploid cells reproduce by mitosis, creating two identical daughter cells. Mitosis is a means of fast and efficient non-sexual cell division. Mitosis is particularly important for cell growth and tissue healing. Epithelial cells are continually shed and replaced thanks to mitosis.
Why Are Haploid Cells Important?
Haploid cells are important for sexual reproduction. Living organisms have adapted a number of clever ways to ensure survival of the species even in the harshest environments. Haploid organisms have one set of chromosomes and only reproduce asexually. Humans have haploid reproductive cells.
Haploid cells are produced by meiosis and contain just one set of chromosomes. During reproduction, two haploid cells (ovum and sperm) merge. Each provides one set of chromosomes to create a diploid cell. Embryo development proceeds under conditions conducive to growth.
The human genome is comprised of 46 chromosomes; 23 chromosomes originating from the mother and 23 from the father. Sexual reproduction through meiosis gives rise to variations within a population that makes some organisms more fit than others to handle prevailing conditions. If genes did not recombine in meiosis, the new plant or animal would be a clone of itself.
Diploid vs. Triploid Organisms
Many triploid organisms can exist quite well with extra chromosomes. Salmon, salamanders and goldfish are among the triploid animal species possessing three sets of chromosomes. Oysters sold as food have two or three chromosome sets.
Triploid oysters are especially tasty, fast-growing and disease-resistant. However, they are also sterile.
Fisheries initially induced triploidy through chemical exposure, heat or pressure. Scientists at Rutgers then developed tetraploid oysters that could fertilize diploid oyster eggs to produce the more commercially desirable triploid oysters. The process is chemical-free and doesn’t involve gene modification.
Alternating Generations in Plants
Life cycles of plants can include both a haploid and diploid stage. For example, diploid ferns growing in the forest release haploid spores into the air from the underside of fronds. Spores develop into gametophyte plants with reproductive parts that produce haploid sperm and eggs.
In the presence of moisture for mobility, a sperm fertilizes an egg, and the zygote (diploid cell) grows by mitosis into a new fern.
Stages in Cell Division
Organisms can be broadly categorized as eukaryotic or prokaryotic largely based on whether a nucleus containing DNA is present. In eukaryotic organisms, DNA and histones (proteins) coil together, forming chromosomes.
Each chromosome in a diploid cell is part of a homologous pair. Reproductive germ cells are diploid like somatic cells until they undergo the reductionist process of meiosis to form sperm and an egg.
Chromosomes replicate in the first stage of meiosis and become sister chromatids joined together at the centromere. Next, sister chromatids find their homologous counterpart and exchange bits of DNA before the parent cell divides into two haploid daughter cells. In the second stage of meiosis, chromosomes in the daughter cells divide, giving rise to four haploid cells.
Cell Division Missteps
Although mistakes in chromosomal replication and separation are generally corrected at cell division checkpoints, serious errors may can still occur, causing mutations, tumors or gene impairment.
When chromosomes do not separate correctly, one cell can end up with an extra chromosome. This can cause genetic disorders. For example, if you have an extra copy of chromosome 21, you have what's referred to as Down syndrome.
Organisms that inherit chromosomes from two different species commonly have an atypical number of chromosome sets and may be sterile.
- Oxford Academic: GigaScience: The First Near-Complete Assembly of the Hexaploid Bread Wheat Genome, Triticum Aestivum
- National Human Genome Research Institute: Chromosomes
- U.S. National Library of Medicine: Genetics Home Reference: What Is DNA?
- National Park Service: Ferns
- Element Seafood: Love, Sex and Oysters: What Is a Triploid?
- CK-12: Sexual Reproduction