However, organisms inherit all of their mitochondrial DNA from the female parent. This occurs because only egg cells, and not sperm cells, keep their mitochondria during fertilization. DNA is made of chemical building blocks called nucleotides. These building blocks are made of three parts: a phosphate group, a sugar group and one of four types of nitrogen bases.
To form a strand of DNA, nucleotides are linked into chains, with the phosphate and sugar groups alternating. The four types of nitrogen bases found in nucleotides are: adenine A , thymine T , guanine G and cytosine C. The order, or sequence, of these bases determines what biological instructions are contained in a strand of DNA. The complete DNA instruction book, or genome, for a human contains about 3 billion bases and about 20, genes on 23 pairs of chromosomes.
DNA contains the instructions needed for an organism to develop, survive and reproduce. To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies. Each DNA sequence that contains instructions to make a protein is known as a gene. The size of a gene may vary greatly, ranging from about 1, bases to 1 million bases in humans.
Genes only make up about 1 percent of the DNA sequence. DNA sequences outside this 1 percent are involved in regulating when, how and how much of a protein is made.
Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell. Other chapters in Help Me Understand Genetics. Genetics Home Reference has merged with MedlinePlus. Learn more.
The somatic cells of humans, fruit flies, and many other animals and plants are diploid cells. A diploid cell contains pairs of homologous chromosomes. For example, since human cells have 23 different types of chromosomes, each cell receives 46 chromosomes, and after DNA replication, each cell has 92 chromatids.
When a human somatic cell undergoes mitosis , or cell division, the sister chromatids separate, resulting in two daughter cells with 46 chromosomes each. In organisms that reproduce sexually, the gametes reproductive cells like eggs and sperm are haploid cells.
This means they have only one copy of each chromosome—23 individual chromosomes, instead of 23 homologous pairs. Gametes are produced via meiosis , a process in which an original diploid cell divides into four haploid daughter cells. Most prokaryotic cells reproduce via cellular fission.
See more from our free eBook library. A video by the Amoeba Sisters about chromosomes and cell division. Definitions of diploid cells, haploid cells, and chromatids from the National Human Genome Research Institute. Eukaryotic Chromosomes. How do scientists visualize DNA? Figure This karyotype depicts all 23 pairs of chromosomes in a human cell, including the sex-determining X and Y chromosomes that together make up the twenty-third set lower right.
Watch this video for a closer look at the relationship between chromosomes and the DNA double helix. What are karyotypes used for? Who is James Watson? What do we know about Francis Crick? Topic rooms within Genetics Close. No topic rooms are there. Browse Visually. Other Topic Rooms Genetics. Student Voices. Creature Cast.
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