In sexual reproduction, organisms inherit half of their nuclear DNA from the male parent and half from the female parent. 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. This fiber is further coiled into a thicker and more compact structure. At the metaphase stage of mitosis, when the chromosomes are lined up in the center of the cell, the chromosomes are at their most compacted.
They are approximately nm in width, and are found in association with scaffold proteins. In interphase, the phase of the cell cycle between mitoses at which the chromosomes are decondensed, eukaryotic chromosomes have two distinct regions that can be distinguished by staining.
There is a tightly packaged region that stains darkly, and a less dense region. The darkly staining regions usually contain genes that are not active, and are found in the regions of the centromere and telomeres. The lightly staining regions usually contain genes that are active, with DNA packaged around nucleosomes but not further compacted.
Concept in Action. Watch this animation of DNA packaging. The DNA molecule is a polymer of nucleotides. Each nucleotide is composed of a nitrogenous base, a five-carbon sugar deoxyribose , and a phosphate group. There are four nitrogenous bases in DNA, two purines adenine and guanine and two pyrimidines cytosine and thymine. A DNA molecule is composed of two strands. Each strand is composed of nucleotides bonded together covalently between the phosphate group of one and the deoxyribose sugar of the next.
From this backbone extend the bases. The bases of one strand bond to the bases of the second strand with hydrogen bonds.
Chromosome 21, the shortest human chromosome, consists of 48 million base pairs and contains between and genes. Overall, prokaryotic cells have shorter chromosomes with fewer genes.
For example, the bacterium Carsonella rudii has only , base pairs and genes in its entire genome. These are called intergenic regions. Even within genes, there are regions of noncoding DNA called introns. Noncoding regions of DNA are important because they provide binding sites for proteins that help activate or deactivate the process of transcription. They can also provide protection for the coding regions.
For instance, telomeres consist of repetitive sequences that protect the genetic information on each DNA molecule from being damaged during cell division.
See more from our free eBook library. An article from Nature about the genome of Carsonella rudii. Eukaryotic Chromosomes. Prokaryotic Chromosomes. Figure 7: To better fit within the cell, long pieces of double-stranded DNA are tightly packed into structures called chromosomes.
What does real chromatin look like? Compare the relative sizes of the double helix, histones, and chromosomes. Figure 8: In eukaryotic chromatin, double-stranded DNA gray is wrapped around histone proteins red. Figure 9: Supercoiled eukaryotic DNA. 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.
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