Each copy of the homologous pair of chromosomes originates from a different parent; therefore, the copies of each of the genes themselves may not be identical.
The variation of individuals within a species is caused by the specific combination of the genes inherited from both parents. For example, there are three possible gene sequences on the human chromosome that codes for blood type: sequence A, sequence B, and sequence O. Because all diploid human cells have two copies of the chromosome that determines blood type, the blood type the trait is determined by which two versions of the marker gene are inherited.
It is possible to have two copies of the same gene sequence, one on each homologous chromosome for example, AA, BB, or OO , or two different sequences, such as AB. Minor variations in traits such as those for blood type, eye color, and height contribute to the natural variation found within a species.
The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosomes; other than a small amount of homology that is necessary to reliably produce gametes, the genes found on the X and Y chromosomes are not the same. Prokaryotes have a single loop chromosome, whereas eukaryotes have multiple, linear chromosomes surrounded by a nuclear membrane.
Human somatic cells have 46 chromosomes consisting of two sets of 22 homologous chromosomes and a pair of nonhomologous sex chromosomes.
This is the 2 n, or diploid, state. Human gametes have 23 chromosomes or one complete set of chromosomes. This is the n or haploid state.
Genes are segments of DNA that code for a specific protein. Duplicated chromosomes are composed of two sister chromatids. Chromosomes are compacted using a variety of mechanisms during certain stages of the cell cycle. Several classes of protein are involved in the organization and packing of the chromosomal DNA into a highly condensed structure. The condensing complex compacts chromosomes, and the resulting condensed structure is necessary for chromosomal segregation during mitosis.
Eukaryotic chromosomes are thousands of times longer than a typical cell. Explain how chromosomes can fit inside a eukaryotic nucleus. Human somatic cells have 46 chromosomes: 22 pairs and 2 sex chromosomes that may or may not form a pair. This is the 2 n or diploid condition. Human gametes have 23 chromosomes, one each of 23 unique chromosomes, one of which is a sex chromosome. This is the n or haploid condition.
Skip to main content. Cell Division and Cell Cycle. Search for:. Cell Division Learning Objectives By the end of this section, you will be able to: Describe the structure of prokaryotic and eukaryotic genomes Distinguish between chromosomes, genes, and traits Describe the mechanisms of chromosome compaction.
Link to Learning This animation illustrates the different levels of chromosome packing:. Additional Self Check Questions 1. Compare and contrast a human somatic cell to a human gamete. Answers 1. Each chromosome contains hundreds and sometimes thousands of genes, segments of DNA that code for a polypeptide or RNA, and a large amount of DNA with no known function.
The DNA double helix is wrapped around histone proteins to form structures called nucleosomes. Nucleosomes and the linker DNA in between them are coiled into a nm fiber. During cell division, chromatin is further condensed by packing proteins.
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