How does chromosome work

Copy Number Variation and Human Disease. Genetic Recombination. Human Chromosome Number. Trisomy 21 Causes Down Syndrome.

X Chromosome: X Inactivation. Chromosome Theory and the Castle and Morgan Debate. Developing the Chromosome Theory. Meiosis, Genetic Recombination, and Sexual Reproduction. Mitosis and Cell Division. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination. Sex Chromosomes in Mammals: X Inactivation.

Sex Determination in Honeybees. Citation: Clancy, S. Nature Education 1 1 How does DNA recombination work? It occurs frequently in many different cell types, and it has important implications for genomic integrity, evolution, and human disease. Aa Aa Aa. Figure 1: McClintock and Creighton's work in maize shows physical evidence of recombination.

Models of Recombination. Recombination Enzymes. Strand invasion by these 3' ssDNA overhangs into a homologous sequence is followed by DNA synthesis at the invading end. After gap-repair DNA synthesis and ligation, the structure is resolved at the HJs in a non-crossover black arrow heads at both HJs or crossover mode green arrow heads at one HJ and black arrow heads at the other HJ.

The repair product from SDSA is always non-crossover. Mechanism of homologous recombination: mediators and helicases take on regulatory functions.

Nature Reviews Molecular Cell Biology 7, References and Recommended Reading Barzel, A. Cell Research 18 , 99— Liu, Y. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction. Explore This Subject. Chromosome Analysis. Chromosome Structure. Mutations and Alterations in Chromosomes. Chromosome Number.

Chromosome Theory and Cell Division. Sex Chromosomes. Topic rooms within Chromosomes and Cytogenetics Close. No topic rooms are there. Or Browse Visually. Other Topic Rooms Genetics. Student Voices. Creature Cast. Simply Science. Green Screen. Green Science. Though the accessory chromosome was unpaired, it still replicated and entered stages of mitosis in the same manner as all other chromosomes, which prompted Sutton to declare it a "true chromosome" and not simply an accessory.

Sutton postulated that all chromosomes have a stable structure, or "individuality," that is maintained between generations, and he used this property to follow the behavior of individual chromosomes through the various stages of meiosis, including synapsis. Most notably, Sutton recognized that his observations were consistent with the , whose findings had only recently been rediscovered.

In fact, Sutton closed his paper with the statement, "I may finally call attention to the probability that the association of paternal and maternal chromosomes in pairs and their subsequent separation during the reducing division as indicated above may constitute the physical basis of the Mendelian law of heredity. Sutton subsequently went on to explain the basis for the ongoing variation among heritable traits. He noted that the position of each chromosome at the midline during metaphase was random, and that there was never a consistent maternal or paternal side of the cell division.

Each chromosome was, therefore, independent of the others. When they separated into gametes, the set of chromosomes in each daughter cell could contain a mixture of the parental traits, but not necessarily the same mixture as that of other daughter cells.

The newly discovered chromosomal independence during meiosis meant that the number of possible chromosomal combinations for each gamete could be calculated based on the number of chromosomes in the organism : specifically, there are 2 n possible combinations of chromosomes in gametes, with "n" representing the number of chromosomes in the gamete. Furthermore, considering all the possible pairings of one gamete with another, the variation in zygotes is 2 n 2 , which results in some fairly large numbers.

Indeed, Sutton provided examples of the potential variation among hypothetical organisms with gamete chromosome numbers ranging from 1 to 19 Table 1. He also correctly assumed that there was more than one trait present on each chromosome, so the actual total variation was even higher than any of those included in the table. During the early years of the twentieth century, Thomas Hunt Morgan and his colleagues at Columbia University identified hundreds of Drosophila genes and made many pivotal discoveries about genetic transmission.

Cytological examination showed that Drosophila possesses four pairs of chromosomes, including a pair of sex chromosomes. Female flies normally have two identical X chromosomes, whereas males have a single X chromosome and a smaller, gene-poor Y chromosome Figure 2. Unlike humans, however, sex in fruit flies is determined by the number of X chromosomes, rather than by the presence of the Y chromosome. One day, Morgan's associates discovered a male fly with unusual white eyes in their cultures.

Breeding experiments quickly established that the white eye color was caused by a recessive mutation in the gene responsible for normal red eye color, and furthermore, that the gene was probably located on the X chromosome. The chromosome theory predicted that male flies would always display the eye color encoded on their single X chromosome, but that female flies would develop white eye color only when they inherited two mutant versions of the eye color gene.

Thousands of matings confirmed this prediction, but on rare occasions, the group discovered "exceptional" white-eyed females among the progeny of a heterozygous female fly and a normal male, in apparent contradiction of the chromosome theory. Benson, K. Morgan's resistance to the chromosome theory. Nature Reviews Genetics 2 , — doi Bridges, C. Non-disjunction as proof of the chromosome theory of heredity.

Genetics 1 , 1—52 link to article. Brown, S. Entomological contributions to genetics: Studies on insect germ cells linked genes to chromosomes and chromosomes to Mendelian inheritance. Archives of Insect Biochemistry and Physiology 53 , — Cannon, W. Cytological basis for the Mendelian laws. Bulletin of the Torrey Botanical Club 29 , — Studies in plant hybrids: The spermatogenesis of hybrid cotton.

Bulletin of the Torrey Botanical Club 30 , — Mendel, G. Genetics 35 , 1—29 Sturtevant, A. A History of Genetics. Sutton, W. On the morphology of the chromosome group in Brachystola magna. Biological Bulletin 4 , 24—39 The chromosomes in heredity. Biological Bulletin 4 , — link to article. Wilson, E. Mendel's principles of heredity and the maturation of the germ cells. Science 16 , — Winkelmann, A. Wilhelm von Waldeyer-Hartz : An anatomist who left his mark.

Clinical Anatomy 20 , — Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities.

Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes. Telomeres of Human Chromosomes. Chromosomal Abnormalities: Aneuploidies. Chromosome Abnormalities and Cancer Cytogenetics. Copy Number Variation and Human Disease.

Genetic Recombination. Human Chromosome Number. Trisomy 21 Causes Down Syndrome. X Chromosome: X Inactivation. Chromosome Theory and the Castle and Morgan Debate. Besides the linear chromosomes found in the nucleus, the cells of humans and other complex organisms carry a much smaller type of chromosome similar to those seen in bacteria. This circular chromosome is found in mitochondria, which are structures located outside the nucleus that serve as the cell's powerhouses.

Scientists think that, in the past, mitochondria were free-living bacteria with the ability to convert oxygen into energy. When these bacteria invaded cells lacking the power to tap into oxygen's power, the cells retained them, and, over time, the bacteria evolved into modern-day mitochondria.

The constricted region of linear chromosomes is known as the centromere. Although this constriction is called the centromere, it usually is not located exactly in the center of the chromosome and, in some cases, is located almost at the chromosome's end. The regions on either side of the centromere are referred to as the chromosome's arms. Centromeres help to keep chromosomes properly aligned during the complex process of cell division.

As chromosomes are copied in preparation for production of a new cell, the centromere serves as an attachment site for the two halves of each replicated chromosome, known as sister chromatids. Telomeres are repetitive stretches of DNA located at the ends of linear chromosomes. They protect the ends of chromosomes in a manner similar to the way the tips of shoelaces keep them from unraveling.

In many types of cells, telomeres lose a bit of their DNA every time a cell divides. Eventually, when all of the telomere DNA is gone, the cell cannot replicate and dies. White blood cells and other cell types with the capacity to divide very frequently have a special enzyme that prevents their chromosomes from losing their telomeres. Because they retain their telomeres, such cells generally live longer than other cells. Telomeres also play a role in cancer.

The chromosomes of malignant cells usually do not lose their telomeres, helping to fuel the uncontrolled growth that makes cancer so devastating.

In fact, each species of plants and animals has a set number of chromosomes. A fruit fly, for example, has four pairs of chromosomes, while a rice plant has 12 and a dog, In humans and most other complex organisms, one copy of each chromosome is inherited from the female parent and the other from the male parent.

This explains why children inherit some of their traits from their mother and others from their father. The pattern of inheritance is different for the small circular chromosome found in mitochondria. Only egg cells - and not sperm cells - keep their mitochondria during fertilization. So, mitochondrial DNA is always inherited from the female parent.