A Wisdom Archive on homologous chromosomes

Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle that mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. Interphase is divided into three phases: Growth 1 (G1) phase: Characterized by increasing cell size from accelerated manufacture of organelles, proteins, and other cellular matter. Synthesis (S) phase: The genetic material is replicated. Growth 2 (G2) phase: ...

Read more here: » Meiosis: Encyclopedia II - Meiosis - Process

In biology, meiosis is the process that transforms one diploid cell into four haploid cells in eukaryotes in order to redistribute the diploid's cell's genome. Meiosis forms the basis of sexual reproduction and can only occur in eukaryotes. In meiosis, the diploid cell's genome, which is composed of ordered structures of coiled DNA called chromosomes, is replicated once and split twice, producing four sets of haploid cells each containing half of the original cell's chromosomes. These resultant haploid cells will fertilize with ... Including:

• Meiosis - Occurrence of meiosis in eukaryotic life cycles
• Meiosis - Chromosome segregation in meiosis
• Meiosis - Process
• Meiosis - Meiosis I
• Meiosis - Meiosis II
• Meiosis - Significance of meiosis
• Meiosis - Nondisjunction
• Meiosis - Meiosis in humans

Read more here: » Meiosis: Encyclopedia - Meiosis

As a diploid cell is formed from the genetic recombination of two haploid cells, within a diploid cell there are normally two sets of chromosomes that code for the same information, one from the mother's haploid cell and the other from the father's. These chromosomes are called homologous chromosomes. Homologous chromosomes need not be genetically identical. For example, one particular locus (location) on one of the father's chromosomes may code for green eyes, while the same locus on the mother's chromosome may code for brown eyes. This genetic var ...

Read more here: » Meiosis: Encyclopedia II - Meiosis - Chromosome segregation in meiosis

A diploid cell contains a full set of chromosome pairs, each pair containing one chromosome from each parent. These chromosome pairs are called homologous chromosomes. Homologous chromosomes need not be genetically identical. For example, one particular locus (location) on one of the father's chromosomes may code for green eyes, while the same locus on the mother's chromosome may code for brown eyes. This genetic var ...

Read more here: » Meiosis: Encyclopedia II - Meiosis - Chromosome segregation in meiosis

Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes than the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count, resulting in an unwieldy genome that would cripple the reproductive fitness of the species. Polyploidy, the state of having three or more sets of chromosomes, also r ...

Read more here: » Meiosis: Encyclopedia II - Meiosis - Significance of meiosis

The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed disjunction. When the separation is not normal, it is called nondisjunction. This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II phases of ...

Read more here: » Meiosis: Encyclopedia II - Meiosis - Nondisjunction

In females, meiosis occurs in precursor cells known as oogonia that divide twice into oocytes. These stem cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The proce ...

Read more here: » Meiosis: Encyclopedia II - Meiosis - Meiosis in humans

Meiosis occurs in all eukaryotic life cycles involving sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. The organism will then produce the germ cells that continue in the life cycle. The rest of the cells, called somatic cells, function within the organism and will die with it. The organism phase of the life ...

Read more here: » Meiosis: Encyclopedia II - Meiosis - Occurrence of meiosis in eukaryotic life cycles

In genetics, homology is used in reference to protein or DNA sequences, meaning that the given sequences share a common ancestor. Sequence homology may also indicate common function. Homology is an all-or-nothing quality; there is no such condition as "degrees of homology." Sequence regions that are homologous may be called conserved, consensus or canonical sequences and represent the most common choice of base or amino acid at each position. Homology among proteins and DNA is often concluded on the basis of seque ...

Read more here: » Homology biology: Encyclopedia II - Homology biology - Homology of sequences in genetics

 
Definition and meaning of homologous chromosomes:

 
homologous chromosomes - the pair of chromosomes in a diploid individual that have the same overall genetic content. One member of each homologous pair of chromosomes is inherited from each parent

(Source: US National Oceanic and Atmospheric Administration (NOAA) )
 
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Interspecific hybrids are bred by mating two species, normally from within the same genus. The offspring display traits and characteristics of both parents. The offspring of an interspecific cross may be sterile. Sterility is attributed to the different number of chromosomes the two species have, for example donkeys have 62 chromosomes, while horses have 64, mules have 63. Mules and other sterile interspecific hybrids cannot produce viable gametes because the extra chromosome cannot make a homologous pair at meiosis, meiosis is dis ...

Read more here: » Hybrid: Encyclopedia II - Hybrid - Interspecific hybrids

Main article: Chromosomal crossover Crossing over of one of the chromosomes inherited from each of one's parents occurs during meiosis in that parent. After chromosomal replication during gametogenesis, the four available chromatids are in tight formation with one another. During this time, homologous sites on two chromatids can mesh with one another, and may exchange genetic information. Immediately after replication, the tetrad formed by replication contains two pairs of two identical chromatids; after crossing over, each of the four chromatids carries a unique set of genetic information.

Read more here: » Genetic recombination: Encyclopedia II - Genetic recombination - Crossing over

A gene is only a DNA code sequence; the slightly different variations of that sequence are called alleles. Alleles can be significantly different and produce different product RNAs. Combinations of different alleles thus go on to generate different traits through the information flow charted above. For example, if the alleles on homologous chromosomes exhibit a "simple dominan ...

Read more here: » Trait: Encyclopedia II - Trait - Mendelian expression of genes in diploid organisms

The heritable unit that may produce a trait is called a gene. A gene is a strand of DNA that is part of a very long and compacted string of DNA called a chromosome. An important reference point along this string is the centromere; the distance from a gene to the centromere is referred to as the gene's locus or map location. A chromosomal region known to control a trait while the responsible gene within not being identified is referred to as a quantitative trait locus. The nucleus of a diploid cell contains two of each chromosome, with homologous (mostly ...

Read more here: » Trait: Encyclopedia II - Trait - Genetic origin of traits in diploid organisms

Episomes are plasmids that can integrate themselves into the chromosomal DNA of the host organism (Fig. 3). For this reason, they can stay intact for a long time, be duplicated with every cell division of the host, and become a basic part of its genetic makeup. This term is no longer commonly used for plasmids, since it is now clear that a regions of homology with the chromosome such as a transposon makes a plasmid into an episom ...

Read more here: » Plasmid: Encyclopedia II - Plasmid - Episomes