zygotene - ορισμός. Τι είναι το zygotene
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Τι (ποιος) είναι zygotene - ορισμός

PROGRESSION THROUGH THE PHASES OF THE MEIOTIC CELL CYCLE, IN WHICH CANONICALLY A CELL REPLICATES VIA TWO NUCLEAR DIVISIONS
Meiotic; Meioses; Meosis; Mieosis; Zygotene; Leptotene; Pachytene; Diplotene; Diakinesis; Meiosis I; Meiosis 1; Meiosis II; Meiosis 2; Anaphase I; Prophase I; Telophase I; Prophase II; Metaphase II; Telophase II; Anaphase II; Diplonema; Meisois; Metaphase I; Tetrad (chromosomal formation); Reduction division; Haploid stage; Zygonema; Zygotene Stage; Pachynema; Meiome; Recombined; Prophase 1; Metaphase 1; Anaphase 1; Telophase 1; Prophase 2; Metaphase 2; Anaphase 2; Telophase 2; Meiotic division; Syzygy (meiosis); Meiotic prophase I
  • A video of meiosis I in a [[crane fly]] [[spermatocyte]], played back at 120× the recorded speed
  • In meiosis, the [[chromosome]]s duplicate (during [[interphase]]) and [[homologous chromosome]]s exchange genetic information ([[chromosomal crossover]]) during the first division, called [[meiosis I]]. The daughter cells divide again in [[meiosis II]], splitting up [[sister chromatid]]s to form haploid [[gamete]]s. Two gametes fuse during [[fertilization]], forming a diploid cell ([[zygote]]) with a complete set of paired chromosomes.
  • Meiosis Prophase I in mice. In Leptotene (L) the axial elements (stained by SYCP3) begin to form. In Zygotene (Z) the transverse elements (SYCP1) and central elements of the synaptonemal complex are partially installed (appearing as yellow as they overlap with SYCP3). In Pachytene (P) it's fully installed except on the sex chromosomes. In Diplotene (D) it disassembles revealing chiasmata. CREST marks the centromeres.
  • Overview of chromatides' and chromosomes' distribution within the mitotic and meiotic cycle of a male human cell

zygotene         
['z??g?(?)ti:n]
¦ noun Biology the second stage of the prophase of meiosis, following leptotene, during which homologous chromosomes begin to pair.
diplotene         
['d?pl?ti:n]
¦ noun Biology the fourth stage of the prophase of meiosis, following pachytene, during which the paired chromosomes begin to separate.
Origin
1920s: from diplo- + Gk tainia 'band'.
leptotene         
['l?pt?(?)ti:n]
¦ noun Biology the first stage of the prophase of meiosis, during which each chromosome becomes visible as two fine threads (chromatids).
Origin
early 20th cent.: from lepto- + Gk tainia 'band, ribbon'.

Βικιπαίδεια

Meiosis

Meiosis ( (listen); from Ancient Greek μείωσις (meíōsis) 'lessening', since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote.

Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities.

In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA may be cut and then repaired, which allows them to exchange some of their genetic information. A subset of recombination events results in crossovers, which create physical links known as chiasmata (singular: chiasma, for the Greek letter Chi (Χ)) between the homologous chromosomes. In most organisms, these links can help direct each pair of homologous chromosomes to segregate away from each other during meiosis I, resulting in two haploid cells that have half the number of chromosomes as the parent cell.

During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis. In some cases, all four of the meiotic products form gametes such as sperm, spores or pollen. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis.

Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi. It is an essential process for oogenesis and spermatogenesis.