![]() When the haploid sperm (n) and egg (n) combine during fertilization this forms a diploid zygote (2n). Cells with only one copy of each chromosome are haploid (n). Human sex cells (eggs and sperm) contain only one copy of each chromosome. Cells that contain two copies of each chromosome are called diploid (2n, where n is the number of different chromosomes in a single set). Each homologous pair consists of one maternal chromosome and one paternal chromosome. Human somatic cells contain pairs of homologous chromosomes. Interphase is followed by mitosis (in the somatic cells) or meiosis (in reproductive cells), which is when replicated chromosomes and cytoplasm separate, during the process of karyokinesis and cytokinesis respectively.Ĭhromosomes that are the same length, have the same centromere location and the same gene sequences and positions are called homologous chromosomes. Spindle fibers form from and radiate outward from the centrosomes to attach to and move chromosomes during cell division.įigure 2. In animal cells, interphase is also when the centrosome (consisting of two centrioles) is replicated. When the sister chromatids physically separate, later during the cell cycle, they are then considered to be individual chromosomes. While the two sister chromatids are physically joined together they are still considered one replicated chromosome (Figure 2). At this point in the cell cycle the sister chromatids are held together tightly at the centromere. By the end of S phase, each chromosome has made an exact copy and consists of two sister chromatids. At the beginning of S phase, chromosomes are single and unreplicated. In between the two gap phases, the DNA replicates in preparation for cell division. ![]() Interphase includes two gap phases, G1 and G2, where the cell increases in size and synthesizes new organelles, enzymes, and other proteins that are needed for cell division. The nuclear membrane is present, and visible, as is the nucleolus. During interphase, chromosomes are not visible because they are decondensed (present only as a tangled mass of thin threads of DNA with associated proteins, called chromatin). Cells spend most of their time in this phase. Interphase is the time during which the cell performs its normal functions and prepares for cell division. The first phase of the cell cycle is interphase. In eukaryotic cells, the time and phases from the beginning of one cell division until the beginning of the next cell division is called the cell cycle (Figure 1). ![]()
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