Eukaryotic Cell Cycle and M Phase

• Eukaryotic cells contain chromosomes located in the nucleus, which comprise chromatin

Eukaryotic Cell Cycle

• Cell cycle include interphase, and M phase(cell division)
• During the cell cycle, a cells grows, replicates DNA to prepare for division and the cell divides to form 2 daughter cells
• 2 main stages of cell cycle are interphase and M phase
• Interphase is the time between cell divisions when the cell grows and works
  • Interphase consists of G1, S phase, and G2
  • G1 phase (gap 1) - cell growth
  • S phase - DNA replicates
  • G2 phase (gap 2) - prepares to divide
• M phase (cell division) includes nucleus division (mitosis) and Cytokinesis (splitting of the cytoplasm)

M Phase

• In eukaryotes, cell division includes Mitosis and Cytokinesis
• Nucleus cell division occurs in 4 recognizable stages- Prophase, Metaphase, Anaphase, Telophase (PMAT)
• Cytokinesis is the division of the cytoplasm and follows PMAT

Prophase

• The duplicated condense and become visible during prophase, which is the first phase of mitosis.
• Centrioles move to opposite sides of the nucleus and help organize the spindle
• The spindle forms and DNA strands attach at a point called centromeres.
• The nucleolus disappears and the nuclear envelope breaks down.

Metaphase

• During metaphase, the centromeres of the duplicated chromosomes move across the equator of the cell
• Spindle fibers connect the centromere of each chromosome to the two poles of the spindle at the end

Anaphase

• During anaphase, sister chromatids separate into individual chromosomes and begin to move to opposite poles of the cell.
• Chromosomes are puled apart by spindle fibers

Telophase

• During telophase, the chromosomes spread out into a tangle of chromatin
• A nuclear envelope forms around each cluster of chromosomes
• Spindle fibers break apart, and a nucleolus becomes visible in each daughter nucleus.

Diploid Cells

• Cells that contain both sets of homologous chromosomes are diploid, meaning "two sets."
• The term "diploid number of chromosomes" is sometime represented as asymbol 2N
• A body cell in an adult fruit fly has eight chromosomes.
  • Four of the chromosomes come from its male parent, and four come from its female parent
  • These two sets of chromosomes are homologous, it means that each of the four chromosomes from each parent has a corresponding chromosome from female parent
• For fruit flies, the diploid number is 8 and is written as 2N = 8 where N twice the number of chromosomes in a sperm or egg cell

Haploid Cells

• Some cells contain only a single set of chromosomes, and therefore a single set of genes.
• Such cells are haploid, meaning “one set.”
• The gametes of sexually reproducing organisms are haploid.
• For fruit fly gametes, the haploid number is 4, which can be written as N = 4.

Phases of Meiosis

• Meiosis is a process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
• Meiosis usually involves two distinct divisions, called meiosis I and meiosis II.
• By the end of meiosis II, the diploid cell becomes four haploid cells.

Meiosis 1

• Prior to meiosis I, the cell undergoes chromosome replication called interphase I.
• Each replicated chromosome consists of two identical chromatids at the center

Prophase I

• Cells being to divide, and the chromosomes pair up
• As homologous chromosomes pair up, they undergo crossing-over.
  • First, the chromatids of the homologous chromosomes cross over one another
  • The crossed sections of the chromatids are exchanged
• Crossing-over produces new combinations of alleles in the cell

Meiosis II

• The two cells produced by meiosis I enter a second meiotic division
• Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II

Comparing Meiosis and Mitosis

• Mitosis is asexual reproduction, whereas meiosis is sexual reproduction.
• The sorting and recombination of genes in meiosis greater variety of possible gene combinations than mitosis
• Mitosis does not change the chromosome number of the original cell, Meiosis reduces the chromosome number by half
• Mitosis is a single cell division and produces two genetically identical diploid daugher cells. Meiosis requires 2 rounds of cell division and produces four genetically different haploid daughter cells