BIOL 130 - Chapter 7 The Cell Cycle

what is the eukaryotic cell cycle?

need image slide 4

interphase (i): cell grows, performs routine life processes, and prepares to divide.

• the first gap (g1)

• synthesis (s)

• second gap (g2)

mitotic phase (yellow M)

• mitosis

• cytokinesis

  • two cells result

some cells do not divide and they enter into g0 phase

growth phase 1

• first step of interphase

• cell spends most of its life in the first gap (also referred as growth), where a cell undergoes rapid growth and performs its routine functions.

• during this phase, the biosynthetic and metabolic activities of the cell occur at a high rate.

• the synthesis of amino acids and hundreds of thousands or millions of proteins that are required by the cell occurs during this phase.

• proteins produced include those needed for dna replication.

• if the cell is not dividing, the cell enters the g0 phase from this phase.

g0 phase

• resting phase where the cell has left the cycle and has stopped dividing.

• non dividing cells in multicellular eukaryotic organisms enter g0 from g1.

• these cells may remain in g0 for long periods of time, even indefinitely, such as with neurons.

• cells that are completely differentiated may also enter g0.

• some cells stop dividing when issues of sustainability or viability of their daughter cells arise, such as with dna damage or degradation, a process called cellular senescence.

  • cellular senescence occurs when normal diploid cells lose the ability to divide, normally after about 50 cell divisions.

synthesis phase

• second step of interphase

• dividing cells enter the synthesis phase from g1.

• for two genetically identical daughter cells to be formed, the cell’s DNA must be copied through DNA replication.

• when the dna is replicated, both strands of the double helix are used as templates to produce two new complementary strands.

• these new strands then hydrogen bond to the template strands and two double helices form.

• during this phase, the amount of DNA in the cell has effectively doubled, though the cell remains in a diploid state.

growth phase 2

• the third step of interphase

• the second gap (growth) phase is a shortened growth period in which many organelles are reproduced or manufactured.

• parts necessary for mitosis and cell division are made during g2, including microtubles used in the mitotic spindle.

cell growth checkpoint

• occurs toward the end of growth phase 1

• checked whether the cell is big enough and has made the proper proteins for the synthesis phase

• if not, the cell goes through a resting period until it is ready to divide

dna synthesis checkpoint

• occurs during the synthesis phase

• checked whether dna has been replicated correctly

• if so, the cell continues on to mitosis

mitosis checkpoint

• occurs during the mitosis phase

• checks whether mitosis is complete

• if so, the cell divides and the cycle repeats

what happens if any checkpoints fail in the cell cycle?

• cancer may follow = unrestrained growth of cells

about mitosis

• except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin

• only when a cell is about to divide and its dna has replicated does dna condense and coil into the familiar x-shaped form of a chromosome, like the one shown

• because DNA has already replicated, each chromosome actually consists of two identical copies. the two copies of a chromosome are called sister chromatids. sister chromatids are joined together at a region called a centromere (2)

what are the stages of mitosis?

• the phase of the eukaryotic cell cycle that occurs between DNA replication and the formation of two daughter cells.

• four substages:

  • prophase

  • metaphase

  • anaphase

  • telophase

prophase

• first stage of mitosis

• also called prometaphase in later stage

• the spindle starts to form during the prophase of mitosis

• the spindles start to attach to the kinetochores of centromeres of sister chromatids during prometaphase

metaphase

• second stage of mitosis

• chromosomes consisting of sister chromatids line up at the equator or middle of the cell

• the blue lines are spindles and the orange rectangles at the cell poles are centrioles

• some spindles from opposing centrioles attach with each other and some spindles attach to the kinetochores of the sister chromosomes from their respective sides

• each chromosome is attached to two spindles

anaphase

• third phase of mitosis

• sister chromatids break apart and move to the opposite pole with the help of spindles

• the newly separated sister chromatids are called chromosomes now

telophase

• fourth stage of mitosis

• the chromosomes decondense back into chromatin, spindles start to disappear two nuclei form in a cell

cytokinesis

• final stage of eukaryotic cell division

• you can see a microfilament ring forming at the center of the elongated animal cell

• this creates a depression called cleavafe furrow

• this invagination ultimately separates the cell cytoplasm into two cells

meiosis

• the creation of non identical cells

• during this process, homologous chromosomes separate and go to different daughter cells.

• this diagram shows just the nuclei of the cells

• notice the exchange of genetic material that occurs prior to the first cell division, called “crossing over”

• this is the process that produces haploid gametes

• this is a type of cell division in which the number of chromosomes is reduced by half (haploid)

• it occurs only in certain special cells of an organism

• in mammals, this process occurs only in gamete producing cells within the gonads, ovaries and testicles, the sex organs.

• during meiosis, homologous (paired, diploid) chromosomes separate and haploid cells form that only have one chromosome from each pair.

making sperm v eggs

sperm

• mitosis, meiosis i, meiosis ii, differentiation

eggs

• similar except

• oocyte “arrests”, stops progressing in meiosis, before birth at prophase i

• the oocytes stay there until puberty when meiosis continues during ovulation

• if fertilized, meiosis will finish

• note the difference in size, the egg receives half of the genes but most of the cytoplasm and organelles

• the polar body is tiny

nondisjunction in meiosis

• occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number

trisomy 21 and karyotypes

• otherwise known as down syndrome

• a karyotype is a picture of a cell’s chromosomes

• there are two of each chromosome, but three copies of chromosome 21