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The cell cycle

• Every cell has a life cycle—the period from the beginning of one division to the beginning of the next.

• The cell’s life cycle is known as the cell cycle.

• The cell cycle is divided into two periods: interphase and mitosis.

Interphase: the growing phase

• Interphase is the time span from one cell division to another.

• The Three Stages of Interphase 

  • Interphase can be divided into three stages: G1, S, G2.

• The most important phase is the S phase(replicates its genetic material.)

• During interphase, every single chromosome in the nucleus is duplicated.

  • These identical strands of DNA are now called sister chromatids

  • The chromatids are held together by a structure called the centromere.

  • You can think of each chromatid as a chromosome, but because they remain attached, they are called chromatids instead.

  • To be called a chromosome, each needs to have its own centromere.

  • Once the chromatids separate, they will be full-fledged chromosomes.

Cell Cycle Regulation

• G1 and G2- During these stages, the cell performs metabolic reactions and produces organelles, proteins, and enzymes.

• G stands for “gap,” but we can also associate it with “growth.”

• These three phases are highly regulated by checkpoints and special proteins called cyclins and cyclin-dependent kinases (CDKs).

• Cell cycle checkpoints are control mechanisms that make sure cell division is happening properly in eukaryotic cells.

• In eukaryotes, checkpoint pathways function mainly at phase boundaries (such as the G1/S transition and the G2/M transition).

• When damaged DNA is found, checkpoints are activated and cell cycle progression stops. The cell uses the extra time to repair damage in DNA. If the DNA damage is so extensive that it cannot be repaired, the cell can undergo apoptosis, or programmed cell death.

• Cell cycle checkpoints control cell cycle progression by regulating two families of proteins:

  • cyclin-dependent kinases (CDKs)

  • cyclins.

• To induce cell cycle progression, an inactive CDK binds a regulatory cyclin. Once together, the complex is activated, can affect many proteins in the cell, and causes the cell cycle to continue.

• To inhibit cell cycle progression, CDKs and cyclins are kept separate. CDKs and cyclins were first studied in yeast, unicellular eukaryotic fungi.

Cancer

• Cancer occurs when normal cells start behaving and growing very abnormally and spread to other parts of the body.

• Mutated genes that induce cancer are called oncogenes.

• They are genes that can convert normal cells into cancerous cell healthy version is called a proto-oncogene.

• Tumour suppressor genes produce proteins that prevent the conversion of normal cells into cancer cells. They can detect damage to the cell and work with CDK/cyclin complexes to stop cell growth until the damage can be repaired.

• They can also trigger apoptosis if the damage is too severe to be repaired.

Mitosis: the dance of the chromosomes

• Mitosis, or cellular division, occurs in four stages:

  • prophase, metaphase, anaphase, and telophase.

• During prophase, the nuclear envelope disappears and chromosomes condense.

  • Apindle apparatus  form. 

• Next is metaphase, when chromosomes align at the metaphase plate and mitotic spindles attach to kinetochores.

  • Chromosomes form a line in the middle

• In anaphase, chromosomes are pulled away from the center. 

  • Chromtins pull part of

• Telophase terminates mitosis, and the two new nuclei form.

  • The process of cytokinesis, which occurs during telophase, ends mitosis, as the cytoplasm and plasma membranes pinch to form two distinct, identical daughter cells.

• Interphase Once daughter cells are produced, they reenter the initial phase—interphase —and the whole process starts over. The cell goes back to its original state. Once again, the chromosomes decondense and become invisible, and the genetic material is called chromatin again.

Purpose of Mitosis

• Mitosis achieves two things:

  • The production of daughter cells that are identical copies of the parent cell maintaining the proper number of chromosomes from generation to generation

  • The impetus to divide occurs because an organism needs to grow, a tissue needs repair, or asexual reproduction must take place.

An Overview of Meiosis

• Meiosis is the production of gametes.

• Meiosis is limited to sex cells in special sex organs called gonads.

• In males, the gonads are the testes, while in females they are the ovaries.

• The special cells in these organs—also known as germ cells—produce haploid cells (n), and they combine to restore the diploid (2n) number during fertilization. female gamete (n) + male gamete (n) = zygote (2n)

Meiosis is likely to produce sorts of variations than is mitosis, which therefore confers selective advantage on sexually reproducing organisms.

A Closer Look at Meiosis

• Meiosis actually involves two rounds of cell division: meiosis I and meiosis II.

• Before meiosis begins, the diploid cell goes through interphase. Just as in mitosis, double-stranded chromosomes are formed during S phase.

• Meiosis I

  • Meiosis I consists of four stages: prophase I, metaphase I, anaphase I, and telophase I.

  • Meiosis I ensures that each gamete receives a haploid (1n) set of chromosomes.

• Prophase I

  • As in mitosis, the nuclear membrane disappears, the chromosomes become visible, and the centrioles move to opposite poles of the nucleus.

  • The major difference involves the movement of the chromosomes. In meiosis, the chromosomes line up side-by-side with their counterparts (homologs). This event is known as synapsis.

  • Synapsis involves two sets of chromosomes that come together to form a tetrad (a bivalent). A tetrad consists of four chromatids. Synapsis is followed by crossing-over, the exchange of segments between non sister-homologous chromosomes (One’s in the middle)

  • What’s unique in prophase I is that pieces of chromosomes are exchanged between homologous partners. This is one of the ways organisms produce genetic variation.

• Metaphase l

  • As in mitosis, the chromosome pairs—now called tetrads—line up at the metaphase plate.

  • By contrast, you’ll recall that in regular metaphase, the chromosomes line up individually.

  • One important concept to note is that the alignment during metaphase is random, so the copy of each chromosome that ends up in a daughter cell is random.

• Anaphase I

  • During anaphase I, each pair of chromatids within a tetrad moves to opposite poles. The homologs will separate with their centromeres intact.

  • The chromosomes now move to their respective poles.

• Telophase I

  • During telophase I, the nuclear membrane forms around each set of chromosomes.

  • Finally, the cells undergo cytokinesis, leaving us with two daughter cells.

• Meiosis II

  • The purpose of the second meiotic division is to separate sister chromatids

• During prophase II, chromosomes once again condense and become visible.

• In metaphase II, chromosomes move toward the metaphase plate. This time they line up single file, not as pairs.

• During anaphase II, chromatids of each chromosome split at the centromere, and each chromatid is pulled to opposite ends of the cell.

• At telophase II, a nuclear membrane forms around each set of chromosomes and a total of four haploid cells are produced.

Gametogenesis

• Meiosis is also known as gametogenesis.

• If sperm cells are produced, then meiosis is called spermatogenesis.

• If an egg cell or an ovum is produced, this process is called oogenesis.

• Oogenesis produces only one ovum, not four. The other three cells, called polar bodies, get only a tiny amount of cytoplasm and eventually degenerate since the female wants to conserve as much cytoplasm as possible for the surviving gamete, the ovum.

Meiotic Errors

• Nondisjunction—chromosomes failed to separate properly during meiosis.

• This error, which produces the wrong number of chromosomes in a cell, usually results in miscarriage or significant genetic defects.

• Individuals with Down syndrome have three—instead of two—copies of the 21st chromosome.

• Nondisjunction can occur in anaphase I (meaning chromosomes don’t separate when they should), or in anaphase II (meaning chromatids don’t separate).

  • Either one can lead to aneuploidy-the presence of an abnormal number of chromosomes.