The Cell Cycle and Mitosis

Revision Draw and label an animal cell Give the function of nuclear pores, mitochondria, golgi apparatus, Endoplasmic reticulum What is the structure of DNA? How is DNA packaged?

Chromosome Structure Chromosomes are visible during cell division. Each chromosome consists of two chromatids joined somewhere along its length at the centromere. Genetic information (genes/alleles) carried on each chromatid is identical. Chromosome Centromere Chromatid

Identify the gender and condition of the individual with the following karyotypes. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Gender: Male (XY) Condition: Normal Gender: Male (XY) Condition: Gender: Condition: Gender: Female (XX) Condition: Trisomy 21 Gender: Female (XX) Condition: Gender: Condition:

The Cell Cycle The cell cycle describes the sequence of cell growth and cell division. It includes: Interphase cell growth synthesis of organelles DNA copying and checking of genetic information Mitosis chromosomes divide Cytokinesis cytoplasm divided between the daughter cells

… The Cell Cycle Nuclear division Interphase – Cytokinesis The cell cycle describes the events during one complete cycle of division, they are represented in the chart: Nuclear division This interphase and includes prophase, metaphase, anaphase and telophase Interphase – It is a period of intense activity to make new cellular structures Cytokinesis completes the cellular division DNA replication This must happen at some time during interphase

Cell Mitosis Mitosis is the formation of two new IDENTICAL daughter cells from an original cell. It is how organisms grow and is also the basis of simple Asexual reproduction. The interesting stuff happens In the nucleus. Cell Nucleus

Mitosis Mitosis is a continuous sequence but is divided into four stages: prophase metaphase anaphase telophase Essentially chromatids are separated by contraction of spindle fibres. chromatids are pulled to opposite poles of the cell. the cell then divides. Each chromatid contains identical genetic information so each daughter cell also contains identical information.

Nucleus The nucleus of all ordinary somatic (body) cells contains chromosomes. In diploid organisms (like us) there are two copies of each chromosome. In human body cells there are 23 pairs of chromosomes making a total of 46 chromosomes.

Early Prophase During prophase the chromosomes become more distinct they coil up shorten thicken take up stain more intensely The centriole divides Nucleolus becomes less prominent.

Late Prophase The chromosomes have become more distinct and are seen to consist of two chromatids joined by a centromere. The centrioles migrate to opposite poles of the cell. The nucleolus continues to shrink and disappears. The nuclear envelope disintegrates.

Mitosis – The following slides show stages in Allium Prophase (early) Chromosomes coil and condense Nuclear envelope present Nucleolus is evident Prophase (late) Chromosome clearly visible as two chromatids joined at the centromere Nuclear envelope disappears Nucleolus disappears

Metaphase Each centriole is at a pole. Centrioles grow/produce spindle fibres. Spindle fibres attach to the centromere of the chromosomes. Each centromere is attached to both poles. Chromosomes pulled to the metaphase plate or equator.

… Mitosis … Metaphase Spindle forms – some fibres attach to the centromeres, others run from pole to pole. Chromosomes are pulled to the equator of the cell (metaphase plate) by contraction of the fibres. Centromere splits and cell then enters anaphase.

Anaphase Spindle fibres contract. The centromere divides. Chromatids (daughter chromosomes) are pulled to opposite poles of the cell. Pulled centromere first. Each half of the cell receives one chromatid from each chromosome.

… Mitosis … Anaphase Chromatids move to opposite poles of the cell. They are pulled centromere-first by the contracting spindle fibres.

Telophase Chromatids reach the poles of the spindle. They begin to uncoil. They become less distinct. Nuclear envelope starts to reform. Q. What are the chromatids known as when they reach the poles of the spindle? Daughter chromosomes

… Mitosis … Telophase Chromatids (now often called daughter chromosomes) reach the poles of the spindle. Nuclear envelope reforms. Nucleolus reforms. Cell moves into cytokinesis or cell division.

Cytokinesis The cell divides! In animal cells: starts by constriction from the edges of the cell (invagination). In plant cells: a cell wall is laid down. Daughter cells have the same chromosome number and genetic makeup as each other and the parent cell – DNA replication precedes mitosis.

… Mitosis … Cytokinesis As shown here, in plant cells a cell wall is laid down in the position of the metaphase plate.

Interphase This stage comes between successive cell divisions. It is not really part of mitosis, but mitosis couldn’t happen without it. DNA replication occurs (allowing for the double stranded chromosome which later divides). Cellular structures are made (subsequently divided between the two daughter cells). A significant proportion of time is spent checking genetic information.

Mitosis Prophase Metaphase Interphase Anaphase Telophase

Prophase The chromosomes condense into visible, shorter and thicker threads. The chromosomes appear as two strands or chromatids joined by the centromere. During prophase in animal cells the two centrioles move towards opposite poles of the nucleus. Centrioles are absent in plant cells. At the end of prophase the double chromosomes start to move towards the middle (equator) of the nucleus The nuclear membrane fragments and disappears.

Metaphase This is a brief phase The chromosomes are arranged in the equatorial plane of the cell with their centromeres attached to the spindle fibres. In side view they appear as a line across the middle of the spindle.

Metaphase This is a brief phase The chromosomes are arranged in the equatorial plane with their centromeres attached byspindle fibres to the microtubule ends. In side view they appear as a line across the middle of the spindle.

Anaphase The double stranded chromosomes are pulled apart separating each chromatid. They are drawn towards the poles by the contracting spindles by their centromeres. By late anaphase the cell contains two equal groups of chromosomes each near the respective pole of the spindle.

Anaphase The double stranded chromosomes are pulled apart separating each chromatid. They are drawn towards the poles by the contracting spindles by their centromeres. By late anaphase the cell contains two equal groups of chromosomes each near the respective pole of the spindle.

Anaphase The double stranded chromosomes are pulled apart separating each chromatid. They are drawn towards the poles by the contracting spindles by their centromeres. By late anaphase the cell contains two equal groups of chromosomes each near the respective pole of the spindle.

Telophase The two sets of chromosomes (now just single chromatids) reach new poles and become enclosed in new nuclear membranes The chromosomes become longer and thinner and return to their interphase form Cytokinesis occurs- a cleavage furrow develops, deepens and eventually cuts the cell in two. The two new genetically identical cells then pass into the G1 phase of the cell cycle.

Telophase The two sets of chromosomes (now just single chromatids) reach new poles and become enclosed in new nuclear membranes The chromosomes become longer and thinner and return to their interphase form and the nucleoli reappear. Cytokinesis occurs- a cleavage furrow develops, deepens and eventually cuts the cell in two. The two new genetically identical cells then pass into the G1 phase of the cell cycle.

Cytokinesis Cytokinesis occurs- a cleavage furrow develops, deepens and eventually cuts the cell in two. The two new genetically identical cells then pass into the G1 phase of the cell cycle.

Cytokinesis Cytokinesis occurs- a cleavage furrow develops, deepens and eventually cuts the cell in two. The two new genetically identical cells then pass into the G1 phase of the cell cycle.

Identify the stage of mitotic division that the cells indicated by guidelines are in …. State a reason for your identification. B A C H F D E G A = Cytokinesis A B = Prophase (late) B C C = Prophase (early) D = Anaphase (late) D E = Anaphase (early) E F F = Metaphase G = Interphase G H = Telophase H And the correct order of the stages is: And the correct order of the stages is: (G) C B F E D H A (G)

Key points: DNA is copied during Interphase – NOT prophase! The four stages are VERY basically; a. Prophase – nuclear membrane disintegrates and doubled chromosomes condense, shorten and BEGIN to move to the equator. b. Metaphase – the chromosomes line up along the equator attached to spindles by their centromeres c. Anaphase – the chromatids of each replicated double chromosome are ripped apart at the centromere and pulled towards different ‘poles’. d. Telophase – chromosomes (chromatids) arrive at the poles and new nuclear membranes form followed by cytokinesis – actual cytoplasm splitting to form two new IDENTICAL daughter cells

Mitosis An overview