Cell Reproduction

Need For Reproduction Reproduction is the life process in which living things produce other living things of the same species. Reproduction is not necessary for the life of one individual organism. It is necessary for the continued survival of a particular group of organisms.

Cell Division All cells arise from other cells by cell division. When cells grow to a certain size they must either divide or die. If a cell continued to grow without dividing, the surface area of the cell would become too small to hold the cell’s contents. (Problem of Surface Area/Volume Ratio)

Mitosis Cell Division Cell division is a complex series of changes in the nucleus of a cell that leads to the production of two new cells. The new cells are called daughter cells. The nuclei of the daughter cells are identical to each other and to that of the parent cell. The daughter cells grow and increase in size until they divide and produce two more daughter cells. This process continues, resulting in organism growth and reproduction.

Mitosis All cells in the body (except sex cells) are produced by the process of mitotic cell division. Mitosis involves a complex series of changes in the nuclei of body cells that produce identical (same) daughter cells. They have the same number and type of chromosomes as the parent cells. Primary purpose is to increase the number of cells Daughter cells are genetically identical to the parents Occurs during growth and asexual reproduction.

The Cell Cycle – Stages of Mitosis Although the events of mitosis are an ongoing process, they are generally described in terms of separate phases, or stages. Interphase Prophase Metaphase Anaphase Telophase

Interphase Interphase is the period between cell divisions. During interphase, the single-stranded chromosomes replicate (make an extra copy). The chromosomes can not be seen during interphase.

Interphase Sometimes called the “resting stage” between divisions BUT cells are metabolically very active The amount of DNA in the nucleus doubles New organelles such as mitochondria are made.

Prophase In prophase, the double- stranded chromosomes become visible, and the nuclear membrane disappears. A spindle apparatus, consisting of fibers, forms between opposite poles of the cell.

Prophase Protein microtubules develop from each centriole forming spindle fibres. (In plant cells there are no centrioles and the spindle forms independently)

Prophase Towards the end of prophase each chromosome can be seen to consist of two chromatids held together by a centromere.

Prophase At the end of prophase the nucleolus disappears and the nuclear envelope breaks down.

Metaphase During metaphase, the chromosomes move toward the middle of the cell and line up at the cell equator (midline).

Metaphase Chromosomes line up on the equator of the spindle. They attach themselves to the spindle by their centromere

Anaphase During anaphase, the double-stranded chromosome separates and move to opposite poles of the cell. There is a complete set of chromosomes at each pole of the cell.

MITOSIS Anaphase This movement results from the contraction of the spindle fibres. As they shorten they pull the chromatids apart.

Anaphase The centromeres divide The free chromatids move to the poles

Telophase and Cytoplasmic Division The last stage of mitosis is telophase. In telophase, a nuclear membrane forms around each set of chromosomes, forming two identical nuclei. At the end of mitosis, the cytoplasm divides, forming two new identical daughter cells.

Telophase The chromatids have reached the poles and are now regarded as distinct chromosomes again. A nuclear envelope forms around each group of chromosomes

Telophase The chromosomes uncoil returning to chromatin The cytoplasm divides by cytokinesis.

CYTOKINESIS Animal cells The centre of the cell ‘pinches in’ to form a division furrow. As the division deepens, the cell surface membrane on each side joins up. Two separate cells result.

CYTOKINESIS Plant cells Vesicles produced by the Golgi body collect on equator of cell These vesicles fuse to form a cell plate. The cell plate eventually stretches right across the cell forming the middle lamella. Cellulose builds up on lamella to form cell walls.

CYTOKINESIS

The Cell Cycle

Meiosis The process of meiosis involves two cell divisions and produces cells that are different from the parent cell. Meiosis produces cells that have one-half the number of chromosomes as the parent cells. If meiosis did not take place, the fertilized egg would have double the amount of chromosomes it needs. Meiosis takes place during sexual reproduction when sex cells, called the egg and sperm, are produced in sex organs.

Meiosis One-half the number of chromosomes is called the hapolid, (or n) number. In humans, the haploid number (n) is 23. When the egg and sperm unite during fertilization, the species normal chromosome number called the diploid or (2n) number is restored (brought back). The diploid number in humans is 46. n (sperm) + n (egg) = 2n

Meiosis Cell division that is vital for sexual reproduction Takes place in reproductive organs Results in formation of haploid gametes The four daughter cells are not genetically identical

Meiosis and genetic variation There are two main ways in which genetic variation occurs at meiosis: 1. Random segregation of chromosomes 2. Crossing over

Meiosis and genetic variation Random segregation During metaphase 1 homologous chromosomes arrive at the equator They arrange themselves in a random order on the equator

Meiosis and genetic variation Random segregation In a cell with just two pairs of chromosomes there are four different types of outcomes for the way the chromosomes can separate.

Meiosis and genetic variation Random segregation In human cells with 23 chromosomes there are 2 23 different possible ways the chromosomes can segregate! That is over different types of gamete

Meiosis and genetic variation During fertilisation any male gamete can join with any female gametes. So thousands of millions of new genetic combinations are possible.

Meiosis and genetic variation Crossing over During prophase 1 of meiosis homologous chromosomes come together in pairs

Meiosis and genetic variation Crossing over Each chromosome is divided into two chromatids The homologous chromosomes twist around each other This creates tension, which may cause breaks to occur along the length of the chromatids

Meiosis and genetic variation Crossing over During cross over, corresponding fragments may get swapped over. This “cutting and sticking” means that genetic material is exchanged. This creates new genetic combinations and variation in the gametes is increased.

Differences between mitosis and meiosis MitosisMeiosis one division

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomes remains the same

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomesthe number of remains the samechromosomes is halved

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomesthe number of remains the samechromosomes is halved homologous chromosomes do not pair up

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomesthe number of remains the samechromosomes is halvedhomologous chromosomes do not pair uppair up to form bivalents

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomesthe number of remains the samechromosomes is halvedhomologous chromosomes do not pair uppair up to form bivalents chiasmata do not form and cross over never occurs

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomesthe number of remains the samechromosomes is halvedhomologous chromosomes do not pair uppair up to form bivalents chiasmata do not form andchiasmata form and cross over cross over never occursoccurs

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomesthe number of remains the samechromosomes is halvedhomologous chromosomes do not pair uppair up to form bivalents chiasmata do not form andchiasmata form and cross over cross over never occursoccurs daughter cells are genetically identical

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomesthe number of remains the samechromosomes is halvedhomologous chromosomes do not pair uppair up to form bivalents chiasmata do not form andchiasmata form and cross over cross over never occursoccursdaughter cells are genetically identicaldifferent from the parent cells

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomesthe number of remains the samechromosomes is halvedhomologous chromosomes do not pair uppair up to form bivalents chiasmata do not form andchiasmata form and cross over cross over never occursoccursdaughter cells are genetically identicaldifferent from the parent cells two daughter cells are formed

Differences between mitosis and meiosis MitosisMeiosis one divisiontwo divisions the number of chromosomesthe number of remains the samechromosomes is halvedhomologous chromosomes do not pair uppair up to form bivalents chiasmata do not form andchiasmata form and cross over cross over never occursoccursdaughter cells are genetically identicaldifferent from the parent cells two daughter cells are formedfour daughter cells are formed

Comparison of Mitosis and Meiosis CharacteristicMitosisMeiosis Number of daughter cells24 Number of cell division12 Daughter cells are diploid or haploid DiploidHaploid Daughter cells are identical or different IdenticalDifferent Parent cell is diploid or haploid Diploid