Stem Cells

Starter Define these words: Gene Locus Allele Mitosis Meiosis Homologous chromosomes Chromatid Diploid A.One of the two copies of a chromosome that are joined together by a centromere prior to cell division B. A type of cell division where the chromosome number is halved C.One of the different forms of a particular gene D.A section of DNA that codes for a polypeptide E.A type of cell division where the daughter cells have the same number of chromosomes as the parent cell F.A term referring to a nucleus which contains two pairs of chromosomes. G.A pair of chromosomes (one maternal and one paternal) that have the same gene loci. H.The position of a gene on a chromosome

Stem Cells Learning Objectives Define ‘Stem Cell’ Understand and give examples of how stem cells can be modified for use in medicine Success Criteria Produce a written summary for and against stem cell research

Specialized cells In a single-celled organism, all the functions necessary for life must be carried out in one cell. In contrast, multicellular organisms can delegate jobs to particular groups of cells. Cells that have adapted to a specific function are known as specialized cells. Specialized cells are grouped into tissues, which combine to make organs and organ systems.

Tissues, organs and organ systems

Stem cells

Rest phase Cells often stop dividing once they are fully differentiated, so where do they fit into the cell cycle? Specialized cells move from G1 into a resting phase known as G0. G0 is also the phase in which stem cells wait until their associated body cells need replacing. A cell may remain in G0 for the rest of its lifetime, or it may just rest in this state temporarily. G0

Plant structure

Cell differentiation

Chromosome number A human somatic (body) cell contains 46 chromosomes. These consist of 23 pairs of homologous chromosomes. Sex cells, or gametes, have only one copy of each chromosome: they are haploid. A somatic cell, containing two of each, is called diploid. Each pair contains one chromosome from each parent. Other species have different numbers of these homologous pairs.

Haploid gametes These gametes are genetically unique because, unlike somatic cells, they were formed by a special form of cell division called meiosis. All somatic cells in a multicellular organism are genetically identical because they are the result of mitosis. They are all descended from a single cell – a zygote. A zygote is formed when two haploid gametes fuse.

Cell division division of the nucleus and then the cell Mitosis For Growth Produces 2 identical daughter nuclei Same number of chromosomes to parent cell No variation Meiosis For gamete production Produces 4 different daughter nuclei Different number of chromosomes to parent cell Causes Variation

Meiosis I and II Meiosis I introduces genetic diversity by randomly dividing a cell’s genes in two. It results in two haploid cells. Meiosis is the process of cell division underlying sexual reproduction. It is a two-stage process: Meiosis II is similar to mitosis. It splits each chromosome into its two chromatids and places one in each daughter cell. It results in four haploid gametes.

Genetic variation Sexual reproduction creates genetic diversity within a population, which is vital to a species’ survival. During meiosis I, homologous pairs of chromosomes swap parts of their genetic material. This is crossing over. Two processes during meiosis determine the unique genetic make- up of the four daughter cells: The chromosomes from each pair are randomly allotted to the daughter cells by independent assortment.

Meiosis: true or false?

Variation from meiosis

Chromosome mutations