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

Stem cells and cloning

Keyword definitions• Clones• Plasmid• Artificial cloning• Binary Fission• Budding• Stem cells• Totipotent• Meristem cells

Stem Cell Research• Read the article.• Discuss and decide two

advantages and two disadvantages of Stem Cell therapy

Stem cells

Differentiation and Specialisation

Differentiation• Size limits – cells have a

physical limit to the size it can reach

• Large organisms have a small surface area to volume ratio – so need specialised cells forming tissues and organs to carry out particular functions

• Differentiation – number of organelles, shape of

cell, contents of cell

Specialisation• Cells becoming specialised to be

able to carry out a particular role.

• Produce a table to show how the following cells become specialised and their role following specialisation

• Erythrocytes, neutrophils, epithelial cells (squamous and ciliated), sperm cells, palisade cells, root hair cells, xylem and phloem.

Specialized cellsIn 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.

Rest phaseCells 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

Tissues, organs and organ systems

Plant structure

Cell differentiation

Jan 2010

Chromosome numberA 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 divisiondivision 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 variationSexual 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