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OCR 21OCR 21stst Century Century

B5 Growth and B5 Growth and DevelopmentDevelopment

W Richards

The Weald School

12/27/14B5.1 How do organisms develop?B5.1 How do organisms develop?

12/27/14Cell specialisationCell specialisation

White blood cell

Egg cell (ovum)

Ciliated epithelial cell

Nerve cell (neurone)

During the development of a multi-celled organism cells differentiate to form specialised cells:

12/27/14Cells, tissues, organs and systemsCells, tissues, organs and systemsBasically, all living things are made up of cells…

A group of CELLS makes up a TISSUE

A group of TISSUES makes up an ORGAN

A group of ORGANS makes up a SYSTEM

A group of SYSTEMS make up an ORGANISM

12/27/14Another exampleAnother exampleHere’s another example in humans:

Muscle cells

Muscle tissue

Organ

System

Organism

12/27/14FertilisationFertilisation

The human egg and sperm cell (“GAMETES”) contain 23 chromosomes each and are created by meiosis.

When fertilisation happens the gametes fuse together to make a single cell called a ZYGOTE. The zygote has 46 chromosomes (23 pairs) and continues to grow through mitosis.

12/27/14Embryos Embryos

When the zygote grows all of the initial cells are identical (up to the 8-cell stage). These are called “embryonic stem cells”. From this stage on the cells grow through mitosis and start to specialise.

12/27/14Adult Stem CellsAdult Stem Cells

White blood cell

Egg cell (ovum)

Ciliated epithelial cell

Nerve cell (neurone)

It is also possible to have adult stem cells – these are unspecialised cells that can become specialised later (but they can’t form ALL types of cell)

Adult stem cells can be found in places like bone marrow.

12/27/14Plant Growth IntroductionPlant Growth IntroductionPlants grow due to their cells dividing through mitosis. The cells then specialise into root hair cells, palisade cells etc. Unlike animals, plants continue to grow for the rest of their lives.

12/27/14Plant Growth - MeristemsPlant Growth - MeristemsPlant growth occurs in areas called meristems. These meristems are “mitotically active”:

This meristem causes the plant to grow upwards.

This meristem causes the plant to grow in width.

Cells from the meristem behave like stem cells – they can develop into any kind of cell. Cloned plants can be produced from these cells.

12/27/14Cloning Plants by tissue cultureCloning Plants by tissue culture1) Scrape of a few cells from the desired plant

2) Place the scrapings in hormones (“auxin”) and nutrients

3) 2 weeks later you should have lots of genetically identical plants

12/27/14Plant HormonesPlant HormonesThe growth of roots and shoots is controlled by hormones:

In the roots hormones slow down growth of the cells in the lower region, which makes the root bend down.

In the shoots the hormone auxin is “destroyed” by light, so the shoot will bend towards the light as the cells on the shaded side grow quickest.

12/27/14Some DefinitionsSome Definitions

Shoots grow towards light (positive phototropism) and against gravity (negative geotropism).

Roots grow away from light (negative phototropism) and in the direction of gravity (positive geotropism).

12/27/14B5.2 How does an organism produce B5.2 How does an organism produce new cells?new cells?

12/27/14Cell growth 1 - MitosisCell growth 1 - Mitosis

Each daughter cell has the same number of

chromosomes and genetic information as the parent

– it’s a “clone”.

The chromosomes are copied…

The chromosomes separate…

The nucleus divides…

12/27/14Cell growth 2 - MeiosisCell growth 2 - Meiosis

Each daughter cell has half the number of chromosomes

of the parent.

12/27/14Mitosis vs. MeiosisMitosis vs. MeiosisMitosis:

1. Used for growth and repair of cells

2. Used in asexual reproduction

3. Cells with identical number of chromosomes and genetic information are produced (“clones”)

Meiosis:

1. Used to produce haploid gametes for sexual reproduction

2. Each daughter cell has half the number of chromosomes of the parent

During meiosis copies of the genetic information are made and then the cell divides twice to form four daughter cells.

12/27/14Fertilisation RevisionFertilisation Revision

The human egg and sperm cell (“GAMETES”) contain 23 chromosomes each and are created by meiosis.

When fertilisation happens the gametes fuse together to make a single cell called a ZYGOTE. The zygote has 46 chromosomes (23 pairs) and continues to grow through mitosis.

12/27/14B5.3 How do Genes control growth?B5.3 How do Genes control growth?

12/27/14Genes, Chromosomes and DNAGenes, Chromosomes and DNA

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Some facts:

- DNA has a “double ______” structure

- This contains instructions on what a cell does, how the organism should work etc

- The instructions are in the form of a ______

- The code is made up from the four ____ that hold the strands together with hydrogen bonds; A always pairs with T and C with G

- The bases represent the order in which _____ acids are assembled to make specific ________

How genes work - How genes work - DNADNA

Words – helix, amino, code, bases, proteins

12/27/14Making proteinsMaking proteins1) DNA “unravels” and a copy of one strand is made

2) The strand copy is made to produce RNA

3) The copy (with its code) then moves towards the ribosome

4) The ribosome “decodes” the code which tells the ribosome how to make the protein

In other words, genes do NOT leave the nucleus but a copy of the gene (the mRNA) carries the genetic code to the cytoplasm.

12/27/14Adult Stem CellsAdult Stem CellsCells inside an organism contain the same genes. So how can cells specialise if they have the same genes and make the same proteins?

The reason is that many of the genes in the nucleus are “switched off” so that the cell only produces the proteins it needs.

It is possible to “switch on” different genes to make the cell produce different proteins – this is the basis of stem cell research.

12/27/14Stem cell researchStem cell researchAs well as adult stem cells from bone marrow, stem cells can also come from...

Egg and sperm

Embryo

Cloned embryos

These stem cells have the potential to develop into any kind of cell. The rest of the embryo is destroyed. Most of these embryos come from unused IVF treatments.

The ethical issue:

Should these embryos be treated as humans?

12/27/14Cloning AnimalsCloning AnimalsHere’s how Dolly the sheep was cloned:

Clone

Host mothersIt is possible to “switch on” inactive genes in the nucleus of a body cell to form cells of all tissue types. Are cloning and stem cell research the way forward???