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OCR Categories Archives: B5: Growth and Development

B5.3 How do genes control growth and development within the cell?

B5.3 How do genes control growth and development within the cell?

DNA is a nucleic acid in the shape of a double helix. Long strands of DNA make up chromosomes – these are found in the nucleus of a cell. DNA is a chemical code – our bodies need proteins for growth and development, and the DNA controls which proteins are made. The code consists of four different chemicals, or bases, that always pair up in the same way.

T always pairs with A

G always pairs with C

The order of these pairs of bases along the DNA molecule codes for all different proteins. A section of DNA that codes for one particular protein is called a gene.

To enable genes to code for proteins, the bases A, T, G and C get together not in pairs but in triplets. This is how it works:

 

  • Each protein is made up of large numbers of amino acid molecules
  • Each triplet of bases codes for one particular amino acids
  • So amino acids are made in the number and order dictated by the number and order of base triplets
  • Finally, the amino acid molecules join together in a long chain to make a protein molecule. The number and sequence of amino acids determines which protein results

The DNA has sequences of genes, which code for proteins – however the proteins themselves are manufactured in the cytoplasm of the cell. Therefore there is a mechanism for transferring the information stored in the genes into the cytoplasm.

The DNA molecule is too large to leave the cell so the relevant section of DNA is unzipped and the instructions are copied onto smaller molecules which can pass through the nuclear membrane of the nucleus into the cytoplasm.

The smaller molecules are called messenger RNA (mRNA) – these leave the nucleus and carry the instructions to the ribosomes, which follow the instructions to make the specific protein.

All body cells, including stem cells contain exactly the same genes. However although all the body cells in an organism contain the same genes, many genes in a particular cell are not active (switched off) because the cell only produces the specific proteins it needs.

In specialised cells only the genes needed for the cell can be switched on but in embryonic stem cells any gene can be switched on during development to produce any type of specialised cell

 

Adult stem cells and embryonic stem cells have the potential to produce cells needed to replace damaged tissues. For example, stem cells can be used to replace brain tissue in a patient with Parkinson’s disease, or to grow new skin tissue following a burn.

To produce the large number of stem cells needed for, it is necessary to clone cells from five-day-old embryos. The stem cells are collected when the embryo is made up of approximately 150 cells – the rest of the embryo is destroyed. At the moment, unused embryos from IVF treatments are used for stem cell research

There is an ethical issue to whether it is right to use embryos to extract stem cells in this way – should embryos be classed as people?

One view is that if an embryo is left over from IVF (and would therefore never grow into a human being) it would be acceptable for stem cell research to be carried out on it as long as the parents gave their consent. However, another view is that destroying an embryo amounts to destroying a life. The Government regulates and makes laws on such matters.

Mammalian cloning – in carefully controlled conditions of mammalian cloning it is possible for scientists to reactivate (switch on) inactive genes in the nucleus to form cells of all tissue types. This gives the potential to grow new tissue that is genetically the same as the patient

 

 

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B5.2 How does an organism produce new cells?

B5.2 How does an organism produce new cells?

Mitosis and meiosis are two ways that cells reproduce

MITOSIS is the process by which a cell divides to produce two new cells with identical sets of chromosomes to the parent cell – the new cells will also have all the necessary organelles. The purpose of mitosis is to produce new cells for growth and repair and to replace old tissues.

Mitosis leads to the production of two new cells, which are identical to each other and to the parent cell. Mitosis can only take place when a cell is ready to divide this means that cells go through a cell cycle.

The cell cycle consists of a growth stage (G1) where the cell gets bigger and the number of organelles increase, then a synthesis stage (S) where the DNA is copied, followed by another very short growth stage (G2) immediately before mitosis (M)

Both new cells need to have a full complement of organelles and DNA to function properly. Therefore the number of organelles needs to increase and the DNA has to be copied.

The chromosomes are copied when the two strands of each DNA molecule separate and new strands form alongside them.

Meiosis only takes place in the testes and ovaries – it is a special type of division that produces gametes (sex cells i.e. eggs and sperm) for sexual reproduction.

Gametes contain half the number of chromosomes of the parent cell – this is important because it means that when the male and female gametes fuse, the number of chromosomes will increase back to the full number. The resulting zygote has a set of chromosomes from each parent.

 

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B5.1 How do organisms develop

B5.1 How do organisms develop

Cells are the building blocks of all living things – multicellular organisms are made up of collections of cells. The cells can become specialised to do a particular job.

Groups of specialised cells working together are called tissues and a group of tissues working together are called organs.

When an egg is fertilised by a sperm it becomes a ZYGOTE.

The zygote then divides many times by mitosis to form an embryo – up to (and including) the eight-cell stage all the cells are identical and can produce any sort of cell required – embryonic stem cells

After the eight-cell stage, most of the embryo cells become specialised and from different types of tissue

Some cells remain unspecialised – these are adult stem cells. At a later stage they can become specialised however, unlike embryonic stem cells, adult cells cannot become any type of cell.

Plants have cells that are like stem cells in animals – the cells are in areas called meristems. Only cells within meristems can divide repeatedly (mitotically active)

Cells in the meristem are unspecialised but they can develop into any type of plant cell. Under normal hormonal conditions, unspecialised plant cells can become specialised to form different types of tissues (including xylem and phloem) within organs (including flowers, leaves, stems and roots)

There are two types of meristems:

  • Apical meristems – those that result in increased height and longer roots
  • Lateral meristems – those that result in increased girth

Xylem is made from specialised cells to transport water and soluble mineral salts from the plant roots to the stem and leaves, and to replace water lost during transpiration and photosynthesis

Phloem is made from specialised cells to transport dissolved foods made by photosynthesis throughout the plant for respiration or storage

When a stem is deliberately cut, special plant hormones can be added – these can send messages to the meristems to start to produce roots. As the cutting already has a stem and leaves, it will then grow into a clone of the parent plant. This may be done to reproduce a plant with desirable features.

A cut stem from a plant can develop roots and then grow into a complete plant which is clone of the parent, and that the rooting can be promoted by the presence of plant hormones (AUXINS). Auxins mainly affect cell division at the tip of a shoot, because that is where the meristems are.

The growth and development of plants are affected by the environment e.g. phototropism and geotropism (where roots and shoots grow towards and away from the source of gravity)

PHOTOTROPISM is a response by the plant to light – a plant’s survival depends on its ability to photosynthesis therefore plants need strategies to detect light and to respond to changes in intensity. This is demonstrated by the way in which plants will grow towards a light source.

The cells furthest away from a light source grow more, due to the presence of auxin which is sensitive to light. Auxin is produced at the shoot tip and migrates down the shoot

A – When the tips are removed, no auxin is made so the stems do not grow

B – When the tips are covered, auxin moves to all parts of the stem causing all parts to grow

C – When the tips are lit from one side auxin accumulates on the shaded side causing it to grow more than the illuminated side

 

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