Sunday, 3 March 2013

9.3.6 Phytochrome and the control of Flowering

Flowering Cues: 
Plants have to coordinate the production of flowers to coincide with the best reproductive opportunity . There are many environmental cues that affect flowering however the photoperiod isthe most reliable indicator on time of the year.

The photoperiod is the period of day light in relation to dark. In northernly and southern regions, this photoperiod is a reliable indication of the time of year and therefore one of the most reliable indicators of the seasonal changes.

Short and Long day Plants:
Short day plants (SDP) typically flower in the spring or autumn when the length of day is short.
Long day plants (LDP) typically flower during the summer months of longer photoperiod.

Critical Night Length:
Experiments have shown that the important factor determining flowering is the length of night rather than the length of day.
Therefore, SDP have a critical long night and LDP will have a critical short night.

Phytochrome System:
The receptor of photoperiod is located within the lead but the cellular location of the receptor remains unclear. The chemical nature of the receptor is the molecule Phytochrome and phytochrome can be converted from one form to another by different types of light

9.3.3 Dicotyledonous Seed Structure and Function

Testa protects the plant embryo and the cotyledon food stores

Radicle is the embryonic root

Plumule is the embryonic stem

Cotyledonous is the food store for the seed.

Micropyle is a hole in the testa through which the water can enter  the seed prior to germination

Scar is where the ovule was attached to the carpel wall


9.3.2 Pollination and Fertilisation

Pollination refers to the transfer of pollen grains from the anther to the stigma

Fertilisation refers to the fusion of the male gamete nuclei with the female gamete to form a zygote. Pollen grains often contain an additional nuclei used in the fertilisation of the food store cells.

The Seed are the fertilised ovules. The seeds are moved away from the parental plant before germination to reduce competition for limited resources with parental plant. There are a variety of seed dispersal mechanisms including fruits, winds, water and animals.


9.3.1 Structure of an animal-pollinated dicotyledonous Plant

  • Sepals cover the flower structure while the flower is developing and in some species these are modified to petals
  • Petals surround the male and female flower parts and its function is to attract animal pollinators.
  • Stigma is the surface on which pollen lands and the pollen tube grows down to the ovary.
  • Style connects the stigma to the ovary.
  • Ovary contains the ovules of the plant.
  • Filament supports the anther that contain the pollen and together they are called the stamen.

Wednesday, 5 September 2012

Outline of Cell Theory

The invention of microscopes in the 17th century lead to the discovery of cells: Robert Hooke first coined the term 'cell' after observing the structure of cork and other plant tissues in 1655.
In 1674 Anton van Leeuwenhoek made the first observation of single-celled organism (microorganisms)And in 1838 Schleden and Schwann developed the Cell Theory:
  • Cells are the smallest unit of life and nothing smaller can survive independentally
  • All living things are made of cells
  • Existing cells have come from other cells

(Source: , IB Biology Course Companion)

Saturday, 17 March 2012

5.20 Cloned transgenic animals

Evaluate the potential for using cloned transgenic animals, for example to produce commercial quantities of human antibodies or organs for transplantation.

Animals which are cloned are genetically identical and transgenic refers to an organism having DNA from two or more organisms.

Commercial production of antibodies (example using transgenic cows):

  1. We want to obtain an egg cell from the cow, and from a human we will take a cell and we are going to remove using restriction enzymes we are going to cut a gene associated with antibody production. 
  2. In the egg cell we knock out the cow antibody gene and then we are going to add the human gene using ligase enzyme. 
  3. The cow cell is then developed by mitosis to form clone of cells - an embryo. 
  4. This is transferred to a surrogate mother which will then produce genetically identical calves and in this particular example, the gene for antibodies is expressed and the human antibodies are collected from the milk of the animal in a large commercial scale.

5.19 Mammal Cloning

Describe the stages in the production of cloned mammals involving the introduction of a diploid nucleus from a mature cell into an enucleated egg cell, illustrated by Dolly the sheep

The original sheep in which the scientists take the genetic information from would have the same genes as its clone (Dolly).  This is done by:
  • Removing a diploid cell with the full set of genetic information. This nucleus contains all the genetic information to form a clone.
  •  At the same time, we need to obtain a cell, which has a tendency to divide, so another sheep would be injected with hormones to produce eggs, but we don't want the genetic information so we remove it (enucleation). 
  • So we take the cell with the genetic information that we do wish to copy and we take the egg cell which divides and we fuse them together and by doing so we combine the genetic information that we want with a cell that needs to divide
  • The combination of the two results in many cell divisions by mitosis and forms a ball of cells called the blastula, and this is essentially an embryonic sheep
  • This embryo is placed into another sheep - the surrogate sheep.
  • The embryo will grow into a foetus and then it will be born and this sheep that is born is called Dolly and it has the same sets of genetic information as the first sheep

Sunday, 11 March 2012

5.18 Commercial Plant Growing

Understand how microprogation can be used to produce commercial quantities of identical plants (clones) with desirable characteristics.

If a plants has commercially desirable characteristics, people would want to make many copies of it, however, the two ways of doing this:

  • Sexual reproduction will lead to variation and a loss of qualities. 
  • Therefore, we want to use cloning technique (micropropagation) so that we get many plants of the same quality and commercially that keeps the product the same so it can be sold.

5.17 Micropropagation

Describe the process of micropropagation (tissue culture) in which small pieces of plants (explants) are grown in vitro using nutrient media.

We begin with a plant which has characteristics that we consider desirable and we want to produce more plants of the same kind, the problem is if we use sexual reproduction, the plant will show genetic variation, instead we will have to cloning technique called micropropagation.

  1. We begin by taking tissue from the shoot tip or the root tip 
  2. The next step is under aseptic conditions (free from contamination), we are going to cut this tissue into many small parts
  3. Then transfer the tissue to a petri dish which will contain nutrient agar.
  4.  In addition to the minerals, there will also be rooting compounds and other plant hormones which will encourage the growth of each of the small parts into small clone of the original plant and then each of these can be then grown on into a seedling.
  5.  In the process of doing so, we create a lot of copies of the original plants, and these plants are known as 'clones' which will have the same genes.

5.16 Transgenic Organism

Recall that the term 'transgenic' means the transfer of genetic material from one species to a different species.

The previous objectives 5.13 and 5.15 both show examples of transgenic organisms:

  • In 5.13, the bacterial cell had become transgenic since it still had bacterial DNA but also plasmids that carried human insulin genes.
  • In 5.15, the maize had  become transgenic since we introduced the BT gene to the maize DNA.