8 DNA And Meiosis
8.1 Structure Of DNA
- DNA is a chemical that determines your inherited characteristics.
- It holds vast quantities of information.
- DNA stands for deoxyribonucleic acid.
- It is composed of simple monomers called nucleotides.
Nucleotides
The organic base varies between nucleotides.
It can be one of two groups:
1. Single ring base
2. Double ring base.
It can be one of two groups:
1. Single ring base
2. Double ring base.
- DNA is a polynucleotide.
- Mononucleotides are joined together by condensation reactions (removal of water).
- This forms a bond between the deoxyribose sugar and the phosphate group.
- Multiple condensation reactions form a polynucleotide strand of DNA.
DNA Structure
Base Pairing
DNA contains organic bases with either single or double ring structures.
- DNA structure was discovered by Watson and Crick in 1953.
- It is composed of two interlinking polynucleotide strands; linkages are caused by bonding interactions between the organic bases on each polynucleotide strand, these are hydrogen bonds.
Base Pairing
DNA contains organic bases with either single or double ring structures.
- Nucleotides with double ring structures will be longer in length (they have more atoms and so require more space).
- For the interlinking of strands to be successful, the base pairings must always be composed of one single ring and one double ring base.
- Base pairings are always as follows; Adenine and Thymine (A+T); Cytosine and Guanine (C+G).
- A+T form two hydrogen bonds.
- C+G form three hydrogen bonds.
- The DNA strand is strong because of this complementary base pairing.
- This means that the percentage of each complementary base will be the same as it's paired partner.
The Double Helix
The double polynucleotide strand is twisted into a coil. The sugar-phosphate chains become wound around each other and this creates the sugar-phosphate backbone.
8.2 The Triplet Code
Genes are sections of DNA that contain coded information for making polypeptides.
The coded information is the specific sequence of organic bases along the DNA molecule.
The order of the organic bases determine the order in which amino acids are strung together to create a polypeptide strand.
The polypeptides coil (and can join together) to make proteins.
Proteins have many different functions in the body:
Genes--->Proteins--->Enzymes.
There are 20 amino acids that are used to build proteins living in organisms.
Because genes are the code for a proteins's sequence, and the only variation with a DNA strand is the organic bases, the bases must code for individual amino acids.
The coded information is the specific sequence of organic bases along the DNA molecule.
The order of the organic bases determine the order in which amino acids are strung together to create a polypeptide strand.
The polypeptides coil (and can join together) to make proteins.
Proteins have many different functions in the body:
- Control all metabolic reactions (these are called enzymes) - digestion, respiration and photosynthesis.
- Growth and repair
- Fibrous proteins
- Antibodies
- Haemoglobin
- Hormones
- Fibrin.
Genes--->Proteins--->Enzymes.
There are 20 amino acids that are used to build proteins living in organisms.
Because genes are the code for a proteins's sequence, and the only variation with a DNA strand is the organic bases, the bases must code for individual amino acids.
- 3 organic bases arranged along the chain makes up an amino acid (hence the name triplet code).
- There are 3 codes that tell the ribosome making the polypeptide chain. These are TAA, TAG and TGA.
- Methionine is the amino acid that codes to start.
- Amino acids don't just have one code, they have several. This is called degenerate code. For example, there are 6 codes for Arginine - CGT, CGC, CGA, CGG, AGA and AGG.
- When the bases are arranged in 3s to make amino acids, they are non-overlapping.
10 The Variety Of Life
10.4 Plant Cell Structure
Plant cells are eukaryotic - they have a distinct nucleus, membrane organelles, etc. (Take a look back at 3.10 to boost your memory on eukaryotic and prokaryotic cells).
13 Exchange And Transport
13.1 Exchange Between Organisms And Their Environment
What needs to be exchanged between an organism and its environment?
- respiratory gases (oxygen, carbon dioxide)
- nutrients (glucose, fatty acids, amino acids, vitamins, minerals)
- excretory products (urea, carbon dioxide)
- heat.
Exchange:
- passive (diffusion and osmosis) - no energy is needed
- active (active transport) - energy is needed.
Surface area to volume ratio:
Exchange takes place at the surface of an organism, but the materials absorbed are used by the cells that mostly make up its volume.
- respiratory gases (oxygen, carbon dioxide)
- nutrients (glucose, fatty acids, amino acids, vitamins, minerals)
- excretory products (urea, carbon dioxide)
- heat.
Exchange:
- passive (diffusion and osmosis) - no energy is needed
- active (active transport) - energy is needed.
Surface area to volume ratio:
Exchange takes place at the surface of an organism, but the materials absorbed are used by the cells that mostly make up its volume.
For exchange to be effective, the surface area of the organism must be large compared with its volume.
Small organisms, e.g. amoeba, have a surface area large enough compared with their volume, to allow efficient exchange across their body surface.
Large organisms have larger volumes compared to their surface area and therefore simple diffusion of materials across the surface can only meet the needs of relatively inactive organisms. Even if the surface area could supply enough material, it would still take too long for it to reach the middle of the organism if diffusion alone was the method of transport.
Large organisms have larger volumes compared to their surface area and therefore simple diffusion of materials across the surface can only meet the needs of relatively inactive organisms. Even if the surface area could supply enough material, it would still take too long for it to reach the middle of the organism if diffusion alone was the method of transport.
Evolved features of organisms which enables organisms to overcome this problem:
- a flattened shape so that no cell is ever far from the surface, e.g. a flatworm
- specialized exchange surfaces with large surface areas to increase the surface area to volume ratio, e.g. lungs in mammals; gills in fish.
Factors affecting the rate of diffusion:
- the concentration gradient; the greater the difference in concentration between two regions, the greater the amount which diffuses in a given time
- the distance over which diffusion occurs; the shorter the distance over which diffusion occurs, the greater the rate of diffusion across is
- the area across which diffusion occurs; the larger the area, the greater the rate of diffusion
- the structure through which diffusion occurs; pores in a barrier may enhance diffusion
- the size and type of diffusing molecule; smaller molecules diffuse faster than larger molecules; molecules soluble in the substances of a barrier diffuse faster through it.
- a flattened shape so that no cell is ever far from the surface, e.g. a flatworm
- specialized exchange surfaces with large surface areas to increase the surface area to volume ratio, e.g. lungs in mammals; gills in fish.
Factors affecting the rate of diffusion:
- the concentration gradient; the greater the difference in concentration between two regions, the greater the amount which diffuses in a given time
- the distance over which diffusion occurs; the shorter the distance over which diffusion occurs, the greater the rate of diffusion across is
- the area across which diffusion occurs; the larger the area, the greater the rate of diffusion
- the structure through which diffusion occurs; pores in a barrier may enhance diffusion
- the size and type of diffusing molecule; smaller molecules diffuse faster than larger molecules; molecules soluble in the substances of a barrier diffuse faster through it.
13.2 Gas Exchange In Single-celled Organisms And Insects
Gas exchange in insects
- Insects do not use blood to transport their respiratory gases from a singe gas exchange surface.
- Insects have tubes taking air close to all their body cells.
- An insect has a tracheal system consisting of tubes leading from the outside to the inside of its body.
- On the outside of the insect's body are small holes on each side of the segments, through which gases can diffuse.
- These holes are called spiracles, they can open and close to control the level of ventilation.
- Spiracles lead into a system of large tubes called tracheae. Each trachea has rings of chitin in its walls and is impermeable to gases.
- The tracheae branch into smaller tubes called tracheoles, and can pass between and even into the insect's cells.
- Tracheoles contain little or no chitin and are permeable to gases and water.
- Carbon dioxide diffuses from cells to the tracheoles and oxygen diffuses from tracheoles directly into cells.
Gas Exchange
In the tissues, the partial pressure of oxygen is lower than it is in the tracheoles, so oxygen diffuses from the tracheole into the tissue. This creates a diffusion gradient between the tracheoles and the air surrounding the insect.
Ventilation
In the tissues, the partial pressure of oxygen is lower than it is in the tracheoles, so oxygen diffuses from the tracheole into the tissue. This creates a diffusion gradient between the tracheoles and the air surrounding the insect.
Ventilation
- In small insects, gas transport is entirely diffusion.
- In larger insects, the tracheal system is actively ventilated by pumping movements of the abdomen.
- These insects compress their abdomen and squeeze air from their tracheal tubes. Fresh air moves into the tubes when the body returns to its normal size.
- High activity in an insect creates lactic acid accumulation in the cells. This decreases the water potential. By osmosis, water in the tracheoles is drawn into the cells, causing more air to enter the tracheoles.
- Thus more oxygen comes into close contact with the tissues just at the time when it is required.
13.3 Gas Exchange In Fish
Bony fish have a small surface area to volume ratio; they need specialized gas exchange surfaces, i.e. gills.
Structure Of The Gills
- The gills are fragile and complex.
- They lie in the pharynx, the area between the buccal cavity and the oesophagus.
- Each gill is made up of two piles of soft, thin plates called filaments. The filaments are attached to a bony gill arch.
- These filaments lie on top of each other; if not kept apart by water, they stick together and their exposed surface area becomes too small for efficient gas exchange.
- To increase the surface area even more, each filament is covered with flaps called gill plates or gill lamellae.
The Countercurrent Exchange Principle
- The great number of blood capillaries in the gills provide a large surface area for diffusion.
- Water passes over the gill plates in the opposite direction to the flow of blood through them, so that, across the whole gill plate, blood meets with a higher concentration of oxygen than it's own. This is known as countercurrent flow.
- It increases the efficiency with which the fish's gills extract oxygen from the water.
- Blood and water flowing in the same direction would quickly reach equilibrium, so the blood would extract much less of the oxygen available in the water. This is known as parallel flow.
Ventilation
Bony fish achieve a continuous flow of water over their fills using a pumping system. Water is drawn through the mouth and into the buccal cavity by muscular contractions. This water passes over the gills and leaves through the operculum. When the fish opens it's mouth, the volume inside the buccal cavity increases.
Bony fish achieve a continuous flow of water over their fills using a pumping system. Water is drawn through the mouth and into the buccal cavity by muscular contractions. This water passes over the gills and leaves through the operculum. When the fish opens it's mouth, the volume inside the buccal cavity increases.