AS Revision - DNA Replication

Mitosis: (used for growth and repair, including the growth of the foetus)

2 daughter cells produced

Identical to original (parent) cell and to each other, unless a mutation occurs.

Importance:

• Increase in number of cells
• 2 haploid cells fuse to make a diploid cell
• Differentiation
• Replacing cells (when they're damaged or die new cells must form to be genetically identical to function effectively)

Interphase: (cell not dividing, when replication of DNA occurs)

• cell continues its normal function
• prepares to divide
• Cells DNA is unravelled and replicated - genetic content is doubled
• Organelles replicated so there's spares
• ATP content increased

Prophase:

• Chromosomes condense (they get shorter and fatter)
• Centrioles start moving towards opposite ends of the cell forming a network of protein fibres across it (forms the spindle)
• Nuclear envelope breaks down
• Chromosomes are free in the cytoplasm

Nuclear envelope: membrane around the nucleus

Centrioles: tiny bundles of protein

Metaphase:

• Chromosomes (each have two chromatids) line up along the middle of the cell
• become attached to the spindle via their centromere

Anaphase:

• Centromeres divide
• separating each pair of sister chromatids
• spindle contracts
• Pulls chromatids to opposite ends of the cell (first the centromere)

Telophase:

• Chromatids reach opposite poles on the spindle
• uncoil and become long and thin again
• They're now called chromosomes again
• Nuclear envelope forms around each group of chromosomes so there are now 2 nuclei
• Cytoplasm divides
• FORMING: 2 daughter cells genetically identical to the original cell and to each other

Mitosis is now finished and each daughter cell starts interphase and the whole cycle repeats.

Mitotic index = the number of cells carrying out mitosis (visible chromosomes)/ total number of cells on the slide

Cancer:

(result of uncontrolled cell division)

• cell growth and cell division controlled by genes
• cancer is a tumour that invades surrounding tissue
• cells keep on dividing to make more tissue (the tumourr)

Chemotherapy:

• prevent the synthesis of enzymes needed for DNA replication
• Not specific- can kill normal cells, steps taken to reduce the impact on normal cells
• A large portion of the tumor is removed using surgery
• Repeated treatments (ie not one big dose), treatment breaks in between. 

G1- cell growth and protein production

specifically for AQA Biology AS Unit 2

Antibiotics | Biology A-Level Revision

• In normal cells water enters by osmosis - too much water, the cells burst (osmotic lysis)
• Bacterial cells have a tough cell wall which prevents the cell from expanding
• Antibiotics stop the bacterial cell wall from forming correctly so the wall is weaker. 
• This weakened cell wall means that the cell can burst due to osmotic lysis so the bacteria doesn't spread

Although there's a new problem, antibiotic resistance:

• Shortly after antibiotics were discovered, small groups of people were developing resistance due to a chance mutation. 
• The mutation caused the production of a new enzyme which broke down the penicillin. 
• The gene that coded for this mutation then is passed to the next generation (through vertical gene transmission) 
• The antibiotic resistance gene carries plasmids so it can pass onto other bacterial species via horizontal gene transmission. 

Why do bacteria need to adapt?

Similarly to what Darwin said, only the fittest survive, so to increase their chances of survival, they need to adjust to suit their environment. What this does mean however is that antibiotics can quickly become useless as the bacteria adapt to become resistant to them.

Conjugation

1. A donor cell produces a thin conjugation tube between its cell and the recipient cell. 
2. The donor cell replicates its plasmids
3. The plasmid is unwound so it's now straight and can fit through the conjugation tube
4. The replicated plasmid moves through the conjugation tube and enters the recipient cell
5. The plasmid which has just entered the recipient cell now returns to a circular shape
6. The bacteria has now replicated. 

Key terms:

1. Mutations: A random change in the quantity or structure of DNA (one base is added,deleted or replaced)
2. Conjugation: The fastest method of evolution where DNA of existing individuals is combined - horizontal gene transmission
3. Plasmids: A circular piece of DNA

Vertical Gene Transmission - DNA passed to the next generation of the SAME species

Horizontal Gene Transmission - DNA passed to different species via CONJUGATION

DNA (deoxyribonucleic acid) - Profile

What is it?


As James Watson and Francis Crick famously said - "the secret of life". It's a vital part of any species, how it functions and the reactions that are carried out.



When was DNA discovered?



1953, by James Watson and Francis Crick - although as with any scientific discovery they were working as part of a team. Rosalind Franklin is often credited for her contribution to the discovery of DNA.



What's it made from?


The backbone of DNA if you like is made from sugar and phosphate. The 'rungs' if you like are made from bases - adenine, cytosine, guanine and thymine which form complementary base pairings. As its full name suggests, there's also deoxyribose sugar which makes up the structure.

These complementary base pairings combined with the deoxyribose sugar and phosphate make up the 'nucleotide', lots of these join together make up one of the DNA strands. Because there are lots of these joined together - DNA is a polynucleotide.





How much DNA do we have?


A LOT! Although sequencing human DNA is in its very early stages, we do know that human DNA has around three billion base pairings.



Can we do anything with DNA?
Surprisingly, yes. This year Barack Obama announced plans to sequence the genome of around 1 million people in the US in the hope scientists can use this information to create advanced treatments, targeting specific regions of our DNA.


How do we make more of it?


You'll see in more detail exactly how DNA is replicated in my Christmas countdown posts, but in short, DNA is copied to a molecule called RNA. This is because the organelles for protein synthesis are found in the cytoplasm but DNA is bound in the nucleus. This process of copying the DNA to RNA is called transcription. Once the RNA has left the nucleus, it joins to a ribosome where it can then make a protein. This is called translation.

Gas Exchange in Fish | AS Biology Revision

• Fish have a small surface area to volume ratio
• This means the surface can't supply enough respiratory gases - so fish developed gills

Fish Gills

• Fish gills are behind their head
• Made up of many gill filaments stacked in a pile
• Gill lamellae at right angles to the filaments to increase surface area
• Water taken in through their mouth
• Water then forced over the gills and out through the opening on each side of their body

Countercurrent Flow (parallel flow wouldn't work!)

• Flow of water over the lamellae is in the opposite direction o the flow of blood within the lamellae
• Ensures the largest diffusion gradient between the water and blood at all times
• Blood loaded with oxygen meets water high in oxygen- diffusion oxygen from water to the blood 

CONCENTRATION GRADIENT IS MAINTAINED ALONG THE LENGTH OF THE GILLS WITH THE COUNTERCURRENT PRINCIPLE

Gill adaptions

• Large surface area (from the lamellae and filaments) - increases diffusion
• Short distance between the water and the blood
• Countercurrent Flow
• Circulation (replaces blood saturated with oxygen)

AS Physics - Electron Diffraction

(for AQA AS Physics)

• Electron diffraction shows the wave nature of electrons (how electrons can behave like waves)
• Diffraction patterns are observed when accelerated electrons in a vacuum tube interact with the spaces in a graphite crystal - this confirms electrons show wavelike properties
• According to the wave theory - the spread of the lines in the diffraction pattern increases if the wavelength of the wave is greater
• In experiments: a smaller accelerating voltage (the electrons are slower which gives widely spaced rings
• Increase in the electron speed - the diffraction pattern circles squash together towards the middle. (fits with the de Broglie equation  - if velocity is higher the wavelength is shorter and the spread of the lines smaller)

Wavelength for electrons accelerated in a vacuum tube is about the same size as electromagnetic waves in the X-ray part of the spectrum

Processes Explained: The Movement of Water Through Plants

Water needs to travel up through the roots of a plant for photosynthesis in order for a plant to survive. Roots, like everything have cells, these are root hair cells. These root hair cells are long and a thin extension of the root epidermal cell. They only live for a few weeks before they die and are replaced by others growing near the tip. Because roots absorb water, minerals and ions they constantly loose water by transpiration so this needs to be replaced.
Every root hair cell is adapted to its function because:

• They have a large surface area because there's thousands of branches which come off of each root. 
• The root has a thin surface layer (made up of the cell membrane and the cellulose cell wall) which allows material to move across the plant easier. 

In damp conditions when there's more water, a solution forms which contains minerals and mostly water. 

In areas of high water potential in the soil means water moves by osmosis from the soil into the root hair. 

Once it's in the root hair cell it takes one of two paths - 

• the apoplastic pathway
• the symplastic pathway

To be continued...

This is based on AQA AS Biology Chapter 13 Movement of water through plants.