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

What is Speciation? | A Level Biology Revision

Speciation is pretty much the evolution of new species from existing species. A species being a group of organisms with similar genes which are able to interbreed to produce fertile offspring. If populations become seperated (geographical isolation) then they are no longer able to interbreed. This might be for a number of reasons  - lets take a huge mountain for example. Selection pressures will be different in both locations, for example ease of getting food or temperature may differ. Within these slightly different environments some are better able to compete than others - this  causes the allele frequencies to change in each population. The genes then, over time become so different they now can't interbreed.

Other keywords:

Stabilizing selection - this is selection which tends to favor the average (gets rid of the extremes)
Directional selection - this favors one extreme

This is specifically for AQA A Level Biology Unit 4

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)

A level Physics: Wave Particle Duality

• Light is part of the electromagnetic spectrum
• The theory of the electromagnetic waves predicted the existence of waves beyond visible light (before we knew about x-rays and so on)
• The discovery of x rays and radio waves confirmed their existence (and their predictions)
• Many scientists in the late 19th Century thought that all aspects of physics could be explained through Newton's laws of motion. 

Interference and Diffraction show light as a wave

• Light produces interference and diffraction patterns - these are alternating bands of dark and light
• Can only be explained using waves interfering constructively or interfering destructively

Constructive Interference - when two waves overlap in phase

Destructive interference - when two waves are out of phase

Although - Particles DON'T show wave-like properties all the time

Diffraction only happens if a particle interacts with an object about the same size as the de Broglie wavelength

de Broglie came up with the wave-particle duality theory

shorter wavelength = less diffraction effects (this fact is used in electron microscopes)
Diffraction effects blur detail on an image 

(specifically for AQA A-level physics specification A)

A-Level Revision - The Cell Surface Membrane

These revision notes refer to AQA AS Biology Chapter 3.5

 The cell surface membrane is another name for the cell membrane or plasma membrane.
- It surround the cell boundary between the cell cytoplasm and the environment (this is where there is different conditions inside and outside of the cell).

The Fluid Mosaic Model

Why is this called the fluid-mosaic model?
Fluid - The Molecules move relative to one another
Mosaic - means being made up of different substances, in this case proteins and phospholipids.

How do substances pass through a cell membrane?
(a very common exam question)

- the phospholipids form a bilayer
- the phospholipids are arranged with hydrophilic heads on the outside and hydrophobic tails on the inside
- There are two types of proteins, some are passing through and some are confined to one layer
- There are other molecules present, for example: cholesterol or glycoproteins
- substances are moved DOWN the concentration gradient (from a high to low concentration)
- water and ions move through the channel proteins
- lipid soluble molecules are passed between the phospholipids
- carrier proteins are involved in facilitated diffusion.