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 a synapse?

When electrical impulses are fired around your body, they do so by travelling around in neurones as chemical/electrical impulses.


However, your neurones don't cover every inch of your body. To get over this issue, there are small gaps between the neurones - this is called the synapse. Technically speaking, it's a junction between the neurone and the next cell. , the gap itself is called the synaptic cleft. The neurone before the gap is given the name of a presynaptic neurone - it has a swelling on it called the synaptic knob. On the presynaptic neurone there's synaptic vesicles filled with chemicals called neurotransmitters.


When an action potential reaches the end of the neurone, neurotransmitters are released into the synaptic cleft. They diffuse across this gap and attach to the postsynaptic membrane - here they bind to specific receptors. When the neurotransmitter binds to the receptors, this could trigger an action potential, causing muscle contraction (perhaps) in the next cell. Alternatively, a hormone could be secreted if it's a gland cell.




Because these receptors are only on the post synaptic membrane, this makes action potentials unidirectional - i.e they travel in only one direction. If nerve signals travelled in multiple directions, things would get very confusing very quickly!

What are genetic mutations?

 

Mutations happen all the time - most are absolutely harmless, some are harmful - but what are mutations?


A mutation is a change, deletion or addition of a base in a DNA code. Your DNA is a long 'string' if you like of complementary bases - these code for amino acids and in turn, these code for proteins. If one of the bases are substituted this doesn't always lead to a change in amino acid formed. This is because your DNA is degenerate. This means that more than one combination of three bases code for one amino acid.


A deletion of a base will change the amino acid coded for. This is because it causes a 'frame shift' - this moves all the bases up in the sequence - different amino acids are coded for. This in turn, affects the hydrogen bonding between the complementary base pairs. Because the hydrogen bonds are altered - this affects the tertiary structure. This means the protein produced isn't functional.


The real issues occur if mutations occur within the genes which control cell division - these are called, the "tumour suppressor genes" and the "proto-oncogenes". If mutations occur within the proto-oncogenes, then cell division is stimulated. It is uncontrolled cell division which leads to the formation of tumours and potentially cancers.

 

What is tRNA? | A- Level Revision

• tRNA is a type of RNA
• single polynucleotide strand
• folded into a clover shape
• hydrogen bonds between specific base pairs hold the molecule in space

At one end:

• specific sequence of 3 bases called an anticodon

At the other end:

• amino acid binding site (complementary base pairings)

Function:

• tRNA found in the cytoplasm
• involved in translation
• carries the amino acids (used to make proteins) to the ribosomes

Panda Genetic Fingerprinting

Using Genetic Fingerprinting to save the Panda might seem like a crazy idea but it's an idea that might have legs! The giant panda is one of the rarest animals in the world and is considered to be on the brink of extinction. Giant pandas have been kept and bred in zoos for many years now. In many areas, Panda's are on breeding programmes so they could, one day, be released into the wild.

Unfortunately, zoo life doesn't come worry free. There's the issue that with small populations, genetic variation is reduced. This basically means they could well struggle when they're released into the wild. Having said that, Pandas find it difficult to reproduce in captivity.

Scientists can only guarantee that a female will get pregnant through pretty much carrying out Panda IVF. When they do so, they use sperm from several males. One problem with this is that, who's the father? Identifying who the father is can then help the scientists to identify how genetically different pandas are.

Their method of obtaining DNA samples is strange, to say the least!! They collect around

10 Panda faeces can be collected from the wild. The faeces contain DNA from the panda, from the bamboo on which they feed and from bacteria. The DNA is then put through the 

polymerase chain reaction (PCR). The primers used attach only to the panda DNA because of complementary base pairings.

The DNA from this, then goes through genetic fingerprinting. This is where the genetic sequences are read (ie GATC etc etc). Collating an idea of what happens in the wild, pandas can then be individually identified.

How this happens, is quite a complex scientific process. DNA is first cut using a restriction enzyme. This cuts the DNA at specific points. Gel Electrophoresis then separates the strands according to length (so mass as well). A process called Southern blotting then makes the strands single stranded. A DNA probe (which is complementary to part of the DNA strand) is then added. The probe will act as some kind of marker - it could be radioactively marked or fluorescent.

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

What is cholesterol?

 

We always hear about cholesterol - pretty much why too much of it's bad, and they're not wrong. There are two types of cholesterol, both good and bad. It's a soft and waxy material found in the membranes of cells. From here, it's transported into your blood stream. Partly, this is a substance produced by your body and partly absorbed from animal products you eat, this includes eggs, meat and dairy products.

The bad cholesterol is obviously the stuff we should try and limit, this is because it can clog blood vessels. The issue here, is that it increases your risk of heart attacks and strokes. Good cholesterol however removes the bad cholesterol. It takes it to the liver to be destroyed. This means that if you've got a lot of good cholesterol in your system, it actually works to protect your body from heart disease.

What are mutagenic agents?

Mutations are spontaneous errors where DNA is misread during replication. There are some things which can increase the rate of mutations. These include UV, ionising (high energy) radiation, some chemicals such as benzene and some viruses are all examples of mutagenic agents. They work by

acting as a base. Chemicals called base analogs can then act as a substitute for a base during DNA replication.

Maria Sharapova - The Debate

There's a whole number of ways someone can flip Maria's failed drugs test. For pretty much her entire career, she's been at the centre of tennis. She's won five grand slams and an Olympic silver medal at London 2012. She's 28.








If you take Sharapova's statements literally, then it was an honest mistake - stupid at least. From what she said at last Monday's press conference, she says she received the list as an email attachment from Wada. She says she didn't open up the email. There are so many ways you can look at this, how much trust do you place in your doctors? This is where sports psychology comes in - does this place unnecessary stress onto someone who has to perform day in, day out?










Having said that, if you look more closely at what this drug actually is, it appears as though it's one which you only take for about 2 weeks at a time - why was she taking it for 10 years?










Legally speaking, this now falls to the ITF, WADA and her team to investigate what happened and the circumstances leading up to this. She'll be given a suspension (pretty much for certain), it just falls to this investigation process to determine whether she deliberately took the drug knowing it was banned or took it without knowing it was banned which would class as negligence. According to WADA it was added to the banned list because of "evidence of its use by athletes with the intention of enhancing performance".





Whatever you believe, we'll doubtless hear the truth eventually!