AQA BIOL2~3rd June 2013~AS Biology (Now Closed)

these are my notes so far i hope they help and if anyone things wrong or misplaced please let me know thanks

The cell cycle is a process that all body cells of multicellular organisms use to divide and grow. It begins with a cell that has been produced by cell division and ends with the cell dividing.
Interphase is the process of cell growth and DNA replication. It consists of gap phase 1, synthesis and gap phase 2. During interphase the ATP content will increase. During interphase the nuclear membrane is intact and the chromosomes are not visible.
DNA replication occurs before cell division so that the new cell has a full amount of DNA. First the enzyme DNA helicase breaks the hydrogen bonds between the polynucleotide strands. This leads to the helix unwinding to produce two separate strands. The separate strands then act as a template for the new strands. The free DNA nucleotides then bind to the exposed bases by specific base pairing. The nucleotides on the new strand then join together by the enzyme DNA polymerase. Hydrogen bonds then form between the bases of the original and new strand. The DNA molecule now contains a strand from the original DNA molecule and one new strand.
Semi conservative test uses nitrogen isotopes N14 and N15, because of the nitrogenous bases. In your first generation you get semi conservative and in the second generation you get semi conservative and N14.
The conservative model suggests that the original DNA molecule remains intact and that a new molecule made from entirely new nucleotides is formed.
Mitosis is the process of cell division. It result in the formation of two genetically similar (identical) daughter cells. It is used for repairing of damaged tissue, asexual reproduction, and cell growth.
4 stages of mitosis PMAT
Prophase- the chromosomes condense they become shorter and fatter. They also become visible. The centrioles move to the opposite ends of the cell. The nuclear membrane breaks down and the nuclear envelope disappears.
Metaphase- the chromosomes line up in the center of the cell also known as the equator. They attach to the spindles by their centromere.
Anaphase- the centromere splits, separating each pair of sister chromatid. The spindles contract and pull the chromatids to the opposite ends of the cell.
Telophase- the chromosomes reach the opposite ends of the cell. They unwind and become long and thin. The nuclear envelope begins to reform. The cytoplasm splits forming two identical daughter cells.
Cancer is the result of a mutation in the gene that controls cell division. Cell division and cell growth are controlled by genes. Proto-oncogenes produce proteins that stimulate the cell cycle, whereas tumor suppressor genes stop cells from dividing.
A tumor is the abnormal mass of a tissue caused by uncontrolled cell growth.
Treatments for cancer can disrupt the cell cycle because they cannot distinguish between normal and cancer cells. Chemical treatments can result in enzymes used in DNA replication not being synthesized. This prevents the cell from moving to the next stage of the cell cycle and the cell will kill itself. In radiation treatments DNA can become damaged. During synthesis damaged DNA is checked for, if it is damaged the cell will kill itself which prevents further tumor growth. To reduce the impact on normal body cells a large chunk of the tumor can be removed by surgery first so any left to nutrients or oxygen will be more likely to be triggered into the cell cycle where they are more susceptible to the treatment. Also having breaks will mean your body cells are able to recover and since the breaks will be short they won’t allow the tumor to grow to the same length as they were.
If in the table for mitosis interphase has the smallest number of cells then that means that interphase is fast.
Multicellular organisms are made up of many different cell types. These cells are specialised to carry out a function. The structure of these cells is adapted to suit its function. These cells become specialised by a process known as differentiation.
Palisade mesophyll cells are found in the leaf and is where photosynthesis occurs. They have lots of chloroplasts so they can absorb sunlight. They have thin wall so carbon dioxide can enter easily.
Tissues are a group of cells that work together to carry out a function.
Organs are a group of tissues that work together to carry out a function.
A leaf is an example of a plant organ it is made up of the following tissues:
Upper epidermis- covered in a waxy waterproof cuticle to prevent water loss
Lower epidermis- contains stomata which let air in and out for gas exchange
Spongy mesophyll- contains air spaces for gases to circulate
Palisade mesophyll- where photosynthesis occurs
Xylem- carries water towards the leaf
Phloem- carries sugars away from the leaf
Lungs are an example of animal organs. They are made up of the following tissues:
Fibrous connective tissues- contain fibers that force air out when exhaling
Blood vessels- contain capillaries that surround your alveoli.
DNA stands for deoxyribonucleic acid. DNA has hydrogen bonds between its bases and has a sugar- phosphate backbone.
Structure of a nucleotide:
Phosphate- sugar- base
Base pairs will always be adenine with thymine by 2 hydrogen bonds and cytosine and guanine with 3 hydrogen bonds.
Adenine and guanine are both large molecules compared to thymine and cytosine. The distance in the sugars/phosphates must be constant. Therefore if adenine and guanine paired up the distance would be longer compared to thymine and cytosine.
Also cytosine and adenine cannot pair up because of the molecular structure of the two hydrogen bonds would not be able to form.
The function of DNA is to store genetic material. It is wrapped and coiled so can store a large amount of information in a small space in the nucleus. The DNA has a paired structure so it can easily replicate itself. The double helix means that the DNA molecule is stable.
Eukaryotes DNA is linear and exists in the form of chromosomes. The DNA is long so has to be wrapped around protein known as histone. Histone also support the DNA. DNA and histone are then coiled into a compacted chromosome.
Prokaryotic DNA is shorter and circular it is not wrapped around protein but is condensed to fit by super coiling.
Genes are sections of DNA that are found on chromosomes and code for protein. Proteins are made from amino acids. Amino acids are coded by a sequence of bases known as a triplet.
Introns are sections of DNA that do not code for an amino acid. Extrons code for amino acids. Repeated sequences also don’t code for amino acids. Introns are removed during protein synthesis.
Nature and development is controlled by genes because DNA sequence determines the sequence of amino acids. Amino acids produce protein. Enzymes are proteins. Enzymes help control metabolic pathways. Metabolic pathways determine our nature and development. Therefore genes determine our nature and development because they contain all the genetic information needed to produce all our proteins and enzymes.
Alleles are different versions of a gene they can be recessive or dominant.
In humans we have 23 pairs of chromosomes or 46 in total. A homologous pair is a pair of matching chromosomes. Homologous pairs are the same size, contain the same genes but may have different alleles. These alleles that code for the same gene are found at the same point or locus of a chromosome.
Mutations are changes in the sequence of bases in an organisms DNA. It can result in the formation of new alleles. Also the proteins produce would change. A gene that codes for an enzyme may produce a nonfunctioning protein if a mutation occurs because the enzyme may fold up in the wrong way, resulting in a different shaped active site.
Gametes are sex cells. In males they are sperm and in females they are eggs. They are both haploid and fuse together during fertilization to produce a zygote which then divides to produce an organism. They must be haploid so that when they fuse the organism has the right number of chromosomes.
Meiosis is process of cell division which produces gametes. It takes a diploid cell and makes it haploid. It forms 4 non identical daughter cells.
First the DNA unwinds and replicates so there are two copies of each chromosome. The DNA then condenses to form double armed chromosomes which are made up of two sister chromatids. Meiosis 1 is the first division where the chromosomes arrange themselves into homologous pairs. The homologous pairs then split halving the number of chromosomes. Does this mean we now have 23 chromosomes instead of 46? am confused what u mean! You then have meiosis 2 where the sister chromatids that make up the chromosome split. This leads to the formation of 4 genetically different daughter cells.
During meiosis 1 the chromatids cross over and parts of the chromatid will break off. This leads to the formation of different alleles which leads to genetic variation.
Independent segregation of chromosomes will also lead to genetic variation because the homolohous chromosomes can split up in any way. This leads to different alleles being produced.
Genetic diversity in a species is caused by different alleles. All members of the same species will have the same genes. Different species will have different genes. The more shared the DNA is the closer the relation. Genetic diversity is mainly caused by differences in alleles.
Differences in alleles occur due to mutations, or because of gene flow when new genes are introduced to a population from another population.
Genetic bottlenecks lead to large reduction in the population. The survivors will then reproduce and the genetic diversity is reduced because there are fewer alleles. Due to increased inbreeding.
Founder effect is when organisms of a population decide to form a colony. This reduces the gene pool. The gene pool is the total number of alleles in a particular population in a specific time. This increases the chances of developing genetic diseases.
Selective breeding is the process whereby humans select which domestic animal and or strain of plant will reproduce together. This reduces genetic diversity as once the organism with desired characteristics is produced only that type of organism will be bred.
For selective breeding
Produces high yielding animals and plants
Produces animals and plants with high resistance to diseases
Produces plants and animals that can withstand bad conditions
Against selective breeding
Can cause health problems
Can lead to an increase in genetic diseases
Water in plants goes from the soil to the root hair cells to the cortex to the endodermis to the xylem vessel.
Water moves by osmosis along the concentration gradient
Once water has entered the root hair cell it can move by two pathways.
Symplast pathway- water moves through the cytoplasm of the cells in the cortex by osmosis. They move through the cell walls through tiny openings known as plasmodesmata.
Apoplast pathway- water moves by cohesion along the cell wall. Cohesion is the mutual attraction of molecules.
Once water has reached the endodermis by the apoplast pathway it meets the casparian strip and cannot progress further along the cell wall. It is forced into the cytoplasm of the endodermal cells where it meets water from the symplast pathway. The endodermal cells actively transport mineral ions into the xylem. The active transport of mineral ions leads to the water potential of the xylem decreasing. Water can now move from the endodermal cells to the xylem by osmosis along the concentration gradient.
Once the water has reached the xylem pressure is created known as root pressure. This can be used to move water further up the xylem. As the pressure is weak it is not useful in large plants but is useful in smaller plants.
Cohesion- tension theory is used to move water up towards the leaf
Water leaves the leaf as a result of transpiration. This leads to the water molecules forming hydrogen bonds between one another so they stick together this is known as cohesion. The water then forms a continuous unbroken pathway along the mesophyll cells and down the xylem. Water then evaporates form the mesophyll cells in the leaf into the air space beneath the stomata. More water is then pulled up behind it. This water is then pulled up from the xylem. This is known as transpiration pull. This puts the xylem under tension so hence the name cohesion- tension theory.

Yes, we double the chromosome number.
They will probably give you something not human though as that can be practised fairly easily.

They will probably say a fruit fly has 29 pairs of chromosomes, how many chromosomes does it have during anaphase?
So, really the 29 pairs would be simply "n" and during anaphase, it would be 2n. =)

I'm a retaker too, grade boosting haha (I got 1 mark off a last time, but I need to claw some extra UMS because of the ruddy ISAs)

I'm focusing most on the unit 5 things, because they are the priority, but whatever you were shaky on for this unit, go over it loads and loads, and do the questions in the text book for all chapters just to see how well you remember it :P

X

Right, 1 cell turns into two during 4 hours and 18 minutes

2 turn into 4 during another 4 hours and 18 mins

4 turn into 8 during ANOTHER 4 hours 18 mins.

4 hours 18 mins = 258 mins per cycle.
258 mins * 3 = 774 mins
774/60 = 12.9 hours
12.9 hours = 12 hours 54 mins. =)

EDIT:
To make spoilers do do the following(remove the V);

Start with [SpoilerV]Type here

I would put it online but my teacher said she would give it to me when the year 12s finished with their mocks. =l

Have you done your's yet?

Yay found this thread! Hi all

I have a question... a past paper says this, but I don't understand how to get the answer (probably me being a bit thick at before 9 in the morning!)

"The time required for a cell to complete the cell cycle was 4 hours 18 minutes.
Calculate the time required in minutes for this cell to multiply to produce eight cells.
Show your working."

I have the answer from the markscheme, but I don't know how to do that spoiler thing hehe Look away now if you want to work it out before you see the answer!








"Correct answer = 774 minutes/ 12 hours 54mins = 2 marks;;
Incorrect answer but indicates 3 cell cycles involved = one mark;"

Thank you to anyone who can help

I can't copy the question because I'm on my iPad, but could anyone tell me why the answer to 7b(I) on June 2011 is G.Americana and G.monachus, it just doesn't make sense to me!


Posted from TSR Mobile

I can't copy the question because I'm on my iPad, but could anyone tell me why the answer to 7b(I) on June 2011 is G.Americana and G.monachus, it just doesn't make sense to me!


Posted from TSR Mobile

The 99.9% ones are the same species, of course that is to throw you off. =P
The two DIFFERENT species are the ones quoted in the mark scheme. That is because the most hydrogen bonds in total percentage wise formed between these two species due to there being more complimentary base pairings.

EDIT:
To make it easier, I have included a modified version of the question. Green = Same species comparison, red = different.

The 99.9% ones are the same species, of course that is to throw you off. =P
The two DIFFERENT species are the ones quoted in the mark scheme. That is because the most hydrogen bonds in total percentage wise formed between these two species due to there being more complimentary base pairings.

EDIT:
To make it easier, I have included a modified version of the question. Green = Same species comparison, red = different.