Unit 2 – Section 1Variation

Sampling

• Involves taking measurements of individuals selected from the population of organisms that's being investigated.

• In theory these individuals are representative of the population as a whole however there are several reasons why this may not be the case:

▪ Bias – the selection process could be biased by the individuals taking the sample by them making unrepresentative choices either deliberately or unwittingly

▪ Chance – even if bias is avoided the individuals may be unrepresentative due to pure chance

Prevent sampling bias by using random sampling:

• Divide the study area into a numbered grid – this can be achieved by using tape measures

• Using a random numbers table obtain a set of coordinates

• Take a sample at the intersection of each pair of coordinates

Minimising the effect of chance:

• Use a large sample size

▪ The more individuals that are selected the smaller the probability that chance will influence the result

▪ Greater the sample size the more reliable the data will be

• Analysis of the data collected

▪ Using stats tests to determine the extent to which the data has been influenced by chance

▪ Tests allow us to decide whether any variation observed is the result of chance or is more likely to have some other cause

Causes of variation:

• Mutations

▪ These sudden changes in DNA may or may not be passed on

• Meiosis

▪ This forms the gametes and mixes up which chromosomes end up in which gamete so all are different

• Random fertilisation

▪ which sperm fuses with the egg in fertilisation is random therefore adds ot the variety of characteristics the offspring have

Environmental influences:

• exerts an influence on all organisms

• Affect the way the organisms genes are expressed

• Environmental influences include:

▪ Climatic conditions (temperature, light intensity, rainfall etc.)

▪ Soil Conditions

▪ pH

▪ Food availability

Types of Variation

Variation due to genetic factors:

• All organisms fit into a few distinct groups (e.g. blood types A, B, AB and O)

• No intermediate types

• Usually controlled by a single gene

• Represented by a bar graph or pie chart

• Environmental factors have little influence on this type of variation

Variation due to environmental factors:

• Forms a continuum

• Examples in humans are height and mass

• Controlled by many genes

• Environmental factors determine where on the continuum the organism lies

• Represented by a normal distribution curve:

(image from tushar-mehta.com)

Mean and standard deviation & how these relate to the normal distribution curve:

• Mean

▪ maximum height of the curve

▪ average value

▪ Doesn't provide any information about the range of values

• Standard Deviation

▪ measure of the width of the curve

▪ indication of the range of values either side of the mean

Standard Deviation → √ ∑ x − mean2

ndon't forget the top line is actually: (x-mean)2

x= measured value from sample

n = total number of values in sample

Σ = sum of

Unit 2 : Section 2DNA Nucleotide

made up of 3 components:

• Hexose sugar (deoxyribose)

• Phosphate group

• A base of which there are 4:

▪ Cytosine (C)

▪ Thymine (T)

▪ Adenine (A)

▪ Guanine (G)

Pairing of bases (the complimentary base-pair rule)

Base 1 Base 2 Number of hydrogen bonds

A T 2

G C 3

Remembering this:

A and T as letters are both made up of straight lines so go together. G and C are both made up of a curved line so go together.

How we go from small nucleotides to the massive DNA double helix

• Nucleotides bond together in condensation reactions

• In these reactions the dexoyribose sugar from one nucleotide and the phosphate group from another bond together

• This keeps happening and the structure formed from these linkages is called a sugar phosphate backbone. The overall structure is a polynucleotide

• The bases stick out from this and following the complimentary base-pair rule another polynucleotide joins to the first one by hydrogen bonding to the bases.

• This other strand is always upside down

Function of DNA

• Very stable so can pass from generation to generation without change

• Two separate strands joined by hydrogen bonds which allows them to separate for DNA replication

• Very large molecule so carries a lot of genetic information

• The base pair rule protects the genetic information somewhat from corruption

What is a gene?

A section of DNA that codes for one polypeptide (protein)

Triplet code

This means that every 3 bases in DNA codes for one particular amino acid. Some amino acids have multiple codes that make them. Every 3 bases is also called a codon.

The code is also non-overlapping as when the DNA code is read (for example in protein synthesis) 3 bases at a time are read and then you move on to the next set of 3 bases. You don't just move on one base.

Chromosomes

• These are only visible as distinct structures during cell division

• They are formed by the DNA helix combining with proteins

• This then coils more and folds to form loops

• These loops then coil and pack together to form the chromosome

Chromosome's occur in pairs. These are called homologous pairs as the chromosomes determine the same genetic characteristics such as eye colour. This doesn't mean the chromosome’s are identical however.

They occur in pairs as in sexual reproduction one chromosome comes from dad and one from mum so they pair up. This is due to sex cell formation in meiosis as only half the number of chromosomes are in the sex cells. There are normally 46 chromosomes in each body cell however there are only 23 in sex cells.

Alleles

These are different forms of the same gene. If the gene codes for eye colour then the alleles would

be blue, brown, green etc.

Each allele codes for a different polypeptide.

Unit 2 – Section 3

Genetic diversity

• Basically variation in the DNA of organisms

• All members of the same species have the same genes

◦ However there are different forms of these genes (eye colour – lots of different colours = lots of different alleles) these different forms are called alleles

• The greater the number of different alleles a species has the greater the genetic diversity of the species

◦ This means the species has a greater chance of adapting to survive change

The following are all factors that influence genetic diversity:

• Selective Breeding

• The founder effect

• Genetic bottlenecks

Selective Breeding

This involves selecting individuals with desired characteristics and mating them together. Offspring that don't have these desired characteristics are killed or at least prevented from breeding.

Impact on genetic diversity

This reduces genetic diversity as:

• Unwanted alleles are bred out of the population

• Therefore the variety of alleles in the population is deliberately reduced

• This leads to a population which has the desired characteristics but has much reduced genetic diversity

This practice is commonly carried out by farmers to produce high yielding crops and animals

The founder effect

• A few individuals from a population colonise a new region

• These individuals will only carry a small fraction of the gene pool (all the alleles a species has collectively)

• The new population that develops will therefore show less genetic diversity than the original population

• This is often seen on new volcanic islands for example

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Genetic Bottlenecks

• Population suffers a drop in numbers (could be caused by a natural disaster)

• The survivors will only possess a small fraction of the gene pool

• The genetic diversity will therefore be less

• As the breed and re-establish the population the genetic diversity of this new population will be restricted

• This means it is less likely the population can adapt and survive changes

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Unit 2 – Section 4

Haemoglobin

Haemoglobin is a protein found in a wide variety of organisms. It has a primary, secondary, tertiary and quaternary structure:

• Primary structure – consisting of amino acids peptide bonded together in 4 chains• Secondary structure – in which each of the polypeptide chains are coiled into a helix• Tertiary structure – each polypeptide chain is folded into a precise shape which is

important for carry oxygen• Quaternary structure – in which all 4 polypeptides are linked together to form an almost

spherical structure

In addition to this each polypeptide is linked to a haem group which contains the Fe2+ ion. This can bind to one oxygen molecule so each haemoglobin molecule can carry 4 oxygen molecules in humans.

The role of haemoglobin:

The role of haemoglobin is to transport oxygen around the body to where it is needed. To be efficient at this it must:

• Readily 'load' oxygen (association) at the gas exchange surfaces• Readily 'unload' oxygen (dissociation) at the tissues where oxygen is required

Changing affinity and why this is important

Haemoglobin changes it's affinity for oxygen under certain circumstances this means that it can load and unload oxygen where this is necessary:

Region of body Oxygen concentration

Carbon dioxide concentration

Affinity of haemoglobin for

oxygen

Result

Gas Exchange surface

High Low High Oxygen association

Respiring tissues Low High Low Oxygen Dissociation

Different organisms and Haemoglobin

There are different types of haemoglobin and these show different affinities for oxygen and this can be an advantage to organisms living in different habitats

• Haemoglobin with a high affinity for oxygen – take up oxygen more easily but release it less easily – would be an advantage for an organism in a low oxygen environment

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• Haemoglobin with a low affinity for oxygen – associates less easily with oxygen but dissociates faster – advantageous for a very active organism

Oxygen dissociation curves

When haemoglobin is exposed to different partial pressures of oxygen haemoglobin doesn't absorb oxygen evenly. The first molecule is difficult to absorb but then this forces the other 3 molecules to be absorbed a lot easier. The graph of this relationship is known as an oxygen dissociation curve.

In the exam they could give any oxygen dissociation curve to be interpreted keep these in mind:

Where is the curve? Affinity of haemoglobin for

oxygen

Take up of oxygen Release of oxygen

Further to the left Higher Easier Harder

Further to the right Lower Harder Easier

Effects of carbon dioxide concentration

Haemoglobin has a reduced affinity for oxygen when carbon dioxide is present as the greater the concentration of carbon dioxide the more readily the haemoglobin releases the oxygen it's carrying.

• As the gas exchange surface (the lungs) – there are low levels of carbon dioxide as it diffuses out the organism, this means that haemoglobin has a high affinity for oxygen here. As there is also a high concentration of oxygen here means oxygen is readily loaded

• In rapidly respiring tissues – the level of carbon dioxide is high therefore the affinity of haemoglobin for oxygen is reduced and coupled with a low concentration of oxygen in the muscles means that oxygen is readily unloaded from the haemoglobin into the muscle cells

Loading, transport and unloading of oxygen

• At the gas exchange surface carbon dioxide is constantly being removed and due to

this the pH of the blood in this area is raised

• The higher pH changes the shape of haemoglobin meaning that it loads oxygen more

easily

• This shape also increases the affinity of haemoglobin for oxygen so that oxygen isn't

unloaded in the blood on it's way to the tissues

• In the tissues carbon dioxide is produced by respiration and as it diffuses into the blood

it makes the blood pH lower (makes the blood more acidic)

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• This changes the shape of haemoglobin to a shape that means it has a lower affinity

for oxygen

• Haemoglobin therefore releases its oxygen into the respiring tissues

Starch, Glycogen and Cellulose

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Starch• Storage polysaccharide in plants it is found in many parts of the plant in the form of

small grains

• Adapted for this role as:◦ Insoluble so doesn't dissolve in the water that is inside plant cells◦ Compact – so a lot can be stored in a small space◦ When hydrolysed forms α-glucose which can readily be transported around the

plant and used in respiration

• Storage molecule for plants and is never found in animal cells

Glycogen• Very similar in structure to starch• Same adaptations as starch• Sometimes called 'animal starch' as it's the storage molecule in animals and is never

found in plants

Cellulose• Unlike starch or glycogen it's a structural polysaccharide• In order to bond together the adjacent β-glucose molecule has to rotate 180o this

allows hydrogen bonding between the glucose chains and makes it strong• major component of plant cell walls and provides rigidity

Comparison Table

Polysaccharide Monomers Bonding 3-D Structure

Starch α-glucose Glycosidic Coiled

Glycogen α-glucose Glycosidic Coiled

Cellulose β-glucose Glycosidic Sheets

They also all have a 1-4 linkage in the Glycosidic bond as on one glucose the bond starts from carbon 1 and goes to carbon 4 on the neighbouring molecule.

Plant Cell Structure

Leaf palisade cell

• They are long thin cells that form a continuous layer to absorb sunlight for photosynthesis

• They have numerous chloroplasts that arrange themselves in the best position to collect the maximum amount of energy

• A large vacuole that pushes the cytoplasm and chloroplasts to the edge of the cell

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Chloroplasts

Main features of chloroplasts are:

• The chloroplast envelope – double plasma membrane that surrounds the organelle

and is highly selective as to what enters and leaves the organelle

• The grana – these are stacks of up to 100 disc like structures called thylakoids these

are where the first stage of photosynthesis takes place

• Thylakoids – these contain the photosynthetic pigment called chlorophyll, some of

these have extensions called lamella which join them to other thylakoids in adjacent

grana

• Stroma – this is a fluid filled matrix where the second stage of photosynthesis takes

place, within this there are a number of other structures such as starch grains

Cell wall

In plant cells this has the following features:

• Consists of a number of polysaccharides such as cellulose• Thin layer called the middle lamella which marks the boundary between adjacent cells

and cements cells together

Functions of the cell wall are:

• To provide structure and strength to the cell to stop it bursting under the pressure created when water enters by osmosis (osmotic pressure)

• To give structure to the plant as a whole• To allow water to pass along it and therefore contribute to the passage of water

through the plant

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Differences between plant and animal cells

Plant cells Animal cells

Cellulose cell wall and cell surface membrane Only a cell surface membrane

Chloroplasts are present in large numbers in most cells

Chloroplasts are never present

Large single central vacuole filled with cell sap

If there are vacuoles (rare) they are small and scattered throughout the cell

Starch grains used for storage Glycogen granules are used for storage

Unit  2  –  Section  5  

Cell  Division  Two  main  stages:  

1. Nuclear  division  –  two  types  mitosis  and  meiosis  –  this  is  the  process  of  the  nucleus  dividing  2. Cell  division  –  follows  nuclear  division  and  is  the  process  of  the  whole  cell  splitting  into  two  

Semi-­‐conservative  DNA  replication  • The  enzyme  DNA  helicase  breaks  the  hydrogen  bonds  linking  the  pairs  of  DNA  bases  

 • The  DNA  helix  then  separates  into  two  strands  and  unwinds  • Each  exposed  strand  then  acts  as  a  template  and  free  DNA  nucleotides  that  are  in  the  cytoplasm  

join  to  these  template  stands  following  the  complimentary  base  pair  rule  (A-­‐T,  C-­‐G)  • Energy  is  needed  to  attach  these  free  nucleotides  to  the  template  strand  • These  are  then  joined  by  another  enzyme  called  DNA  polymerase  to  from  two  ‘DNA  molecules’  

from  the  one  that  was  present  in  the  cell  

This  is  called  semi-­‐conservative  replication,  as  half  of  the  original  DNA  molecule  is  present  in  both  of  the  new  DNA  molecules  and  by  doing  this  the  two  strands  are  identical.  

 

 

 

 

Mitosis  This  is  the  division  of  the  nucleus  of  the  cell  that  results  in  the  formation  of  two  identical  daughter  cells  with  each  of  them  having  an  exact  copy  of  the  parent  cell’s  DNA  

5  Stages:  

1. Interphase  –  The  DNA  replicates  and  the  cell  gets  ready  to  divide  by  synthesising  proteins  2. Prophase  –  The  chromosomes  become  visible  and  the  nuclear  envelope  disappears  3. Metaphase  –  The  chromosomes  arrange  themselves  at  the  centre  (equator)  of  the  cell  4. Anaphase  –  Spindle  fibres  pull  the  chromatids  towards  the  poles  of  the  cells  5. Telophase  -­‐  Nuclear  envelope  reforms  and  chromosomes  disappear  from  view  and  nucleolus  

reforms  

 

   

Importance  of  mitosis  • Growth  –  When  a  sperm  and  egg  fuse  during  fertilisation  then  the  embryo  needs  to  grow.  All  cells  

need  to  be  genetically  identical  as  they  all  need  to  have  a  full  set  of  genetic  information  to  form  the  new  organism  

• Differentiation  –  Tissues  need  to  be  made  up  of  identical  specialised  cells  so  these  cells  divide  by  mitosis  

• Repair  –  If  cells  are  damaged  it’s  important  they  are  replaced  with  identical  cells  that  have  the  same  structure  and  perform  the  same  function  

The  Cell  Cycle  Cells  don’t  jut  keep  dividing  continuously;  rather  they  go  through  a  cycle  so  it  is  a  controlled  process  

 

 

• G1  –  Part  of  interphase  –  Proteins  to  synthesize  cell  organelles  are  produced  • S  phase  –  Part  of  interphase  –  when  DNA  replication  occurs  • G2  –  part  of  interphase  –  organelles  grow  and  divide  and  energy  stores  are  increased  • Mitosis  

 

Unit  2  –  Section  6  

Definitions  Term   Definition  

Cell  differentiation/cell  specialisation   The  process  by  which  cells  become  adapted  for  their  job  within  the  body  

Tissues   A  collection  of  similar  cells  that  perform  a  specific  function  

Organs   A  combination  of  tissues  that  are  coordinated  to  perform  a  variety  of  functions,  however  they  usually  have  one  major  role  

System   A  collection  of  organs  working  together  to  perform  a  particular  function  more  efficiently  

 

How  do  cells  become  specialised?  • All  cells  have  all  your  genes  • However  only  some  of  these  are  switched  on  (expressed)  • Different  genes  are  switched  on  depending  on  what  type  of  cell  is  going  to  be  created  • Specialised  cells  have  different  shapes  but  also  different  numbers  of  organelles  such  as  

mitochondria  –  as  muscle  cells  will  need  more  because  they  will  need  to  respire  more  to  produce  enough  energy  to  contract  and  relax  during  exercise  

This  happens  because  cells  have  evolved  to  become  more  and  more  suited  for  a  particular  function  and  this  means  they  are  dependent  on  other  cells  to  carry  out  other  functions.  However  as  each  cell  is  adapted  for  its  particular  role  it  means  that  that  can  perform  this  role  more  effectively  so  the  organism  functions  more  efficiently.  

Examples  of  Tissues,  Organs  and  Systems  Type   Examples  Tissue   Epithelial  tissues  

Muscle  Connective  tissue  

Organ   Heart  Lung  Stomach  Leaf  

System   Circulatory  system  Gas  exchange  system  Digestive  system    

 

3.2.2 – Meiosis

Meiosis and Mitosis the difference

• Mitosis – Produces two genetically identical daughter cells with the same number of chromosomes as the parent cell

• Meiosis – Produces 4 daughter cells each with half the number of chromosomes as the parent cell

The stages of meiosis

1. The cell copies the chromosomes to form homologous pairs

2. These then line up at the centre of the cell and are pulled apart by spindle fibres

3. One copy of each chromosome goes into each of the two daughter cells

4. The chromosomes line up at the centre of these cells and are pulled apart again

5. This forms chromatids

6. One chromatid from each chromosome then goes into the two daughter cells

7. Don't forget this effectively happens twice as there are two cells initially with chromosomes in

Meiosis 1