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Inheritance, Variation and Evolution

AQA Biology topic 6

6.1 Reproduction 21/11/2017

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Sexual Reproduction

The human egg and sperm cell contain 23 chromosomes each.

When fertilisation happens the gametes fuse together to make a single cell containing 46 chromosomes (23 pairs) - it contains information from each parent. The same happens in plants with pollen and egg cells.

We have similar characteristics to our parents due to genetic information being passed down in genes through gametes:

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Fertilisation summarised

Egg

Sperm

Fertilisation

23 chromosomes in here

23 chromosomes in here

46 chromosomes in a fertilised

egg

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Cell growth 1 - Mitosis

Each daughter cell has the same number of

chromosomes and genetic information as the parent

– a “clone” is produced.

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Gamete formation - Meiosis 1) A copy of the genetic information is made

2) The cell divides twice to form four gametes, each with a single set of chromosomes

All gametes are genetically different from each other. Gametes will then join at fertilisation to restore the full

number of chromosomes and divides by mitosis from then on.

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Mitosis vs. Meiosis Mitosis:

1. Used for growth and repair of cells

2. Used in asexual reproduction

3. Cells with identical number of chromosomes and genetic information are produced (“clones”)

Meiosis:

1. Used to produce haploid gametes for sexual reproduction

2. Each daughter cell has half the number of chromosomes of the parent

During meiosis copies of the genetic information are made and then the cell divides twice to form four daughter cells.

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Sexual vs. Asexual reproduction

Sexual reproduction:

•2 parents are needed

•Offspring will have “pairs” of chromosomes

•This will cause genetic variation

Asexual reproduction:

•Only 1 parent needed

•Offspring are GENETICALLY IDENTICAL to parent (“clones”)

“Snuppy” – the first cloned dog

(Aug 05)

Sexual vs asexual reproduction (Bio only) 21/11/2017

Advantages of sexual reproduction

Advantages of asexual reproduction

No need to find a mate

More time and energy-

efficient

Increased variation, which may cause an

evolutionary advantage

Selective breeding can be used to increase

food production

Which form of reproduction is

better?

Faster than sexual reproduction

Produces a clone of a good animal

Reproducing Sexually and Asexually 21/11/2017

Some organisms can reproduce using both methods. Some examples:

The malaria parasite reproduces asexually in a human but sexually in a mosquito

Many plants reproduce sexually but can also reproduce asexually by runners (e.g. strawberries) or by bulb division (e.g. daffodils)

Fungi can reproduce asexually using spores or sexually to give variation

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Genes

Section of a chromosome:

Genes for eye colour

Genes for hair colour

Genes for blood group:

Genetic information is stored by genes which are arranged on chromosomes:

Each gene codes for a particular sequence of amino acids, to make a specific protein. The entire genetic makeup of an organism is called its genome.

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Genes, Chromosomes and DNA

DNA is a polymer made up of two strands forming a “double helix” structure. DNA is contained within chromosomes.

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The Human Genome Project

In 1990 an international project was launched called the “Human Genome Project”. The aim was to map the _______ makeup of the human race and includes work from ______ in 18 different countries.

Possible benefits:

• Improved genetic testing

• Improved predictions and screening of ________ diseases

• New gene ________ treatments

• New knowledge of human _______ patterns from the past

Words – scientists, genetic, migration, inherited, therapy

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Some facts:

- It is made of four different nucleotides that consist of a _____ and phosphate group

- It contains instructions on what a ____ does, how the organism should work etc

- The code is made up from the four ____ that hold the strands together with weak hydrogen bonds

- A sequence of three bases represent the order in which _____ acids are assembled to make specific ________

- The DNA polymer is made up of repeating ________ units

- In the complementary strands, a C is always linked to a G and a T is always linked to an A

DNA detail (Bio only)

Words – amino, sugar, bases, cell, proteins, nucleotide

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Making proteins (HT only) 1) DNA “unravels” and a copy of one strand is made

2) The strand copy is made to produce RNA

3) The mRNA copy (with its code) then moves towards the ribosome

4) The ribosome “decodes” the mRNA code which tells the ribosome how to make the protein

5) Amino acids are then joined together to form a polypeptide (protein)

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Mutations (HT only) Cells contain a nucleus and the nucleus contains genes that carry instructions for what that cell should do:

Genes

Some facts:

• Some genes are “switched off” and don’t do anything in that cell

• Genes basically tell the cell which proteins they should be producing

• Sometimes cells can “mutate” which may have a harmful effect on the cell and can be caused by natural or artificial means.

Genetic mutations (Bio HT only) 21/11/2017

A T C G G A T Here’s a sequence of bases in DNA:

Q. What happens if this sequence is changed?

Possible ways this sequence is changed:

1) A base is inserted A T C G G A T A

2) A base is substituted A T C G G A T A

3) A base is deleted A T C G G A T

DNA mutations are the reason why we have genetic variation in the first place. Sometimes these changes won’t affect a protein made from the DNA, sometimes the protein may function differently (changing the phenotype). Changes in non-coding DNA may influence phenotype by altering how genes are expressed.

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Proteins (HT only) Proteins, when unfolded, are basically long chains of amino acids:

Every different protein has its own number and sequence of amino acids which results in differently shaped molecules with different functions.

Some example proteins:

Name of protein Function

Collagen Give structure

Insulin Hormones

Enzymes Help food digestion

Not all parts of DNA code for proteins. Non-coding parts of DNA can switch genes on and off.

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Mutations and effects on Proteins (HT)

Enzymes are biological catalysts. Most enzymes are proteins and, as such, a mutation in DNA will affect them:

An enzyme is basically a protein molecule made up of long chains of amino acids. These molecules are then “folded” to create a certain shape with high “specificity”:

The enzyme’s shape helps another molecule “fit” into it:

This shape can be affected by mutations in the DNA or the proten may lose its strength:

Enzyme Substrate

21/11/2017 Basic genetics - Boy or Girl?

X Y X

XX XY Girl Boy

“Allele”

“Phenotype”

Heterozygous Homozygous

Note that the Y chromosome is “dominant” and the X is “recessive”. The Y chromosome dictates the development of testes.

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Father

Mother

Son

Daughter

Boy or Girl?

During sexual reproduction, children inherit two alleles of each gene (one from each parent).

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Key words Genotype

Phenotype

Allele

Dominant

Recessive

Homozygous

Heterozygous

•This allele determines the development of a

characteristic

•The characteristic caused by the genotype

•This allele will determine a characteristic only if

there are no dominant ones

•This word refers to a pair of chromosomes being

made of two different alleles of a gene

•The genetic make up in a nucleus

•This word refers to a pair of chromosomes being

made of two of the same alleles of a gene

•An alternative form of a gene

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Eye colour In eye colour the brown eye allele is dominant, so we call it B, and the blue eye is recessive, so we call it b:

bb BB Bb

Homozygous brown-eyed

parent

Heterozygous brown-eyed

parent

Blue-eyed parent

What would the offspring have?

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Eye colour Example 1: A homozygous

brown-eyed parent and a blue-eyed parent:

Example 2: 2 heterozygous brown-eyed parents

BB bb X Bb Bb X Parents:

Gametes:

Offspring: Bb Bb Bb Bb BB Bb bb bB

B B b b B b B b

(FOIL)

All offspring have brown eyes 25% chance of blue eyes

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Eye colour Example 3: A heterozygous brown-eyed father and a blue-eyed mother:

Bb

Bb Bb bb bb

bb

b b B b

Equal (50%) chance of being either brown eyed or blue eyed.

Note – in reality, characteristics like this are usually depend on the instructions of multiple genes and other parts of the genome.

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B b

b

b

Another method – the “Punnett square”

Example 3: A heterozygous brown-eyed father and a blue-eyed mother:

B b

b Bb bb

b Bb bb

Father

Mother

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Example questions

1) In mice, white fur is dominant. What type of offspring would you expect from a cross between a heterozygous individual and one with grey fur? Explain your answer with a genetic diagram.

2) A homozygous long-tailed cat is crossed with a homozygous short-tailed cat and produces a litter of 9 long-tailed kittens. Show the probable offspring which would be produced if two of these kittens were mated and describe the characteristics of the offspring (hint: work out the kitten’s genotype first).

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Inherited diseases 1) Cystic fibrosis – a disorder or cell membranes. It’s caused by recessive alleles so both parents need to be “carriers”:

2) Polydactyly – a condition where a person has extra fingers or toes. It’s caused by a dominant allele so can be passed on by a parent who already has it:

Embryos can be screened for alleles that cause these diseases before birth. Do you think this is right?

Ff Ff X

Pp pp X

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Genetic Diagrams Here’s what happens (genetically) when an egg is fertilised:

xx

xx xy xy xx

xy

x y x x

Equal (50%) chance of being a boy or a girl

Mother Father

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x y

x

x

Genetic Diagrams Here’s another way of drawing it:

Father

Mother

6.2 Variation and Evolution 21/11/2017

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Variation “Variation” is the name given to differences between individuals of the SAME species.

Variation is due to GENETIC or ENVIRONMENTAL causes. There is often extensive variation within a species. For example, consider dogs:

1) Ways in which they are the same:

2) Ways in which they are different:

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Environmental differences Some of this variation is due to our parents, but some of it is due to our upbringing and the environment in which we live – this is called “Environmental variation”.

Variation due to inheritance only

Variation due to environment only

Variation due to a bit of both

21/11/2017 21/11/2017 Beneficial Mutations - the peppered moth

All variations arise from mutations. Most mutations have no effect on phenotype whereas some do. If the new phenotype is more suited to its environment then it can lead to a rapid change in the species. For example…

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Evolution

Charles Darwin, 1809-1882

Evolution is the slow change in organisms that happens over a long period of time. All life on Earth has evolved from simple life forms that existed around 3 billion years ago. It happens through a process called “natural selection”, which basically says this:

3) They then have kids who also have the “better” phenotypes

1) Different species show variation

2) The “better adapted” ones survive

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Definition of “Species”

Liger

Geep Zebroid

Mule

Different species can sometimes mate and have offspring but they would be infertile. Some examples:

A “species” is defined as when organisms reproduce with each other to produce fertile offspring.

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Selective breeding

I raise cows. Each type of cow is good at a certain job. The Friesian cow produces large quantities of milk, the Jersey cow produces very nice milk and the Hereford cow produces lot of beef.

If, for example, I want lots of milk I would only breed Friesian cows

with each other – this is SELECTIVE BREEDING. The only trouble is that I’m reducing the

“gene pool” by doing this.

Friesian

Jersey

Hereford

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Examples of Selective Breeding

Other examples can include plants with larger flowers and disease resistance in crops. Some breeds may be particularly prone to diseases or inherited defects due to selective breeding.

Basics of Genetic Engineering

With genetic engineering I can produce milk that contains: • Extra protein • Lower levels of cholesterol • Human antibodies

Genetic engineering is basically the idea of modifying a genome using a gene from a different organism in order to improve characteristics. For example:

Genetic Engineering has also been used to make disease-resistant plants with bigger fruits and to produce insulin on large scales.

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Genetic Modification Advantages Disadvantages

Improving crop yield

Improving resistance to pesticides

Extend shelf-life

Manufacture a certain chemical (e.g. insulin)

Convenience

Current medical research into using genetic modification to overcome inherited disorders

Genetically modified organisms may be expensive

Need for long term studies

Effects may be passed on to other crops, e.g. weed resistance spreading from crops to weeds

Ethical issues about abortion

Risk of miscarriage or “false positives” with amniocentesis

Summary Here are the basic steps:

Identify the desired gene

Remove the gene from the DNA

Cut open the DNA in the other organism

Insert the removed gene using enzymes again

Clone the organism to produce lots of copies

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How Genetic engineering is done (HT only)

Step 1: “Cut out” the part of the human chromosome that is responsible for the desired characteristic.

Step 2: Using another restriction enzyme cut open a ring of bacterial DNA (a “plasmid” or “vector”). Other enzymes are then used to insert the piece of human DNA into the plasmid. Most of the cells don’t take up the vector so the ones that have need to be “marked”.

Step 3: Place the plasmid into a bacterium which will start to divide rapidly. As it divides it will replicate the plasmid and make millions of them, each with the desired characteristic.

21/11/2017 21/11/2017 Cloning Plants (Bio only) Plants can reproduce ASEXUALLY. The offspring are genetically ________ to the parent plant and are called _________. The only variation between then will be due to environmental factors. Two examples:

1) This spider plant has grown a rooting side branch (“stolon”) which will eventually become __________.

2) A gardener has taken cuttings of this plant (which probably has good characteristics) and is growing them in a ____ atmosphere until the ____ develop.

Words – clones, damp, independent, roots, identical

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Cloning Plants by tissue culture (Bio only)

1) Scrape off a few cells from the desired plant

2) Place the scrapings in hormones and nutrients

3) 2 weeks later you should have lots of genetically identical plants

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Cloning Animals 1 (Bio only) Method 1 – “Embryo transplants”

A developing embryo is “split” before the cells specialise and the identical embryos are implanted into host mothers.

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Cloning Animals 2 (Bio only) Method 2 – Fusion:

Animals can be cloned by taking the nucleus from an adult body cell and transferring it to an empty, unfertilised egg and giving it an electric shock to stimulate the egg to form an embryo:

Host mother Clone

6.3 The Development of understanding of Genetics and Evolution

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Evolution (Bio only)

Charles Darwin, 1809-1882

Evolution is the slow change in organisms that happens over a long period of time. All life on Earth has evolved from simple life forms that existed around 3 billion years ago. I came up with this theory after observations around the world and after years of experimentation and observations. Evolution happens through a process called “natural selection”, which looks like this:

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Natural Selection (Bio only) 1) Each species shows variation – here are some long-necked giraffes and short-necked giraffes:

2) There is competition within each species for food, living space, water, mates etc

4) These survivors will pass on their better genes to their offspring who will also show this beneficial variation. The “smaller-necked” giraffe will eventually die out.

Get off my land

Harsh

Yum

3) The “better adapted” members of these species are more likely to survive – “Survival of the Fittest”

Controversy about Darwin’s Work (Bio only) 21/11/2017

I published my work “On the Origin of Species” in 1859. However, it attracted a lot of controversy because:

1) It challenged the theory that God made every animal and plant on the Earth

2) There wasn’t a lot of evidence at the time

3) The mechanism of natural selection was not known until around 50 years later

21/11/2017 An example of Natural Selection – antibiotic resistant bacteria

1) Mutation – some strains of bacteria can develop _______ to the antibiotics.

2) The non-resistant bacteria are _____ by the _______.

3) The resistant bacteria _______ and pass on their mutations to their ______ - an example of ______ ______

Bacteria

Penicillin

No effect!!

Words – offspring, resistance, killed, antibiotics, reproduce,

natural selection

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Darwin vs Lamarck Darwin wasn’t the first to come up with evolution – he was simply the one credited with explaining how it worked (i.e. Natural Selection). An earlier scientist called Lamarck explained evolution by different means:

Jean Baptiste Lamarck

(1744 - 1829

The giraffe has a long neck because it “stretches” its neck to

reach the food, and these long necks are passed on to their

offspring. Organs which aren’t used will eventually disappear.

However, we now know that changes like these cannot be inherited.

Speciation (Bio only) 21/11/2017

Alfred Wallace 1823-1913

I independently proposed the theory of evolution by natural selection at around

the same time as Darwin. I am best known for my work on warning colours in

animals and my theory of speciation – the “formation of a new species”. One

example of speciation is “geographic isolation”:

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Geographic Isolation Different species can be formed by “geographic isolation”, for example, consider an African elephant:

1) Elephants are separated by a geographic feature e.g. a _________

2) Elephants on each side of the mountain have different ______ in their _____ pool

3) Some offspring have characteristics that help them survive

4) Their weaker _______ die out and the offspring are so genetically removed that they’re incapable of ________ with each other – they’re now different ________

Words – species, mutations, mountain, gene, ancestors, reproducing

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Mendel’s Work (Bio only)

Guten tag! My name is Gregor Mendel. I am the father of modern genetics because of the work I did on pea plants in 1865…

Take two plants; one which is pure-bred for tallness and one pure-bred for shortness, and cross them:

X

Mendel’s experiment:

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Modern Genetics

All the plants produced were tall.

Now cross two of these plants…

3 out of every 4 plants were tall, leading Mendel to hypothesise that “for every characteristic there must be two units that determine the characteristic”

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Modern Genetics

Achtung! Unfortunately, nobody knew about chromosomes or genes when I

published my findings so no one believed me until after my death, when more powerful microscopes were available.

Three key developments that led to Mendel’s ideas being accepted:

1) By the late 19th Century behaviour of chromosomes during cell division had been observed

2) By the early 20th Century it was observed that chromosomes and Mendel’s “units” behaved in similar ways – therefore, the units (now called genes) are located on chromosomes

3) In the mid-20th Century the structure of DNA was determined.

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Evidence for Evolution

Fossil records, showing the development of an organism over a long period of time

Antibiotic-resistant bacteria, giving evidence

for natural selection

Now that the mechanism of natural selection has been understood and with evidence like fossils and antibiotic-resistant bacteria, the theory of evolution is widely accepted.

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Fossils Fossils are the remains of organisms from millions of years ago and are found in rocks. They provide evidence of early life and could have been formed in many ways. Four examples:

This fossil of a bat was formed due to hard parts of the

animal not decaying

This bee and orchid pollen were preserved in amber – the amber lacked some of the

conditions needed for decay to happen

This fossil was formed by parts of its body being replaced by minerals

Fossilised footprints

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Fossil records Fossil records can provide a useful way of observing a species’ development:

The problem is, many early forms of life only had soft bodies and the few remaining traces of them have been destroyed by geological activity. This makes it difficult for scientists to know what happened in the distant past.

The “Stenheim skull”, found in

Germany in 1933

Oh no…

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Extinct Species

Dodo

Sabre-toothed tigers and mammoths

“Extinction” is when every member of a species dies out. Some examples:

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Recap – antibiotic resistant bacteria

1) Mutation – some strains of bacteria can develop _______ to the antibiotics.

2) The non-resistant bacteria are _____ by the _______.

3) The resistant bacteria _______ and pass on their mutations to their ______ - an example of ______ ______

Bacteria

Penicillin

No effect!!

Words – offspring, resistance, killed, antibiotics, reproduce,

natural selection

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Using Antibiotics Antibiotics are used to cure bacterial _______ by killing infective bacteria inside the body. The development of new antibiotics is usually slower than the development of new ______ of bacteria.

There are some issues with using antibiotics:

1) Patients should always complete their _______

2) Antibiotics should be used _______ as bacteria can grow _______ to them, e.g. the MRSA “superbug”.

3) Agricultural use of antibiotics should be _______

Words – course, immune, strains, diseases, sparingly, restricted

6.4 Classification of New Organisms

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Classification How would you construct a key to classify these organisms?

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Classifying organisms All organisms are classified into groups. For example:

Organism

Plants Animals

Vertebrates Invertebrates

Reptiles Fish Birds Amphibians

What is the main difference between these?

“Kingdoms”

Dogs Cats

“Species”

Mammals

Notice that the number of similarities increases as you go down this tree

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Why use classification? 21/11/2017

Carolus Linnaeus, 1707-1778

I invented the modern system of naming species. I did this so that species would have the same name in every language and so that we would have a greater ability to understand different species and how they have evolved.

Human – “homo sapien”

Dog – “Canis lupus familiaris”

Wasp – “vespula germanica”

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Classification Organisms are classified using the following levels:

Kingdom

Phylum

Class

Order

Family

Genus

Species

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Classification Consider, for example, a dog:

Kingdom - Animalia

Phylum - Chordata

Class - Mammalia

Order - Carnivora

Family - Canidae

Genus - Canis

Species – C. lupus

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The 5 Kingdoms The 5 Kingdoms that organisms are classified by are:

Animalia

Plantae

Fungi

Protoctista

Prokaryotes

Multicellular, don’t have cell walls or chlorophyll

Multicellular, have cell walls and chlorophyll

Multicellular, have cell walls but no cholophyll

Unicellular, have a nucleus

Unicellular, have no nucleus

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Classification A modern way of doing it…

As microscopes became more powerful and our knowledge of DNA improved, new models of classification were proposed. Due to evidence from chemical analysis I devised a “three-domain” system:

Archaea (primitive bacteria usually living in extreme conditions) Bacteria (true bacteria) Eukaryota (which includes protists, fungi, plants and animals)

Carl Woese, 1928-2012

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Common Ancestors Modern DNA research shows that all forms of life share a lot of their DNA. This is used as evidence to suggest that all forms of desecnded from common ancestors (the Theory of Evolution).

98.8% shared DNA 85% shared DNA

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The “Evolution Tree”

Family Hominidae (Great Apes)

Family Hylobatidae (Lesser Apes)

Subfamily Hominidae

Subfamily Ponginae

Tribe Homini Tribe Panini

Tribe Gorillini

Humans Chimpanzees Gorillas Orangutans Gibbons

Similar species are proposed to have common ancestors but also will have differences due to the habitats they live in.

Scientists use current classification data and older fossil data to help them develop this tree