Inheritance, Variation & Evolution

What is Meiosis?

Formation of four non-identical cells from one cell

What is Mitosis?

Formation of two identical cells from one cell

What does Sexual reproduction involve?

Joining of male and female gametes, each containing genetic information from the mother or father

-Sperm and egg cells in animals

-Pollen and egg cells in flowering plants

How are gametes formed?

By meiosis (non-identical)

How many chromosomes in a normal cell?

46 chromosomes

Two sets of chromosomes (23 pairs)

Each pair has one chromosome from father and one from mother

How many chromosomes does one gamete have?

23 chromosomes

They fuse in fertilisation where genetic information from each parent is mixed, producing variation in offspring

What does asexual reproduction involve?

One parent with no gametes joining

Happens using Mitosis

2 identical cells are formed from one cell

Examples of asexually reproducing organisms?

Bacteria, Some plants and some animals

What is Meiosis?

Formation of four non-identical cells from one cell

How do cells in reproductive organisms divide?

By Meiosis to form gametes

Gametes only have one copy of each chromosome

Process of Gametes dividing

• Cell makes copies of its chromosomes so it has double amt of genetic information

• Cell divides into two cells, each with half the amount of chromosomes (46)

• Cell divides again producig four cells, each with a quarter of amount of chromosomes (23)

  • These are called gametes and they are all genetically different from each other because the chromosomes are shuffled during the process, resulting in random chromosomes ending up in each of the four cells

Gametes joining at fertilisation

Gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes (normal number)

• This Cell divides by mitosis to produce many copies

• More and more cells are produced and an embryo forms

• Cells begin to take on different roles after this stage (differentiation)

Advantages of Sexual Reproduction:

Produces Variation in offspring

Allows us to use selective breeding

Advantages of asexual reproduction

Only one parent is needed

Uses less energy and is faster as organisms do not need to find a mate

In favorable conditions lots of identical offspring can be produced

Advantage of sexual reproduction: Produces variation in offspring

• This means if environment changes it is likely that an organism in the species will have a characteristic that allows them to survive (called a survival advantage)

• Although some individuals may die, variation decreases chance of species becoming extinct

Advantage of sexual reproduction: selective breeding

• Mixes genetic information from two organisms

• Organisms with different desirable characteristics can be bred to produce offspring with even more desirable characteristics

• This speeds up natural selection

  • An example: increase food production by breeding two animals with lots of meat

Examples of Organisms that can use both methods to produce

• Malarial Parasites

• Some fungi

• Some plants

Sexual and Asexual reproduction of Malarial Parasites

• They reproduce sexually in mosquito

• They reproduce asexually in human host (liver and blood cells)

Sexual and Asexual reproduction of some fungi

Many species can undergo both times of reproduction, releasing spores which land and become new fungi

• Spores produced asexually: genetically identical

• Spores produced sexually when conditions change, in order ot increase variation and avoid extinction

Sexual Reproduction: Some Plants

• Many plants reproduce sexually using pollen: must reach egg cells in female parts of another flower

  • This is called pollination, and it forms seeds

Asexual Reproduction: Some Plants

• Strawberry: produce runners. New identical plants grow off the runner

• Daffodils: grow from bulbs. New bulbs can grow from the main one, producing new identical plant

Why is it advantageous for plants to be able to reproduce sexually and asexually?

It means they can reproduce even if the flowers have been destroyed by frost or other animals

What is DNA?

A polymer made up of two strands which wrap around each other like a rope (double helix)
DNA is contained in structures called chromosomes

What is between the two strands of DNA?

Four nitrogenous bases lined up in single rows - these come together to form a series of complementary pairs

What is a gene?

A small section of DNA on a chromosome

A triplet of bases that codes for a specific protein

Each gene codes for a particular sequence of amino acids, together a chain of amino acids can join to make a protein

What is the genome?

All the genes coding for all the proteins with an organism

Why did studying the human genome help?

Improved understanding of genes linked to different kinds of disease

Treatment of inherited disorders

Helped in tracing human migration patterns from the past

Understanding the human genome will have great importance in the future

What does DNA stand for?

Deoxyribonucleic acid: a polymer that contains instructions for the body

What are chromosomes made up of?

Long molecules of DNA found in the nucleus of a cell

What is DNA made up of?

Many small parts called nucleotides

What are nucleotides made up of?

One sugar molecule, One phosphate molecule (form the backbone), and one of the four types of organic bases

What are the four types of organic base?

• A, C, G, T

• DNA: two DNA strands, twisted together.

  • Each base is connected to another base in the other strand

Complementary Base pairing

• A bases only connect to T bases

• C bases only connect to G bases

• This is complementary base pairing

Code of DNA bases

• The order of different bases forms a code

• Each group of three bases codes for an amino acid

• The amino acids are joined together to make a protein

  • Different types and order of amino acids determine which type of protein it is

• There are 20 types of amino acid

  • Therefore order of bases in DNA determine which proteins are used

Non-coding parts of DNA

• Do not code for proteins

• Some of them are responsible for switching genes on or off (controlling wether the gene is used to form a protein or not)

What is protein synthesis?

Process of producing a protein from DNA

‘Expressing’ a gene

If a gene is coded to make a protein, it has been expressed

1. DNA contains genetic code for making a protein but cannot move out the nucleus - too big

2. The two strands pull apart from each other and mRNA nuecleotides match to their complementary base on the strand

3. The mRNA nucleotides themselves are then joined together, creating a new strand: mRNA strand (a template of the original DNA)

4. mRNA then moves out of the nucleus to the cytoplasm and onto structures called ribosomes

5. At ribosomes, the bases on the mRNA are read in threes to code for an amino acid (first three bases code for one amino acid, second three bases code for another etc)

6. The corresponding amino acids are brought to ribosomes by carrier molecules

7. These amino acids connect to form a protein

8. When the chain is complete the protein folds to form a unique 3D structure

What is mRNA

Messenger RNA: a different type of nucleotide

What helps protein carry out its role?

It’s specific shape

Proteins can be:

Enzymes: biological catalysts that speed up rate of reaction

Hormones: chemical messengers that send signals around the body

Structural protein: strong proteins in order to form structures, e.g collagen

What do mutations do?

Change the sequences of bases in DNA

Types of Mutations

1. A base is inserted into the code

   • As they are read in threes this changes how it’s read

   • May change all the amino acids coded for after this insertion

2. A base is deleted from the code

   • Like insertions they change the way it is read

   • May change all amino acids coded for after this insertion

3. A base is substituted

   • This will only change one amino acid in the sequence or it may not change amino acid (as new sequence can sometimes still code for same amino acid)

What does a change in type/sequence of amino acids do?

Affect the way it folds and therefore change the structure

Most mutations do not alter the protein and only do so slightly

Some can have a serious affect and change the shape

Mutations that have a serious effect and can change the shape:

• Substrate will not fit into active sight so it cannot act as a protein

• A structural protein may lose its shape

There can also be mutations in non-coding parts of DNA that control wether the genes are expressd

Why does variation between two organisms arise?

1. The coding DNA that determines the proteins and their activity

2. The non-coding DNA that determines which genes are expressed

What are gametes?

An organisms reproductive cell (egg in female, sperm in males) which has half the number of chromosomes

What is a chromosome?

A structure found in the nucleus which is made up of a long strand of DNA

What is a gene?

A short section of DNA that codes for a protein, and therefore contribute to a characteristics

Some characteristics are controlled by a single gene, such as fur colour in mice and red-green colour blindness in humans

However, most characteristics are the result of many different genes interacting

What are Alleles?

The different forms of the gene - humans have two alleles for each gene as they inherit one from each parent

What is a dominant allele?

Only one (out of 2 alleles) is needed for it to be expressed and for the corresponding phenotype to be observed

What is the Recessive Allele?

Two copies are needed for it to be expressed and for the corresponding phenotype to be observed

What does homozygous mean?

When both inherited alleles are the same (i.e two dominant alleles or two recessive alleles)

What does heterozygous mean?

When one of the inherited alleles is dominant and the other is recessive

What is a genotype?

The combination of alleles an individual has, e.g Aa

What is a phenotype?

The physical characteristics that are observed in the individual, e.g eye colour

What do family trees show?

The inheritance of different phenotypes over generation in the same family

What is a single gene cross?

A single gene cross looks at the probability of the offspring of two parents having certain genotypes and phenotypes

This is done using the alleles the two parents have for a gene and a Punnet square diagram

What do Uppercase and Lowercase letters represent in a Punett square diagram?

Uppercase: dominant characteristics

Lowercase: recessive characteristics

How are genetic disorders caused?

By inheriting certain alleles

What is Polydactyly?

Having extra fingers or toes

Caused by a dominant Allele

What is Cystic Fibrosis?

A disorder of cell membranes, resulting in thick mucus in the airways and pancreas

• Caused by a recessive allele

• Both parents need to either be carriers (have on of the recessive alleles) or one must have CF themselves and the other is a carrier

What is Embryonic Screening?

Allows scientists to observe whether the child will have a genetic condition or not

• If embryo developed in lab: cells will be taken from it and analysed

• DNA from embryos in the womb can also be extracted

Arguments FOR embryonic screening

• Reduces number of people suffering (ethical)

• Treating disorders is very expensive (economic)

• There are many regulations in place to stop it getting out of hand (social)

Arguments AGAINST embryonic screening

Could encourage people to pick characteristics - creating designer babies (ethical)

It is expensive to carry out screening (economic)

May promote prejudice as it suggests that those with genetic disorders will not live a full life or are unwanted (social)

Decisions have to be made about terminating a pregnancy (social)

The procedure can lead to a miscarriage (social)

What do chromosomes in cells control?

22 pairs control characteristics, and the chromosomes in each pair look very similar

The 23rd pair carries sex determining genes and the two chromosomes can look very similar (Y smaller than X)

What happens when cells undergo meiosis to form a gamete?

One sex chromosome goes into each gamete

What chromosomes (X and Y) do females and males have?

Females have two X chromosomes so can only pass on X chromosomes in their eggs

Males have one X chromosome and one Y chromosome, so can pass on both in their sperm

Punnet squares can be used to show?

Sex Inheritance: there is a 50/50 chance of child being male/female

What does the phenotype an organism has depend on?

Genotype: the genes it inherited

Environment: the place it lives in

How are genes passed on?

From the parent, in sex cells

What does the combining of genes from the mother and father create?

Genetic Variation

Who has the same genotype?

Only identical twins

How does environment affect the phenotype of an organism?

The conditions the organism grows and develops in affects it’s appearance

Examples: scars in animals, smaller and yellow leaves in plants

Characteristics resulting from a combination of genetics and environment

Weight

Depends on the food you eat but also how quickly your body breaks it down and how much it stores as fat depends on your genes

How is genetic variation introduced?

By mutations in the sequence of DNA:

-Most have no effect on phenotype

-Some will influence phenotype but it is unlikely that it will bring about a new phenotype

-If the mutation does determine the phenotype and it is advantageous, natural selection will mean it becomes the common phenotype relatively quickly

What is Selective Breeding?

When humans choose which organisms to breed in order to produce offspring with a certain desirable characteristic (e.g animals with more meat, plants with a disease resistance or big flowers)

Process of Selective Breeding

• Parents with desired characteristics are chosen

• They are bred together

• From the offspring those with desired characteristics are bred together

• The process is repeated many times until all the offspring have the desired characteristic

Problems that can come from selective breeding

Inbreeding:

-Breeding those with similar desirable characteristics means it is likely you are breeding closely related individuals

-This results in the reduction of the gene pool, as the number of different alleles reduce (as they mostly have the same alleles)

-This means if the environment changes or there is a new disease, the species could become extinct as they all have the same genetic make-up (so the chance of a few organisms having a survival advantage and not dying is reduced)

-Another problem: small gene pool leads to greater chance of genetic defects being present in offspring, as recessive characteristics are more likely to be present

What is genetic engineering?

Modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic

-Plant cells have been engineered for disease resistance or to have larger fruits

-Bacterial cells have been engineered to produce substances useful to humans such as human insulin to treat diabetes

Process of Genetic Engineering

1. Genes from chromosomes are ‘cut out’ using restriction enzymes leaving ‘sticky ends’ (short sections of exposed unpaired bases)

2. A virus/ bacterial plasmid is cut using the same restriction enzyme to also create sticky ends. This also contains an antibiotic marker gene

3. The loop and gene sticky ends are then joined together by DNA ligase enzymes

4. The combined loop is placed in a vector (a bacterial cell) and then allowed to multiply as it will now contain the modified gene. As the bacteria grows we can see which ones are resistant to antibiotics. The colonies that are resistant will be the bacteria that are also producing the modified gene, as they were inserted together

5. In plants the vector is put into meristematic cells which can then produce identical copies of the modified plant

Genetically Modified Crops

• Engineered to be resistant to insects and herbicides

• This will result in increased yields as less crops will die

Genetic modification in medicine

• It may be possible to use genetic engineering to cure inherited disorders

• It is called gene therapy and involves transferring normal genes (not faulty) into patients so the correct proteins are produced

Perceived benefits of genetic engineering

• Can be very useful in medicine to mass produce certain hormones in microorganisms (bacteria and fungi)

• In agriculture it can be used to improve yields (improving growth rates, introducing modifications that allow the crops to grow in different conditions, introducing modifications that allows plants to make their own pesticide or herbicide)

• Crops with extra vitamins can be produced in areas where they are difficult to obtain

• Greater yields can help solve world hunger, which is becoming an increasingly bigger problem due to population growth

Perceived risks of genetic engineering

• GM crops might have an effect on wild flowers and therefore insects

• GM crops are infertile and these genes could spread into wild plants, leading to infertility in other species, which affects the entire environment

• Growing with herbicides and pesticides can kill insects and other plants, which would reduce biodiversity

• People are worried we do not completely understand the effects of GM crops on human health

• Genetic engineering in agriculture could lead to genetic engineering in humans. This may result in people using the technology to have designer babies

• They pose a selection pressure, which could lead to increased resistance in other species, creating super weeds and pests

What is cloning?

Creating genetically identical copies of an organism

Method of Plant Cloning: Tissue Culture

Important to preserve rare plant species or commercially in plant:

• Plant cells are taken

• They are placed in a growth medium with nutrients and hormones

• They grow into new plants, and are clones as they are genetically identical to the parent

Method of plant cloning: Cuttings

An older, easier method to produce clones

• Cuttings, such as section of the stem, are taken from a plant with a desirable feature

• They are planted and produce clones as they are genetically identical to the parent

Method of animal cloning: Embryo transplants

• Sperm and Egg cells from parents with desirable features are obtained

• In the lab, they are fertilised to form an embryo

• The embryo divides many times and is then inserted into a host mother

• The offspring which is eventually born is genetically identical (with the desirable feature) as they have genetic information from the same mother and father

Method of animal cloning: Adult cell cloning

• The nucleus is removed from an unfertilised egg cell

• The nucleus is removed from an adult body cell and placed in the denucleated egg cell

• Through the stimulation of an electric shock, the egg cell begins to divide

Benefits of cloning

Produces lots of offspring with a specific desirable feature

The study of clones could help research into embryo development

Can help extremely endangered species, or even bring back species that have become extinct

Risks of cloning

The gene pool is reduced through producing clones, meaning it’s less likely that the population will survive if a disease arises with low diversity in the population

Clones have a low survival rate, and tend to have some genetic problems

It may lead to human cloning

Who was Gregor Mednel?

• Trained in mathematics and natural history in Vienna

• Worked in the monastery gardens and observed the characteristics passed on to the next generations in plants

• He carried out breeding experiments on pea plants

• He used smooth peas, wrinkled peas, green peas and yellow peas and observed the offspring to see which characteristics they had inherited

Gregor Mendel’s conclusions

Through keeping record of everything he did and publishing his work in 1866, he concluded:

• Offspring have some characteristics that their parents have because they inherit ‘hereditary units’ from each

• One unit is received from each parent

• Units can be dominant or recessive and cannot be mixed together

Recognition of Gregor Mendel

Not recognised until after his death as genes and chromosomes were not yet discovered, so people could not understand

Discoveries of Genetics after Gregor Mendel

• Late 19th century: chromosomes as a part of cell division were observed

• 20th century: it was understood that chromosomes and units had similar behaviours. It was decided that units (now known as genes) were on the chromosomes

• The Structure of DNA was determined in 1953, which meant we were able to understand how genes worked

What is evolution?

Change in inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species

Theory of Evolution

All species have evolved from simple life forms that first developed more than three billion years ago

Why does evolution occur?

Because of natural selection

• Mutations occur - provides variation between organisms

• If a mutation provides a survival advantage, the organism is more likely to survive to breeding age

• The mutation will then be passed off onto offspring

• Over many generations, the frequency of mutation will increase within the population

What is speciation?

Evolution may cause one population of a species to become so different that they can no longer interbreed to produce fertile offspring - this is called speciation

Who was Charles Darwin?

• Scientist and naturalist

• Put forward theory of evolution

• Supported by experimentation and his knowledge of geology and fossils that he discovered on a round the world expedition

• Published ‘on the origin of species’, 1859