Biology GCSE - 4.6 Inheriten

DNA

Deoxyribonucleic Acid

Polymer made up of two strands wrapped around each other to form a double helix

Nucleotides are the monomers that form DNA (a polymer)

Chromosome

A chromosome is a long DNA molecule which part of the genetic material of an organism

Each cell has 46 chromosomes in it - 23 different cells - 1 from each parent

Located in nucleus

Sex chromosomes

Gametes

23rd pair of chromosomes

Determine whether an individual is male (XY) or female (XX)

Gene

Small section of DNA of a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein

Determines what type of a cell a cell becomes

Genome

The genome of an organism is the entire genetic material of that organism

How could understanding the human genome help science?

• It will allow scientists to search for genes linked to different diseases

• It could help us understand and treat inherited disorders

• It also could be used in tracing human migration patterns from the past

Some genes cause disease (inherited disease) or increase the Risk of getting a disease (BRCA - Breast cancer)

Alleles

Alleles are the different versions of a particular gene

Each individual has 2 alleles of each gene - one from each parent

What does it mean to be “homozygous” for a particular gene?

Both alleles of that gene are identical

What does it mean to be “heterozygous” for a particular gene?

The two alleles of that gene are different

Dominant allele

The allele that is always expressed, even if there is only one copy of that allele

Recessive allele

An allele which is only expressed if there are two copies of it

Genotype

Refers to the specific genes or alleles that an individual has

Phenotype

The characteristics an organism has as a result of their genotype (and environment)

Most characteristics are determined by the combination of multiple genes and the environment

Structure of a Nucleotide

Contain phosphate connected to sugar with a base on the side

Phosphate of one nucleotide bonds to the sugar of the next nucleotide forming a sugar phosphate backbone - form protective outer casing with bases in the middle

Complementary base pairing

4 bases that make DNA: A T C G

A always pairs up with T

C always pairs up with G

Genetic Code

Sequence of bases

3 bases code for an amino acid - a codon

3 Uses of Proteins

• Enzymes - biological catalyst

• Hormone - messages around the body

• Structural proteins - strength to cells/tissues

Each protein is made from a different sequence of amino acids therefore have a unique shape to carry out a particular function

Two parts of Protein Synthesis

Transcription

Translation

Transcription

The process of copying a single gene of DNA to mRNA

Takes place in the Nucleus

• RNA polymerase (an enzyme) binds to DNA just before where the Gene starts

• The two strands of DNA in front of the RNA polymerase break apart so their bases are exposed and close behind the RNA polymerase when it has moved on

• RNA polymerase moves along the strand (template strand) to read the bases one by one and use them to make an mRNA strand (mRNA will be complementary to DNA)

• A on DNA will be complementary to a U on mRNA

• When the RNA polymerase has finished making the mRNA strand, it detaches, leaving an mRNA strand which is free the leave the nucleus and go to Ribosomes

mRNA

Messenger RNA

DNA is so big, it cannot leave the nucleus therefore to use a Gene, the cells must make a copy of it first so it is small enough to leave the nucleus

This copy is called mRNA

How is mRNA different to DNA?

• mRNA is shorter than DNA

• mRNA is single-stranded, whereas DNA is double-stranded

• mRNA contains uracil (U), where DNA contains thymine (T)

Translation

The process by which a ribosome combines amino acids together to form a protein, using the “instructions” of mRNA

• mRNA strand and ribosome both bind together

• tRNA bring amino acids to the ribosomes (transfer RNA)

• tRNA have amino acid at the top and Anti-codon at the bottom - the sequence of three bases which are complementary to three bases on the mRNA which code the amino acid that the tRNA is carrying

• The tRNAs bring the correct Amino acids in the correct order

• The ribosome then joins them together to start building up a chain of amino acids and moves along the mRNA as the tRNA with the amino acids bind to the mRNA allowing the first tRNA molecule to detach, leaving the amino acid there and repeats the process

• The amino acid chain will then detach from the ribosome and fold up to form a protein

Polypeptides

A chain of amino acids

Mutation

A change in the DNA base sequence

Happen spontaneously in our cells - when DNA Is duplicated

What increases the risk of Mutations?

Carcinogens - harmful chemicals

Certain types of radiation - X-rays or Gamma rays

Effect of mutations

If one base mutates to a different base, a different amino acid may be formed causing a different shaped protein to be formed therefore the protein may not be able to function properly

For example, if the protein is an Enzymes, the mutation might change the shape of the active site therefore it cannot form an enzyme substrate complex and not be able to catalyse the reaction

Why most mutations have no effect?

May not have a significant effect as it may change the shape of the protein slightly, but the protein can still work in the same way

Most mutations occur in Non-coding DNA which isn’t part of any Gene and doesn’t code for a protein (has an important role in the expression of genes, when to turn them off and on)

3 different types of Mutations

• Insertion

• Substitution

• Deletion

Substitution Mutations

When one of the bases is changed for another random base

Changes the codon and changes the amino acid that it codes for

Insertion mutations

An extra base is inserted into the sequence which causes each base to be pushed along to the next codon

Worse than substitutions as all of the subsequent bases are shifted along by one which alters all of the subsequent codons - amino acid chain after that point is completely different

Deletion mutations

One of the bases is deleted from the sequence

If one base is deleted, all the subsequent bases will shift one space to the left which alters all their codons - amino acid chain will be completely different

Two main ways organisms can reproduce

• Sexual Reproduction

• Asexual Reproduction

Some plants can do both but most animals rely on sexual and bacteria only reproduce asexually

Sexual reproduction

Involves fusion of male and female gametes therefore mixing of genetic material - fertilisation

Animals - Sperm and Egg

Flowering plants - pollen and egg

The gametes only have half the genetic material of a normal cell so when they fuse with another gamete, they will have 46 chromosomes like a normal cell

Genetic variation in offspring - 2 parents

Meiosis and Mitosis involved

Advantages and Disadvantages of Sexual Reproduction

Advantage: Genetic diversity which means species can survive if there are changes to their environment

Disadvantage: More risky because the organism must find a suitable mate - need to impress mate - time consuming

Asexual Reproduction

• One parent

• No fusion of gametes so no mixing of genetic information

• Produce clones - genetic material identical to parent

• No genetic variation in offspring

• Mitosis is the only type of cell division involved

• Occurs in Bacteria, fungi, bulbs

Advantages and Disadvantages of Asexual Reproduction

Advantages: enables organism to quickly populate a new environment and ensure that successful characteristics are passed on if it swell adapted - takes up less energy

Disadvantages: No genetic diversity therefore if there is a change to their environment, they are less likely to survive. Less chance of adapting to new conditions (e.g. new environment, new competitors

Organisms that reproduce by both methods depending on circumstances

• Malarial parasites reproduce asexually in the human hot, but sexually in the mosquito

• Many Fungi reproduce asexually by spores but also reproduce sexually to give variation

• Many plants produce seeds sexually, but also reproduce asexually by runners, such as strawberry plants, or bulb divisions such as daffodils

Meiosis

Production of Gametes - haploid cells

• Replicate DNA - adds extra arm to chromosome

• 2 Armed chromosomes line up along centre of the cell in pairs (mother and father) - which cell is on the left and which cell is on the right is completely random

• Sections of DNA also get swapped for further variation in cells

• 1st division, chromosomes are pulled apart and cell splits into two. Chromosomes are randomly distributed - different combination of maternal and paternal chromosomes

• 2nd division: Chromosomes line up in the middle of the cell and the two arms of each chromosome are pulled apart and the then cell divides in half

• 4 genetically unique cells haploid gametes

Inherited disorders

Disorders that are caused by the inheritance of certain alleles

• Polydactyly - having extra figures or toes - is caused by a dominant allele

• Cystic fibrosis - a disorder of cell membranes + sticky mucus released in airway of lungs and pancreas - is caused by a recessive allele

IVF

In vitro fertilisation

• Egg cells fertilise with a sperm in a lab

• When the fertilised eggs cells grow into an embryo, they are implanted into a woman’s uterus lining to grow into a foetus

Embryo Screening

• IVF - Genes are looked at before implanting the embryo to see if they are carrying any genetic disorders

• If embryos are found that carry a disorder, parents + doctors may decide to discard embryo and use a different one

Pros and Cons of Embryo Screening

Pros:

• Reduce overall suffering as there will be fewer people with health problems)

• Save money as there will be less strain on the NHS

Cons:

• Ethical argument: Implies that people with genetic disorders are undesirable

• In the future people may start screening for other traits - laws to prevent this

Gregor Mendel Pea plant experiment

Thought that crossbreeding the best plants would lead to more favourable offspring

In a monastery, he experimented with Pea plants + studied how certain traits, height + colour of peas and flowers, was passed on from one generation to the next

Mendel’s observations

One of his observations was that the inheritance of each characteristic is determined by “hereditary units” that are passed on to descendants unchanged. In the late 19th Century behaviour of chromosomes during cell division was observed.

Importance of Mendel’s discovery

Early 20th century, it was observed that Mendel’s “hereditary units” and chromosomes behaved in similar ways. This led to the idea that the “hereditary units” , now called genes, were located on chromosomes.

In the mid 20th century, the structure of DNA was determined and the mechanism of gene function was worked out. This scientific work by many scientists led to the gene theory being developed

Variation

Differences in the characteristics of individuals within a population

Can vary based on genes and environment (e.g.: not sleeping or eating enough - not as tall + more time spent in the sun - darker) or a combination of both

Beneficial mutations

Mutations - change in the DNA - protein may be different but won’t usually change the organism’s phenotype

Sometimes, the Phenotype will change slightly, which is usually bad, but very occasionally beneficial (ability to run faster, more resistance to lung disease)

Survival of the fittest

People with beneficial mutations are more likely to survive therefore more likely to reproduce and pass on their genes to their offspring

Discovered by Darwin - Didn’t know anything about mutations or genes but noticed that traits were being passed on from parent to child - the most useful traits were passed on the most

Natural selection - the fittest individuals were being selected to survive

Evolution

Inheritance of certain characteristics in a population, over multiple generations, could lead to a change in the whole species or even a development of an entire new species as the phenotypes of two different species may become so different that they can no longer produce fertile offspring

All current species must have evolved from species some time in the past

All living species must have evolved from simple life forms which first developed more than 3 billion years ago

Lamarck’s theory

His early theory of evolution suggested that organisms could acquire new traits during their lifetime, and that these traits could be passed to their offspring

Lamarck’s theory was proven incorrect by modern genetics as the characteristics acquired by an organism during its lifetime do not affect the DNA sequence of the organism and cannot be passed down from one generation to the next

Darwin’s theory

• Individual organisms within a species show a wide range of variation for a specific trait.

• Darwin observed variations between species adapted to different environments.

• Older layers of rock contained fossils of less complex organisms, while more recent layers showed more complex organisms.

Alfred Russel Wallace

Alfred Russel Wallace was a scientist, who after travelling around the world and collecting evidence, proposed a theory of evolution based on the process of natural selection.

Known for Studying warning colouration in animals, for example in the Golden Birdwing Butterfly and known for his theory of speciation

Why was Darwin’s and Wallace’s ideas controversial - speciation

On the Origin of Species - Darwin

• Religious reasons - Darwin's theory challenged the religious belief that God created all life forms.

• Lack of evidence - a lack of hard evidence made many scientists hesitant to accept Darwin & Wallace's theory.

• DNA hadn't been discovered yet - the mechanism for inheritance and variation, now understood through genetics, was unknown during Darwin's time

Definition of a Species

A species is defined as a group of genetically similar organisms that are able to interbreed and produce fertile offspring.

Speciation

Means the forming of a new species

Can happen when different populations of the same species become so different that they are unable to interbreed and produce fertile offspring anymore, at which point they are considered a different species

How may Speciation occur?

Combination of both isolation and natural selection

Isolation happens when a physical barrier separates two different populations of a given species

The environment may be different on either side of the barrier

Because the population on either side of the barrier now live in different environments, the two populations experience different selection pressures. This means that the different traits become more favourable in each population, and through the process of natural selection, different characteristics will become more common in the two populations

After a long time, the two populations become so different (genetically, physically or behaviourally) from each other that they will no longer be able to interbreed and produce fertile offspring - separate species now

Selective Breeding

Taking the best plants or animals and breeding them together to get better offspring

Humans have been using selective breeding for thousands of years especially in agriculture and to domesticate animals like wolves

Uses of Selective Breeding

• Disease resistance in food crops

• Animals which produce more meat or milk

• Domestic dogs with a gentle nature

• Large or unusual flowers

Drawbacks of Selective breeding

Best individuals are closely related and may be bred - Inbreeding

Can cause some breeds to be particularly prone to disease or inherited defects

Selective breeding reduces the Gene pool of the population causing less variation which may cause one disease to wipe out a whole crop

How does selective breeding work?

Parents are chosen with desirable characteristics from a mixed population and they are bred together

From the offspring, those with the desired characteristics are bred together again

This continues over many generations until all the offspring show the desired characteristic

What is Genetic Engineering?

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

Can use the genes of one species to modify another species

Process of Genetic Engineering

• enzymes are used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid or a virus

• the vector is used to insert the gene into the required cells

• genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics.

Examples of Genetic Engineering

• Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits - show increased yield

• Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.

• Sheep have been GMed to produce substances like drugs in their milk which can be extracted and used to treat diseases

Pros and Cons of Genetic Engineering

Pros:

• Can easily make crops with desirable characteristics (more edible fruit which is resistant to disease) - more food for less money which is important in developing countries

• Plants can produce special nutrients like golden rice which contains beta carotene which can protect people from going blind

Cons:

• Don’t know how genetically modified plants might affect our health - no current evidence that it is bad

• There is a chance plants make it into the wild - outcompete local plants and change ecosystm

Why is the human genome important?

• It helps us understand how genetic diseases work so preventative steps can be taken. People can get screened to detect health risks early and start treatment sooner.

• It aids the diagnosis and treatment of inherited disorders by letting us quickly identify faulty genes. New personalised drugs and therapies can be tailored to a patient's specific genetic makeup (this is sometimes called personalised medicine).

Drawbacks of Genetic Screening

• Learning about their genetic risks can cause stress and mental health issues for some people.

• Genetic discrimination may lead to unfair treatment in jobs and insurance for those with certain gene variants. Strict regulations are needed to prevent gene-based bias.

• There are concerns about the misuse of genetic data by employers and insurers, emphasising the need for privacy protections.

Tissue culture

Using small groups of cells from part of a plant to grow identical new plants. This is important for preserving rare plant species or commercially in nurseries.

Cuttings

An older, but simple, method used by gardeners to produce many identical new plants from a parent plant.

Cutting off a growing shoot of a plant with desirable characteristics and planting it

Embryo transplants

Splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers.

Adult Cell Cloning

• The nucleus is removed from an unfertilised egg cell.

• The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell.

• An electric shock stimulates the egg cell to divide to form an embryo.

• These embryo cells contain the same genetic information as the adult skin cell.

• When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.

Fossils

Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks.

How can fossils be formed

• from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent

• when parts of the organism are replaced by minerals as they decay

• as preserved traces of organisms, such as footprints, burrows and rootlet traces

Why is the fossil record is incomplete?

Many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth.

Why study fossils?

We can learn from fossils how much or how little different organisms have changed as life developed on Earth.

Extinction

Extinctions occur when there are no remaining individuals of a species still alive.

How antibiotic resistance happens

• Bacteria undergo random mutations which may make them slightly more resistant to certain antibiotics

• Most of the bacteria will be killed by the antibiotics however the more resistant ones will survive, leaving only them to divide without any other competition

• The resistant bacteria divide very quickly and all have the gene of antibiotic resistance

• The Resistant strain of Bacteria means that a person will still be affected the the antibiotics aren’t effective anymore

• The bacteria can then be spread to other people and replicate there and then there will be no effective treatment

Superbugs

Example: MRSA and is resistant to loads of types of antibiotics

Relatively common and effects people in hospitals

How to reduce the rate of development of antibiotic resistant strains

• doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections

• patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains

• the agricultural use of antibiotics should be restricted.

The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.

Carl Linnaeus Linnaean system

Kingdom

Phylum

Class

Order
Family

Genus

Species

Binomial Naming System

Genus species - italics or underlined

Latin - universally understood

Carl Woes Three Domain System

• As evidence of internal structures became more developed due to improvements in microscopes, and the understanding of biochemical processes progressed, new models of classification were proposed.

• Evidence from new techniques - analysing RNA sequences, he realised some species are less related than initially thought

The Domain System (above Kingdom)

• Prokaryotic/Bacteria Kingdom - Single celled (true bacteria)

• Eukaryota kingdom - plants, fungi, protists, animals

• Archaea Kingdom - Extreme conditions - different type of prokaryote - In hot springs, salt lakes (primitive bacteria)

Evolutionary Trees

Method used by scientists to show how they believe organisms are related. They use current classification data for living organisms and fossil data for extinct organisms.