Topic 6 - Inheritance and Genetics

DNA stands for deoxyribonucleic acid and it’s the chemical that all of the genetic material in a cell is made up of.
It contains coded information of ‘instructions’ on how to out an organism together and make it work.
It determines what inherited characteristics a person or organism has.
It is found in the nucleus of animal and plant cells in really long structures called chromosomes.
Chromosomes are really long molecules of DNA.
Chromosomes normally come in pairs.
DNA is a polymer. It’s made up of two strands coiled together in the shape of a double helix.
Inside a cell you have the nucleus, followed by single chromosomes, followed by many DNA molecules with a double helix structure (a double-stranded spiral) within that.
Every living organism has DNA.

A gene is a small section of DNA found on a chromosome that codes for a specific protein.
Each gene codes for a particular sequence of amino acids, which are put together to make a specific protein.
Only 20 amino acids are used, but they make up thousands of different proteins.
Genes simply tell cells what order to put the amino acids together.
DNA also determines what proteins the cell produces (e.g. haemoglobin, keratin).
That in turn determines what type of cells it is (e.g. red blood cell or skin cell).
Every organism also has a genome, which is the entire set of genetic material within it.
Scientists have worked out the complete human genome and this is important for science and medicine for a few reasons:

• It allows scientists to identify genes in the genome that are linked to different types of diseases
• Knowing which genes are linked to inherited diseases could help us to understand the, better and find effective treatments
• Scientists can look at genomes to trace the migration of certain people around the world. All modern humans descended from a common ancestor in Africa, but humans can now be found all over the planet. The human genome is mostly identical in all individuals, but as different populations have migrated away from Africa, they gradually developed tiny differences in their genomes and these differences can help scientists to determine when new populations split off in a different direction and what route they took.

DNA molecules contain a genetic code that determines which proteins are built.
DNA strands are polymers made up of lots of repeating units called nucleotides.
Each nucleotide consists of one sugar molecule, one phosphate molecule and one ‘base’.
The sugar and phosphate molecules in the nucleotides form a ‘backbone’ to the DNA strands.
The sugar and phosphate molecules are alternate.
One of four different bases - A, T, C or G - joins to each sugar.
Each base links to a base on the opposite strand in the helix.
A and T always pair up, and so does C and G; this is called complementary base pairing.
It’s the order of bases in a gene that decides the order of amino acids in a protein.
Each amino acid is coded for by a sequence of three bases in the gene.
The amino acids are joined together to make various proteins, depending on the order of the genes’ bases.
There are parts of DNA that don’t code for proteins and some of these non-coding parts switch genes on and off, so they control whether or not a gene is expressed (used to make a protein).

Proteins are made in the cell cytoplasm on tiny structure called ribosomes.
To make proteins, ribosomes use the code in the DNA.
DNA is found in the cell nucleus an can’t move out of it because it’s really big. So the cell needs to get the code from the DNA to the ribosome.
This is dome using a molecule called mRNA, which is made by copying the code from DNA.
Their are a few different types of RNA, but in this one the m stands for ‘messenger’ RNA.
The mRNA acts as a messenger between the DNA and the ribosome, and carries the code between the two.
RNA also has the base pair of A and U instead of A and T.
The correct amino acids are brought to the ribosomes in the correct order by carrier molecules.

When a chain of amino acids has been assembled, it folds into a unique shape which allows the protein to perform the task it’s meant to do.
Some different types of protein include:

• Enzymes - Act as a biological catalyst to speed up chemical reactions in the body.
• Hormones - Used to carry messages around the body (e.g. insulin is a hormones released into the blood by the pancreas to regulate the blood sugar levels)
• Structural proteins - Are physically strong (e.g. collagen is a structural protein that strengthens connective tissues like ligaments and cartilage).

Sometimes the sequence of DNA bases can change and these are called mutations.
Mutations are changes in genetic code.
Occasionally a gene can mutate and a mutation is a random change in an organism’s DNA that can sometimes be inherited.
Mutations occur continuously and they can occur spontaneously (e.g. when a chromosome isn’t quite replicated properly).
However, the chance of a mutation is increased by exposure to certain substances or some types of radiation.
Mutations can change the sequence of the DNA bases in a gene, which produces a genetic variant (a different form of the gene).
As the sequence of DNA bases codes for the sequence of amino acids to make up a protein, mutations in a gene sometimes lead to changes in the protein that it codes for.
Most mutations have very little or no effect on the protein; some will change to such a small extent that its function or appearance is unaffected.
However, some mutations can seriously affect a protein.
Sometimes, the mutation will code for an altered protein with a change in its shape, affecting its ability to perform its function. For example:

• If the shape of an enzyme’s active site is changed, the substrate can’t bind to it
• Structural proteins lose their strength, so structure and support in the body is lost
  If there’s a mutation in non-coding DNA, it alters how genes are expressed.

There are a few different ways that mutations can change the DNA base sequence:

• Insertions - Where a new base is inserted into the DNA base sequence where it shouldn’t be. Every three bases in a DNA sequence codes for a particular amino acid. An insertion changes the way the groups of three bases are ‘read’, which can change the amino acids that they code for. Insertions can change more than one amino acid as they have a knock-on effect on the bases further in the sequence. For example:
  Original gene- [T A T][A G T][C T T] which is tyrosine, serine and leucine.
  Mutated gene - [T A T][A G G][T C T]T the second G is the insertion, which pushes of the rest of the amino acids over and criadte tyrosine, arginine and serine instead, with a left over T.
• Deletions - Where a random base is deleted from a DNA base sequence. Like insertions, they change the way that the base sequence is ‘read’ and have knock-on effects further down the sequence.
• Substitutions - Where a random base in the DNA base sequence is changed to a different base. This doesn’t have a knock-on effect and simply changes the amino acid created for one group of three.

Reproduction is important for all species and can happen in two different ways.
SEXUAL REPRODUCTION produces genetically different cells.
The genetic information from two organisms (a father and a mother) is combined to produce offspring which are genetically different to either parent.
In sexual reproduction, the mother and father produce gametes (reproductive cells) by meiosis (e.g. egg and sperm cells in animals).
In humans, each gamete contains 23 chromosomes - half the number of chromosomes in a normal cell.
Instead of having two in each chromosome, a gamete has just one of each.
The egg (from the mother) and the sperm (from the father) then fuse together (fertilisation) to form a cell with a full number of chromosomes (half from the mother and half from the father).
Sexual reproduction therefore, involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parents’ genes.
This is why offspring inherit features from both parents, as it’s received a mixture of chromosomes from its mother and father and the chromosomes decide how the offspring turns out.
The mixture of the genetic information produces variation in the offspring.
Flowering plants can reproduce in this way too. They also have an egg cell, but their version of sperm is known as pollen.

ASEXUAL REPRODUCTION produces genetically identical cells.
In asexual reproduction there’s only one parent so the offspring are genetically identical to that parent.
Asexual reproduction happens by mitosis, where and ordinary cell makes a new cell by dividing in two.
The new cell has exactly the same genetic information (e.g. genes) as the parent cell and is called a clone.
In asexual reproduction therefore, there’s only one parent. There’s no fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring. The offspring are genetically identical to the parents - they’re clones.
Bacteria, some plants and some animals produce asexually.

Gametes are produced by meiosis, which is how they end up with half the number of chromosomes than of a normal cell.
Gametes only have one copy of each chromosome, so that when gamete fusion takes place, you get the right amount of chromosomes again (with two copies of each).
To make gametes which only have half the original number of chromosome, cells divide by meiosis.
This process involves two cell divisions and in humans it only happens in the reproductive organs (ovaries in females and testes in males).
The steps of meiosis (cell division in reproductive cells) are as follows:

• Before the cell starts to divide, it duplicates its genetic information, forming two armed chromosomes, with one of arm each chromosome being an exact copy of the other arm. After replication, the chromosomes arrange themselves into pairs
• In the first divide in meiosis, the chromosome pairs line up in the centre of the cell
• The pairs are them pulled apart so each new cell only has one copy of each new chromosome. Some of the fathers chromosomes and some of the mothers go into each new cell
• In the second division, the chromosomes line up again in the centre of the cell and the arms of the chromosome are pulled apart
• You get four gametes, each with only a single set of chromosomes in it. Each gamete is genetically different for, the others because the chromosomes all get shuffled up during meiosis and each gamete only gets half of them, at random.

The cell produced by gamete fusion replicates itself.
After two gametes have fused during fertilisation, the resulting new cells divides by mitosis to make a copy of itself.
Mitosis repeats many times to produce lots of new cells in an embryo.
As the embryo develops, these cells start to differentiate into the different types of specialised cell that make up a whole organism.

Some organisms can reproduce both sexually and asexually depending on the circumstances. For example:

• Malarial parasites - Malaria is caused by a parasite that’s spread by mosquitos.
  When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human. The parasite reproduces sexually when it’s a mosquito and asexually when it’s in the human host.
• Fungi - Many species of fungi can reproduce both sexually and asexually, and these species release spores, which can become new fungi when they land in a suitable place. Spores can be produced sexually and asexually. Asexually-produced spores from fungi are genetically identical to the parent fungus. Sexually-produced spores introduce variation and are often produced in response to an unfavourable change in the environment, increasing the chance that the population will be able to survive the change.
• Plants - Loads of species of plant produce seeds sexually, but can also reproduce asexually. Asexual reproduction can take place in different ways. For example, strawberry plants produce ‘runners’ which are stems that grow horizontally on the surface of the soil away from a plant and at various points new identical plant forms. Another example is plants that grow from bulbs can form new bulbs from the main bulb to divide off. Each new bulb grows into a new identical plant. Strawberries also reproduce sexually by flowers that produce seeds.

Some characteristics are controlled by single genes.
What genes a person inherits controls what characteristics you develop.
Different genes control different characteristics. Some characteristics are controlled by a single gene (e.g. mouse fur colour or red-green colour blindness in humans).
However, most characteristics are controlled by several genes interacting.
All genes exist in different versions called alleles (represented by letters in genetic diagrams).
A person has two versions (alleles) of every gene in the body; one or each chromosome in a pair.
If an organism has two alleles for a particular gene that are the same, them it’s homozygous for that trait.
If the two alleles are different, then it’s heterozygous.
If the two alleles are different, only one can determine what characteristic is present.
The allele for the characteristics that’s shown is the dominant allele (capital letter), and other one is called the recessive allele (lowercase letter).
A person’s genotype is the combination of alleles they have.
Alleles work at a molecular level to determine what characteristics a person has (their phenotype).

Some disorders can be inherited from a person’s parents, and many of these can screened for in embryos.
Cystic fibrosis is caused by a recessive allele and is a genetic disorder of the cell membranes, resulting in a build up of thick, sticky mucus in the air passages and pancreas.
The recessive allele is carried by about 1 person in 25 and because it’s recessive, people with only one copy of the allele song have the disorder (they’re known as carriers).
For a child to have the disorder, both parents must either be carriers or have the disorder themselves.
Another genetic disorder is polydactyly, which is caused by a dominant allele.
It is a genetic disorder where a baby’s born with extra fingers or toes; the condition doesn’t usually cause any other problems and so isn’t life threatening.
The disorder is caused by a dominant allele and so can be inherited if just one parent carries the defective allele.
The parent that has the defective allele won’t be a carrier, and will instead have the condition since the allele is dominant.

Embryos can be screened for genetic disorders.
During IVF (in vitro fertilisation), embryos are fertilised in a laboratory, and then implanted into the mother’s womb.
Before being implanted, it’s possible to remove a cell from each embryo to analyse its genes.
Many genetic disorders can be detected in this way, such as cystic fibrosis.
It’s also possible to get DNA from an embryo in the womb and test that for disorders.
There are lots of ethical, social and economic concerns surrounding embryonic screening:
FOR - It will help to stop people suffering, treating disorders costs the government and taxpayers a lot of money and there are laws to stop it going too far (e.g. parents getting pregnant through IVF are unable to select the sex of their baby, unless it’s for health reasons).
AGAINST - Embryos with ‘defective’ alleles are destroyed after screening, for embryos in the womb this could lead to the decision to terminate the pregnancy, it implies people with genetic problems are ‘undesirable’ and this could increase prejudice, there may come a point where everyone wants to screen their embryos so that they can pick the most ‘desirable’ and ‘attractive’ one and screening is expensive.

Mendel did an experiment through reproducing tall and drawn pea plants to see what characteristics the offspring would carry.
His three main conclusions were:

• Characteristics in plants are determined by hereditary units
• Hereditary units are passed on to offspring unchanged from both parents, one unit from each parent
• Hereditary units can dominant and recessive; if an individual has both dominant and recessive units for a characteristic, the dominant characteristic will be expressed.
  We now know these units as genes.