Biology - Topic 6 - Inheritance, (DNA structure/meiosis/types of reproduction/mutation)
DNA
DNA stands for doxyribonucleic acid. It's the chemical that all of the genetic material in a cell is made up from.
DNA is found in the nucleus of animal and plant cells, in really long structures called chromosomes (normally come in pairs).
DNA is a polymer and is made up of two strands coiled together in a double helix shape.
Genes
A gene is a small section of DNA found on a chromosome.
Each gene tells the cells to make a particular sequence of amino acids which are put together to make a specific protein.
20 amino acids are used in the making of a specific protein.
DNA determines what proteins the cell produces and that, in turn, determines what type of cell it is (e.g. red blood cell, skin cell etc)
Genomes
Genome is the term used for the entire set of genetic material in an organism.
Why is understanding the human genome really important for science and medicine?
Allows scientists to identify genes in the genome that are linked to different types of disease.
Knowing which genes are linked to inherited diseases could help to understand them better and could help to develop effective treatments.
Scientists can look at genomes to trace the migration of certain populations of people around the world.
The Structure of DNA and Protein Synthesis
Nucleotides
DNA strands are polymers made up of lots of repeating units called nucleotides.
There are four different bases; A, T, C or G.
A always links up with T, and C always pairs up with 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.
mRNA
Proteins are made in the cell cytoplasm on tiny structures called ribosomes.
To make proteins, ribosomes use the code in the DNA.
Functions of proteins
Enzymes - act as biological catalysts to speed up chemical reactions in the body.
Structural proteins - they are physically strong.
Hormones - used to carry messages around the body.
Mutations
Different types of Mutation
Insertions
Every three bases in a DNA base sequence codes for a particular amino acid.
Insertions are where a new base is inserted into the DNA base sequence where it shouldn't be.
An insertion changes the way the groups of three bases are 'read', which can change the amino acids that they code for.
Substitutions
Substitution mutations are when a random base in the DNA base sequence is changed to a different base.
Deletions
Deletions are when a random base is deleted from the DNA base sequence.
Deletions change the way that the base sequence is 'read' and have knock-on effects further down the sequence.
Mutations are changes to the genetic code...
1) Occasionally a gene may mutate (a mutation is a random change in an organism's DNA; they can sometimes be inherited).
2) Mutations occur continuously and can occur spontaneously. The chance of mutation is increased by exposure to certain substances or some types of radiation.
3) Mutations 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 that make up a protein, mutations to a gene sometimes lead to changes in the protein that it codes for.
4) Most mutations have very little or no effect on the protein. Some will change it to such a small extent that its function or appearance is unaffected.
5) On the other hand, some mutations can seriously affect a protein. Sometimes, the mutations will code for an altered protein with a change in its shape. This could affect its ability to perform its function.
6) If there's a mutation in non-coding DNA, it can alter how genes are expressed.
Reproduction
Asexual reproduction
Bacteria, some plants and some animals reproduce asexually.
Asexual reproductionhappens by mitosis -- an ordinary cell makes a new cell by dividing in two.
The new cell has exactly the same genetic information as the parent cell.
In asexual reproduction, 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 parent (clones).
Fungi
Many species of fungus can reproduce both sexually and asexually.
These species release spores, which can become new fungi when they land in a suitable place.
Spores can be produced sexually and asexually.
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 survive the change.
Asexually-produced spores form fungi that are genetically identical to the parent fungus.
Malarial Parasites
Malaria is caused by a parasite that's spread by mosquitoes.
When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human.
The parasite reproduces sexually when it's in the mosquito and asexually when it's in the human host.
Plants
Plants can reproduce sexually and asexually.
Asexual reproduction can take place in different ways.
For example, plants that grow from bulbs (e.g. daffodils). New bulbs can form from the main bulb and divide off. Each new bulb can grow into a new identical plant.
Sexual Reproduction
The mother and father produce gametes by meiosis - e.g. egg and sperm cells in animals.
In humans, each gamete contains 23 chromosomes which is half the number of chromosomes in a normal cell.
The egg and sperm fuse together (fertilisation) to form a cell with the full number of chromosomes (half from mother, half from father).
Sexual reproduction is where genetic information from two organisms is combined to produce offspring which are genetically different to either parent.
Sexual reproduction involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parents' genes.
This mixture of genetic information produces variation in the offspring.
Advantages of Asexual over Sexual
2) Many identical offspring can be produced in favourable conditions.
1) There only needs to be one parent.
This means that asexual reproduction uses less energy than sexual reproduction, because organisms don't have to find a mate.
This also means that asexual reproduction is faster than sexual.
Advantages of Sexual over Asexual
1) Offspring have a mixture of two sets of chromosomes. The organism inherits genes from both parents, which produces variation in the offspring.
Selective breeding speeds up natural selection. This allows us to produce animals with desirable characteristics. Selective breeding is where individuals with desirable characteristics are bred to produce offspring with that desirable characteristics. This means that we can increase food production.
2) Variation increases the chance of a species surviving a change in the environment. While a change could kill some individuals, it's likely that variation will have led to some of the offspring being able to survive.
This means they are more likely to breed successfully and pass the genes for the characteristics on. This is known as natural selection.
Meiosis
Gametes
To make gametes which only have half the original number of chromosomes, cells divide by meiosis.
This process involves two cell divisions. In humans, it only happens in the reproductive organs (in females it's the ovaries and in males, the testes.
Gametes only have one copy of each chromosome so that when gamete fusion takes place, you get the right amount of chromosomes again.
The Process
- In the second division, the chromosomes line up in the centre of the cell and the arms are pulled apart.
- The cell duplicates its genetic information, forming two armed chromosomes(called chromatids). After replication, the chromosomes arrange themselves into pairs.
- In the first division, the chromosome pairs line up in the centre of the cell. At this point crossing over can occur - this is where sections of chromosomes can swap as they are positions alongside each other - this increase the mixing of genes and variety
- The pairs are pulled apart so each new cell only has one copy of each chromosomes (some of the father's and some of the mother's go into each new cell).
You get four gametes, each with only a single set of chromosomes in it. Each of the gametes is genetically different from the others because the chromosomes get all shuffled up during meiosis and each gamete only gets half of them, at random.
Nucleotides made up of sugar, phosphate group and a base
The process by which the code gets from the DNA to the ribosome is done using a molecule called mRNA. (messenger RNA)
mRNA is made by copying the code from DNA and mRNA acts as a messenger between the DNA and the ribosome - it carries the code between the two.
mRNA is a template of a small section of DNA (a gene) - it will carry the information needed to produce a protein
mRNA - is a smaller molecule than DNA - so can move out of the nucleus and into the cytoplasm to the ribosome
the template molecule (mRNA) moves to the ribosome - carrier molecules attached to amino acids match to the mRNA base order - and amino acids line up in the correct order to make a protein