Nucleus of a cell contains chromosomes (23 pairs) where genes are located
Genes are bits that code for a specific protein and is a section of a molecule of DNA
A diploid human cell contains 46 chromosomes, organised into 23 pairs. One set comes from each parent (contained in haploid gametes)
A matching pain of chromosomes (one from each parent) forms a homologous pair, they code for the same characteristics/genes chromosomes are made up of 2 parts called sister chromatid which are visible during the replication.
Each of these is tightly coiled around protein (histones) in order to make it organised/compacted. When unwound the structure of DNA can be seen. It is made of two strands, which twist to form a double helix.
There are four different DNA bases:
C cytosine
G guanine
A adenine
T thymine
C + G go in a pair together whilst A + T are together
Nucleotide- unit that makes up DNA
Bases- chemicals in DNA that carry the genetic code
Protein- the type of molecule that genes are coded for
Amino acids- small molecules that join to make proteins
Alleles are different versions of the same gene:
Dominant:
An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself.
Recessive:
An alternative form of the gene that can be expressed only if a dominant allele of that gene is not present. An organism must have two copies of a recessive allele for that allele to be expressed.
Homozygous: Two alleles that are the same for that particular gene
Heterozygous: Two different alleles that are different for that particular gene
Phenotype: outward appearance of an organism which occur as a result of its genes e.g. blue eyes
Genotype: alleles that an organism has for a particular characterisitic, usually written as letters e.g. bb for blue eyes
For Paper Two-
Co-dominance: neither allele is dominant or recessive so you show characteristics from both alleles,
For example blood group A and B are both dominant meaning that an offspring could possibly have blood group AB.
A monohybrid cross is the study of the inheritance of one characteristic.
You need to be able to explain a monohybrid cross in terms of genotypes. This can be done by writing down:
- the phenotype and genotype of the parents
- the gametes which each can produce
- as a result the ways in which the gametes combine (offspring phenotype and genotype)
The best way to do this is to use the same layout each time; use a punnet square or cross diagram to show the possible genotypes of the offspring.
Family Pedigrees
Knowing how inheritance works helps to interpret a family pedigree which is a family tree of genetic disorders.
To find out whether the allele is dominant you take a small branch from the family tree and if BOTH parents have the trait and ONE child doesn't it is dominant because if the trait were recessive all children would have the same trait.
If the allele is recessive it means that NEITHER parents have the trait and ONE child does; this is possible because parents must be carrying the recessive allele and it has be passed on.
Sex determination:
Out of the 23 pairs of chromosomes there are 2 chromosomes which determine whether you are a boy or a girl.
All men have a X and Y chromosome. Y chromosome causes male characteristics.
All women have two X chromosomes. XX chromosome combination causes female characteristics.
Parent phenotype Male Female
genotype XY XX
gametes X Y X X
Offspring
|
|
Female, XX |
|
|
X |
X |
| Male, XY |
X |
XX |
XX |
|
Female |
Female |
| Y |
XY |
XY |
|
Male |
Male |
There is an equal chance of having a boy or a girl
Mitosis:
Produces genetically identical cells
1) Interphase
Nuclear envelope to keep DNA in nucleus
23 pairs of chromosomes from both parents
Replication takes place here
2) Prophase
The DNA forms X-shaped chromosomes after the duplication/replication
3) Metaphase
Spindle fibres align the chromosomes into the centre after the nuclear envelope has been broken down
4) Anaphase
Splitting the chromatid into half from the centromere, so that the two arms of each chromosome go to opposite ends, also each individual part is a daughter chromosome.
5) Telophase
There are two separate nuclei within one cell and the nuclear envelope is reformed
6) Cytokinesis
The cytoplasm is then split into two genetically identical cells
Why does mitosis take place?
- growth
- repair of damaged cells
- cloning
- asexual reproduction to make identical cells
- replacement of dead cells (dust)
Meiosis:
Produces four haploid cells whose chromosomes are NOT identical
First Division-
Before the cell splits up it duplicates it DNA and one arm of each chromosome is the exact copy of the other arm.
Then the chromosomes line up in the middle during metaphase
The pairs are then pulled apart so the new cell only has one copy of each chromosome. Some of both parent goes into each new cell.
One from each pair passes to the daughter cells and has a mixture of mother and father chromosomes causing gene variation of its offspring.
Second Division-
Chromosomes then line up again in the centre of the cell and the arms are pulled apart and split into daughter cells.
Four haploid cells are produced at the end. The gametes are genetically different.
Comparisons of Mitosis and Meiosis:
|
Mitosis |
Meiosis |
Where it happens
|
In most parts
of the body
|
Ovaries, testes and
anther (in plants)
|
Purpose
|
Growth and cell
replacement
|
Production of
gametes
|
No. of chromosomes
in daughter cell
|
46 (23 pairs)
|
23
|
Number of divisions
|
One
|
Two
|
Haploid or Diploid
|
Diploid
|
Haploid
|
Identical daughter
cells?
|
Yes
|
No
|
No. of daughter cells
produced
|
Two
|
Four
|
Variation?
|
None
|
Yes
|
Variation:
Genetic Variation: due to genes which have been passed on. Examples include natural hair colour and gender
Environmental Variation: Influences from outside- the environment Examples include tanned skin and scars
Some variation comes from both the environment and genetically for example height, weight and freckles. If your malnourished it would effect your weight and also the height that you should've genetically inherited if you had eaten properly.
Discontinuous variation: can be categorised e.g. freckles and eye colour
Continuous variation: has to be measured e.g. weight, height and hair length
Mutations:
A mutation is a rare, random change in an organisms DNA that can be inherited.
Mutations change the sequence of the DNA bases. It could stop the production of a protein, or might mean a different protein is produced entirely. This can lead to new characteristics due to the slight change in allele or gene and increase variation.
Harmful:
Can affect offspring in their reproductive cells and they might develop an abnormality or die.
Diseases such as sickle cell and other genetic diseases
Neutral:
Have no effect at all or occur in unimportant parts of the DNA.
Examples- hair colour and coat colour gene change
Benefits:
Can give an organism survival advantages.
Paler or darker skin meaning that you can live in conditions where other people may not live comfortably. Being able to camouflage (wild animals)
Paper Two- Increased risk of mutations:
Can be caused by exposing yourself to ionising radiation (gamma rays, x-rays or ultraviolet light) and chemicals called mutagens (chemicals in tobacco)
Process of evolution and Natural Selection
Theory of evolution- Life began as simple organisms from which more complex organisms evolved (rather than just popping into existence).
Natural Selection:
- Inheritable variation; slight mutations can give you a better chance at surviving in an environment
- Overproduction of offspring; not all of the offspring will survive because they aren't well adapted, the more offspring you have the more variation
- Struggle to survive/ competition; variation of the animal best suited to the environment will survive and reproduce with an organism with similar qualities to have offspring that are very well suited.
Antibiotic Resistance
Bacteria sometimes develop random mutations in their DNA, this leads to changes in the bacteria's characteristics. It means that the bacterium is less affected by a particular antibiotic.
The gene for resistance being passed on to lots of offspring- due to natural selection. This is how it spreads and becomes more common in a population of bacteria overtime.
The main steps in the development of resistance are:
- Random changes or mutations occur in the genes of individual bacterial cells.
- Some mutations protect the bacterial cell from the effects of the antibiotic.
- Bacteria without the mutation die or cannot reproduce with the antibiotic present.
- The resistant bacteria are able to reproduce with less competition from normal bacterial strains.
MRSA is very dangerous because it is resistant to most antibiotics. To slow down or stop the development of other strains of resistant bacteria, we should:
- always avoid the unnecessary use of antibiotics
- always complete the full course
