DNA = DEOXYRIBONUCLEIC ACID !
This giant polymer was discovered by Watson and crick in 1953 - it is the blueprint for how all living things are built. This chemical carries all the genetic instructions used in the production and functioning of proteins, which in turn determines how each cell functions and develops and what traits and characteristics an individual will have.
Its shaped is described as a double helix - made up of two long strands of repeating units called Nucleotides that are twisted into a helical shape in order for it to fit inside the Nucleus of a cell.
Monomer = one = Nucleotide
Polymer = Many = DNA Molecule
If you untwisted the DNA in a human cell it would be 2 m long and contain 6 billion bases - this information provides 25000 instructions enough to build a complete person.
The full sequence of DNA in a cell is called that organisms GENOME !
(chromosome 1 is approx 220 billion base pairs long)
The set of instructions held in this molecule is referred to as the Genetic Code.
One Nucleotide is made up of a phosphate, a deoxyribose sugar and a base:
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Some Definitions to get your head around and learn .....
- DNA - A double helix shaped molecule that carries ALL the genetic information used for a cell to function and develop - to produce proteins. It is stored in the NUCLEUS of a cell
- A GENE - is a special segment of DNA that is found on a chromosome that codes for a particular protein that determines a particular trait/feature/characteristic.
- A CHROMOSOME - is a special structure that is found in cells that is made up of an organised section/strand of DNA that contains many genes - every human body cell has 23 pairs.
CHROMOSOMES:
The DNA is organised and packaged into special structures called Chromosomes, these are only found in the cell nucleus.
Each species has its own unique number of Chromosomes: Eg: Humans have 23 pairs- 46 in total in every body cell. Fruit fly have 4 !
Chromosomes occur as Homologous Pairs - ("Homos" is Greek for the same.) They are identical in size and shape. One of the pair comes from the Father and the other from the Mother.
The DNA is organised and packaged into special structures called Chromosomes, these are only found in the cell nucleus.
Each species has its own unique number of Chromosomes: Eg: Humans have 23 pairs- 46 in total in every body cell. Fruit fly have 4 !
Chromosomes occur as Homologous Pairs - ("Homos" is Greek for the same.) They are identical in size and shape. One of the pair comes from the Father and the other from the Mother.
HOW CHROMOSOMES WORK:
A special length on a chromosome is called a GENE and this controls a special feature of characteristic as it tells the cell to make a particular PROTEIN.
It works like this....
Three BASES in a row is called a TRIPLET CODE
Each TRIPLET codes for one AMINO ACID
A unique series of AMINO ACIDS linked together (like beads on a string) forms a specific PROTEIN
So the exact SEQUENCE of bases in the DNA is converted into each sequence of AMINO ACIDS that are needed for each particular protein
PROTEINS control our appearance and HOW the body functions
Proteins:
A special length on a chromosome is called a GENE and this controls a special feature of characteristic as it tells the cell to make a particular PROTEIN.
It works like this....
Three BASES in a row is called a TRIPLET CODE
Each TRIPLET codes for one AMINO ACID
A unique series of AMINO ACIDS linked together (like beads on a string) forms a specific PROTEIN
So the exact SEQUENCE of bases in the DNA is converted into each sequence of AMINO ACIDS that are needed for each particular protein
PROTEINS control our appearance and HOW the body functions
Proteins:
- Form structures in our bodies like skin, bones and hair Eg: skin contains the protein Collagen, Hair contains Keratin, Hemoglobin carries Oxygen,
- Make up ENZYMES which control ALL the chemical reactions of cells. Eg: Pepsin, lipase and Amylase are all enzymes of our digestive system.
DNA REPLICATION:
The base pairing rule provides a mechanism by which DNA can make a copy = REPLICATE - this means each new cell produced can have an EXACT copy of the Genetic Information needed for that cell to function correctly.
We are continually producing new cells during times of:
STEPS involved in DNA Replication:
The base pairing rule provides a mechanism by which DNA can make a copy = REPLICATE - this means each new cell produced can have an EXACT copy of the Genetic Information needed for that cell to function correctly.
We are continually producing new cells during times of:
- Growth Eg: Embyo development , Puberty,
- Repair Eg: skin cells , bone cells if a break occurs,
- Replacement Eg: bone marrow producing new Red blood cells,
STEPS involved in DNA Replication:
- The DNA Helix UNWINDS - with the help of special enzymes - this separates the two DNA strands out.
- The two separated strands and their exposed bases forms a pattern or TEMPLATE so a new DNA strand can be built (often called daughter strand)
- Free nucleotide bases (that are in the cells cytoplasm) can then be brought in to match up with the original parent strands bases using the COMPLIMENTARY BASE PAIR RULE: T------A AND G -----C
- TWO new DNA molecules form - each one contains one strand from the original DNA strand AND one newly created strand. BECAUSE one of the original parent strands is retained....this process is called SEMI CONSERVATIVE DNA - as the parent strand is CONSERVED in the newly created DNA molecule.
GENES and ALLELES
A GENE = A special length/section of DNA found on a chromosome that codes for a protein that determines what Trait or characteristic is expressed.
Each pair of homologous chromosomes carries the same, (but not necessarily identical genes) Eg: Both chromosomes of a particular pair might contain the gene for eye colour, but one may carry the version for Brown eyes and the may carry the Blue eyed version.
The alternative versions or forms of the same gene is called ALLELE. These have a slightly different order of bases ..so they cause a slightly different trait !
Because half of your chromosomes come from your mother and the other half from your father - it is possible to have two different alleles for the same trait/characteristic. If they are different they are termed Heterozygous if they are same they are termed Homozygous - (see later notes)
A GENE = A special length/section of DNA found on a chromosome that codes for a protein that determines what Trait or characteristic is expressed.
Each pair of homologous chromosomes carries the same, (but not necessarily identical genes) Eg: Both chromosomes of a particular pair might contain the gene for eye colour, but one may carry the version for Brown eyes and the may carry the Blue eyed version.
The alternative versions or forms of the same gene is called ALLELE. These have a slightly different order of bases ..so they cause a slightly different trait !
Because half of your chromosomes come from your mother and the other half from your father - it is possible to have two different alleles for the same trait/characteristic. If they are different they are termed Heterozygous if they are same they are termed Homozygous - (see later notes)
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Karyotype - what is this......
Most living things have chromosomes, or units of genetic information, in their cells. The number and appearance of chromosomes vary among species. A karyotype is the number, size, and shape of chromosomes in an organism.
Most living things have chromosomes, or units of genetic information, in their cells. The number and appearance of chromosomes vary among species. A karyotype is the number, size, and shape of chromosomes in an organism.
Monohybrid Inheritance = Punnet Squares
Punnet Squares are used to determine the probability of an offspring having a particular genotypye.
We can also work out what the expected Phenotype will be.
It is made by comparing all the possible combinations of alleles from the mother with those from the father.
It is important to be aware that Punnet Squares do NOT always match what we see in real life. They simply show the "Predicted Outcome" when 2 parents are crossed.
Each Fertilisation (fusing of egg and sperm) is a RANDOM EVENT or INDEPENDENT EVENT - the punne sqaure just shows the "cahnce"
Punnet Squares are used to determine the probability of an offspring having a particular genotypye.
We can also work out what the expected Phenotype will be.
It is made by comparing all the possible combinations of alleles from the mother with those from the father.
It is important to be aware that Punnet Squares do NOT always match what we see in real life. They simply show the "Predicted Outcome" when 2 parents are crossed.
Each Fertilisation (fusing of egg and sperm) is a RANDOM EVENT or INDEPENDENT EVENT - the punne sqaure just shows the "cahnce"
TEST CROSS
WHAT is it?......
A test cross is when we cross an organism showing a dominant genotype which is unknown - ie: is it homozygous dominant OR heterozygous for that trait or desired characteristic.
WHY DO WE DO IT?
To find out whether the animal in question carrying the dominant gene is in fact Heterozygous or Homozygous for that trait. This can be important in many breeding situations...
see more notes here....
WHAT is it?......
A test cross is when we cross an organism showing a dominant genotype which is unknown - ie: is it homozygous dominant OR heterozygous for that trait or desired characteristic.
WHY DO WE DO IT?
To find out whether the animal in question carrying the dominant gene is in fact Heterozygous or Homozygous for that trait. This can be important in many breeding situations...
see more notes here....
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VARIATION:
Variation occurs due to 3 processes:
SEXUAL REPRODUCTION of course !!!
It requires two parents
All living things inherit their characteristics from their parents. So offspring are a "combo" combination of the characteristics/traits of their parents. Each different combination depends on which allele combinations are inherited !
Remember an allele is an alternative form of the same gene !
Although all cells carry the genetic code, specialised cells are produced for Sexual reproduction = these are called GAMETES (sex cells)
In animals these are egg and sperm, in plants pollen and eggs. Special sex organs produce gametes - testes produce Sperm and ovaries Eggs.
GAMETES differ from normal body cells(somatic cells) in that they only carry ONE COPY of each chromosome - ie: 23 Chromosomes
During Fertilisation when sperm meets egg and their nucleic fuse - the normal functional number o chromosomes for humans get restored :ie
23+ 23 = 46
A ZYGOTE - is a fertilised egg - the very first cell of a new individual.
This zygote grows eventually into a new individual made by many cells produced by the process of Mitosis !(see later notes)
Each new individual has a different combination of chromosomes - half from each parent - so leads to variation in traits = will be genetically unique= !!!
Variation in traits forms the basis for evolutionary changes (natural selection = favorable traits selected for !!)
- sexual Reproduction
- Mutation
- Environmental
SEXUAL REPRODUCTION of course !!!
It requires two parents
All living things inherit their characteristics from their parents. So offspring are a "combo" combination of the characteristics/traits of their parents. Each different combination depends on which allele combinations are inherited !
Remember an allele is an alternative form of the same gene !
Although all cells carry the genetic code, specialised cells are produced for Sexual reproduction = these are called GAMETES (sex cells)
In animals these are egg and sperm, in plants pollen and eggs. Special sex organs produce gametes - testes produce Sperm and ovaries Eggs.
GAMETES differ from normal body cells(somatic cells) in that they only carry ONE COPY of each chromosome - ie: 23 Chromosomes
During Fertilisation when sperm meets egg and their nucleic fuse - the normal functional number o chromosomes for humans get restored :ie
23+ 23 = 46
A ZYGOTE - is a fertilised egg - the very first cell of a new individual.
This zygote grows eventually into a new individual made by many cells produced by the process of Mitosis !(see later notes)
Each new individual has a different combination of chromosomes - half from each parent - so leads to variation in traits = will be genetically unique= !!!
Variation in traits forms the basis for evolutionary changes (natural selection = favorable traits selected for !!)
VARIATION: continued....
The role of mutations in forming new alleles.
A mutation is a permanent change in the base sequence of DNA caused by a mutagen. A mutagen is an agent, such as a chemical substance, UV light or radiation, that causes genetic mutation.
Types of mutationMutation is a permanent / random changes in the DNA/ genetic material. Mutation must occur in gamete-producing cells to enter the gene pool of the population
A mutation is a permanent (unrepaired) change in an organisms DNA base sequence - this introduces a NEW ALLELE combination into the population
They introduce new alleles into a population.
Most mutations are harmful.
Mutations are caused by mutagens.
Beneficial ones tend to occur more often in organisms with short generation times.
Many may be silent – not observed – and may only be selected for or against at a later date.
Neutral mutations make no change at all.
Beneficial mutation = A mutation that gives an organism a survival advantage in its environment.
Harmful mutation = A mutation that effects the survival of the organism.
Silent mutation = a mutation which has no observable effect on the organism.
A mutation in a somatic cell (normal body cell) is referred to as non-inheritable variation and does not get passed on eg: weight lifter gaining muscle, increased flexibility, skin tan in summer, plants bending towards the light, plant leaf length increasing in shade.
A mutation in a sex cell - is referred to as inheritable variation and is passed on to the next generation during sexual reproduction and Meiosis. eg; hair colour, eye colour, coat type in dogs, wool fibre in sheep, disease resistance, sharper teeth, bigger eyes.
WHY is GENETIC VARIATION so IMPORTANT?.....
Genetic variation refers to the variety in a population eg: different alleles possible for each gene- means siblings are similar to each other and parents but not identical. Meiosis, sexual reproduction and Mutations lead to Genetic variation
The advantage of variation to a population is that it may see some individuals survive if environment changes eg drought, insecticides, disease. ie; a disease may affect some individuals but not all, some insects may be affected by an insecticide spray but not all- so those that do survive, have what we refer to as favourable alleles/traits/phenotype and therefore have a survival advantage - get "selected for " - are then able to be reproductively successful and pass on their favourable alleles/phenotype/trait to the NEXT generation passing these favoured (advantageous) alleles on !
When the favourable genes/better suited genes/favored alleles get passed on to the next generation - we say this is part of the process called Natural Selection - (Darwins theory of natural Selection). This passing on of favoured alleles means that over time the whole population will inherit this new advantageous trait/phenotype.
The effect of the environment on Variation
The phentoype of an organism results from the interactions between the genotype (the alleles) AND the environment. We say the
GENOTYPE + ENVIRONMENT = PHENOTYPE
The environment can change the phenotype without altering the genotype
Eg: Hydrangea flowers of same genotype can range from blue to pink to purple due to pH of soil
Plants may have the genotype for tall, upright form, but lack nutrients, sunlight or strong wind - so grow in reduced heights.
Enzymes that control coat colour only function in a range in certain temperatures
The phentoype of an organism results from the interactions between the genotype (the alleles) AND the environment. We say the
GENOTYPE + ENVIRONMENT = PHENOTYPE
The environment can change the phenotype without altering the genotype
Eg: Hydrangea flowers of same genotype can range from blue to pink to purple due to pH of soil
Plants may have the genotype for tall, upright form, but lack nutrients, sunlight or strong wind - so grow in reduced heights.
Enzymes that control coat colour only function in a range in certain temperatures
Sexual Reproduction is all about the fertilisation of Gametes - Sperm + Egg....BUT how do we produce Gametes ??
By a cell process call MEIOSIS:
Meiosis ( from the Greek word "to Make smaller" is a special type of cell division that produces male and female gametes.
Where does it occur ?.....In our sex organs...testes and ovaries !
Purpose - to produce sex cells - gametes for Sexual Reproduction.
It produces four new cells (gametes) that are genetically different to each other, and to the parent cell !
Gametes only contain HALF the number of chromosomes as every other cell in the body - so that when 2 gametes fuse (with 23 chromosomes each) the viable human chromosome number of 46 is restored ! During Meiosis genes gets shuffled around and this leads to VARIATION which is why you look similar to other members of you family but not identical to them. This is due to two important steps that occur during the process of Meiosis.
1. the crossing over of sections of chromosomes whilst the Homologous pairs "pair up" (referred to as a tetrad) and line up at the middle of the cell equator. As they are so close, chromosomes cross over each other and therefore exchange/swap sections of DNA between each other - therefore swap alleles. This exchanging of alleles creates new combos of alleles on each chromosome. So each gamete produced may have a different combination of alleles on it.
2/ Independent assortment - when chromosomes line up at the cell equator they do so in no particular order - totaly random event - no set pattern = "sort independently" of each other - so it is completely random as to which combination of alleles ends up in each new Gamete cell produced.
So each Gamete is very UNIQUE with its own unique combination of alleles on it.
By a cell process call MEIOSIS:
Meiosis ( from the Greek word "to Make smaller" is a special type of cell division that produces male and female gametes.
Where does it occur ?.....In our sex organs...testes and ovaries !
Purpose - to produce sex cells - gametes for Sexual Reproduction.
It produces four new cells (gametes) that are genetically different to each other, and to the parent cell !
Gametes only contain HALF the number of chromosomes as every other cell in the body - so that when 2 gametes fuse (with 23 chromosomes each) the viable human chromosome number of 46 is restored ! During Meiosis genes gets shuffled around and this leads to VARIATION which is why you look similar to other members of you family but not identical to them. This is due to two important steps that occur during the process of Meiosis.
1. the crossing over of sections of chromosomes whilst the Homologous pairs "pair up" (referred to as a tetrad) and line up at the middle of the cell equator. As they are so close, chromosomes cross over each other and therefore exchange/swap sections of DNA between each other - therefore swap alleles. This exchanging of alleles creates new combos of alleles on each chromosome. So each gamete produced may have a different combination of alleles on it.
2/ Independent assortment - when chromosomes line up at the cell equator they do so in no particular order - totaly random event - no set pattern = "sort independently" of each other - so it is completely random as to which combination of alleles ends up in each new Gamete cell produced.
So each Gamete is very UNIQUE with its own unique combination of alleles on it.
Click here to see a cool animation...
SEX DETERMINATION
Mitosis
The process of cell division to produce GENETICALLY IDENTICAL cells - produces cell for Growth, Repair and Replacement of all body cells. It occurs in all our body cells (somatic Cells) except sex cells. It produces the cells needed to heal cuts, wounds, broken bones, replace dead skin cells etc
Each new cell produced has a full set of Chromosomes - 46 and is termed Diploid 2n
Each new cell produced has a full set of Chromosomes - 46 and is termed Diploid 2n
PREDICTING INHERITANCE:
Scientists use a grid-like called a punnet Square to make predictions about how offspring may inherit alleles.
The alleles from the parents are split and are put into their own box.
The punnet square shows only the probability of what might occur and not the actual results. Probability is the "chance" of something occurring. The punnet square only shows the chances of what might occur each time the event is undertaken.
The name we give to predicting inheritance using one gene only is called a Monohybrid Cross
Watch the clips below to learn more about Monohybrid Crosses !
Click here for some more notes if needed...
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Click HERE for more information if needed !
Pure Breeding and the TEST CROSS in punnet Squares:
In order to determine if an individual is Pure breeding...ie: homozygous dominant for the desired Phenotype...a breeding test called a Test Cross can be carried out on the individual in question.
Breeders like to ensure that the animal they are going to be breeding from is not a "carrier " of an undesirable phenotype ie: have the recessive allele & so have a heterozygous genotype and so can reduce the chances of all the offspring expressing the desired Phenotype.
A test cross simply involves mating the organism in question with an individual that is homozygous recessive for that trait !.
For example:
A purple flower could have the alleles - Pp or PP - by mating the flower in question with a homozygous recessive flower ie: a pure bred white flower pp the offspring produced will tell you the alleles of the purple flower in question.
See picture below -
SPECIAL NOTE: To actually get a reliable result from a test cross you may need to produce a large number of offspring.....If you get a few purple offspring there is always a small chance the parent is actually heterozygous (Pp) and that as luck would have it was always the dominant allele (P) that had been passed on. However, if a large number of offspring have been produced and they all have the dominant characteristic (Purple Petals) it is far more likely that the parent is homozygous dominant (PP)
To get a reliable result from a test cross you may need to produce a large number of offspring
PEDIGREE CHARTS:
A pedigree chart is like a family tree and and shows the genetic relationships between members of the same family. They can show many generations and are used to track genetic conditions
A pedigree chart is like a family tree and and shows the genetic relationships between members of the same family. They can show many generations and are used to track genetic conditions
Click here for some more notes and a worked example if needed !!!