DNA stands for deoxyribose nucleic acid
-
Location of DNA
-
Structure of DNA
-
Purpose of DNA
<
>
DNA is a double-stranded molecule. It looks like a ladder that is all twisted up!
|
|
When an organism grows, reproduces, repairs old cells or replaces them, new cells must be made. Every new cell needs DNA in it so that the cell can carry out its function. This means that DNA has to be copied (or replicated) before the new cell is formed so it can start off its cell life with DNA inside its nucleus. The new DNA must be the same as the original. This is achieved by DNA replication.
DNA Replication:
There are three steps:
Each new DNA molecule contains one strand of old DNA (the template) and one strand of newly made DNA. We call this type of DNA replication semi-conservative replication.
- The DNA double helix unwinds and the two strands are separated (this means there are no hydrogen bonds holding the base-pairs together any more and they can move apart).
- The two separated strands act as templates, and enzymes add new nucleotides (remember these are sugar/phosphate/base) which are complimentary to the template strand - e.g. if the template strand is TAGTAGCCG then the newly added nucleotides would be ATCATCGGC (complementary).
- When complete, there are now two identical DNA molecules that each wind back up.
Each new DNA molecule contains one strand of old DNA (the template) and one strand of newly made DNA. We call this type of DNA replication semi-conservative replication.
Why is it important that the new DNA strands are identical to the old ones?
If these new cells contained DNA that was different to the parent cell (therefore different to original DNA strand) it is likely that the new cell would do things it is not meant to do. It would make proteins that it shouldn't! This means the cell won't be doing its job.
Sometimes mistakes can happen during DNA replication
Before any new cell is made (by mitosis or meiosis), DNA must make a copy of itself. Sometimes, the wrong bases end up the wrong places! When this happens, it is called a mutation. A mistake will change the sequence of bases on a gene and can result in a different amino acid being incorporated into a protein, resulting in either a deformed (non-functioning) protein, or perhaps a completely different protein (making a new trait!). Sometimes mutations have no effect at all - it just depends. You'll learn more about this in Level 2 Biology.
|
DNA to PHENOTYPE
Some terms to know and understand:
-
Homologous pairs
-
Gene
-
Allele
<
>
DNA is organised into chromosomes. Humans have 23 pairs of chromosomes (46 chromosomes total) inside every single cell. Chromosomes are found in what are called homologous pairs due to the similarities in their structures - the two chromosomes in each pair have the same genes in the same location and they are the same size and shape.
|
An allele is the code found on the gene - it is the instructions to make a particular protein - the order of bases on that section of DNA.
Many genes only have two alleles that code for what protein is made, either allele A or allele B - the alleles are made from the sequence the bases (letters) are in on that particular gene (section of DNA that codes for a trait). Below are two sections of DNA (one from each chromosome in a homologous pair) and you will notice that there is a different sequence of bases on each. The gene could be for earlobe attachment, and the allele on the left could code for attached earlobes whereas the one on the right could code for unattached earlobes! |
Because chromosomes come in pairs, we have two alleles for every gene...
For every single gene we have in our DNA there is a genotype and a phenotype for it. Let's use the example of eye colour. There is a gene on chromosome 15 that codes for eye colour. To make it simple, let's say there are only two possible alleles for eye colour - brown and blue.
On both of a person's chromosome 15 is the gene for eye colour. The DNA sequence may differ on each, or they may be the same. The characteristic produced from the DNA sequence is what we call an allele, and the alleles may differ between the two chromosomes in the pair. On the first chromosome there could be the allele B for brown eyes (coded for by a specific sequences of base-pairs), but on the second chromosome there could be the allele b for blue eyes.
Blue eyes is recessive to brown eyes, meaning it will not be expressed if the allele for brown eyes (B) is present in the genotype. We know it is recessive because it is written as a lower case b (lower case letters represent recessive alleles). The genotype for a person with one allele for brown eyes and one for blue eyes would be Bb, meaning they have the phenotype of brown eyes. If the homologous pair of chromosome 15 both had the alleles b, making the genotype bb, then the phenotype (physical appearance) would be blue eyes.
It is not possible to have a phenotype without a genotype. Genotype determines phenotype.This is because the genotype is the allele combination that actually dictates the physical appearance of the trait/characteristic.
On both of a person's chromosome 15 is the gene for eye colour. The DNA sequence may differ on each, or they may be the same. The characteristic produced from the DNA sequence is what we call an allele, and the alleles may differ between the two chromosomes in the pair. On the first chromosome there could be the allele B for brown eyes (coded for by a specific sequences of base-pairs), but on the second chromosome there could be the allele b for blue eyes.
Blue eyes is recessive to brown eyes, meaning it will not be expressed if the allele for brown eyes (B) is present in the genotype. We know it is recessive because it is written as a lower case b (lower case letters represent recessive alleles). The genotype for a person with one allele for brown eyes and one for blue eyes would be Bb, meaning they have the phenotype of brown eyes. If the homologous pair of chromosome 15 both had the alleles b, making the genotype bb, then the phenotype (physical appearance) would be blue eyes.
It is not possible to have a phenotype without a genotype. Genotype determines phenotype.This is because the genotype is the allele combination that actually dictates the physical appearance of the trait/characteristic.