DNA is a complex chemical that carries genetic information. DNA Is contained in chromosomes, which are found in the nucleus of most cells. The gene is the unit of inheritance and different forms of the same genes are called alleles.

Chromosomes

Chromosomes are x-shaped objects found in the nucleus of most cells. They consist of long strands of deoxyribonucleic acid, or DNA for short. A section of DNA that has the genetic code for making a particular protein is called a gene.

The gene is the unit of inheritance, and each chromosome has several thousand genes. We inherit particular chromosomes through the egg of our mother and sperm from our father. The genes on those chromosomes carry the code that determines our physical characteristics, which are a combination of our two parents.

The bases in the DNA molecule carry the different codes needed for different amino acids. The code for a particular amino acid is made from three bases in a particular order.

Variation

Variation means differences between individuals. Variation arises from inheritance, environmental factors and mutations. There are two types of variation:

Continuous:

In continuous variation there is a complete range of measurements from one extreme to the other.

Heightis an example of continuous variation - individuals can have a complete range ofheights, for example, 1.6, 1.61, 1.62, 1.625 etc metres high.

Discontinuous:

Variation that is either/or E.g. Roll tongue, attached earlobes.

Chromosomes:

Chromosomes are long coiled lengths of DNA found in the nucleus of the cell. Each chromosomes are made up of many genes. Different species of animals and plants contain different numbers of chromosomes. Humans have 46 chromosomes (23 pairs). A pair of chromosomes are called a homologous pair.

Genes:

Are sections of DNA that code for particular traits such as eye colour, hair colour, ear shape.

Alleles:

The name given to one of the forms that a gene can have. E.g. The gene for tongue rolling has 2 alleles; a dominant form and a recessive form.

DNA:

A molecule that contains the instructions to make a new organism It is found in the nucleus of the cell. It is usually super twisted to form chromosomes.

Chromosomes exist as pairs so that individuals inherit two copies of each gene.

The backbone of the DNA molecule consists of two strands of alternating chemical strands of alternating sugar molecules and phosphate groups, the two strands twisted to form a double helix. Each sugar molecule is attached to one of the 4 bases called adenine, guanine, thymine and cytosine (A, G, T, C for short) The bases are paired up on opposite strands. A is always paired with T, and G is always paired up with C. This is called complementary base pairing - the order of bases in one strand determine the order of bases in another. A nucleotide is a basic building block for DNA made up of phosphate, sugar and base. The two strands are held together by weak hydrogen bonds. A gene consists of hundreds or thousands of bases. A gene codes for a particular protein by its particular base sequence.

DNA must copy or replicate itself before any cell division can take place. The DNA Double Helix is perfectly suited for replication because each strand can serve as a template to produce a strand opposite to itself.

First it is "unzipped" - the two strands are separated. New nucleotides in the cell line up alongside the unpaired bases, A pairing with T and G pairing with C. The new nucleotides are joined together using an enzyme called DNA polymerase.

DNA replication is semi conservative i.e. each strand in the original DNA molecule is used as a template o make a new strand of DNA. Each new DNA molecule contains an original strand and a newly made strand.

Cell division

Cell division occurs through mitosis and meiosis. Biological ideas relating to mitosis and meiosis are limited to:

Purpose
Where they occur
Sequence of events (The name of stages are not required)
Reasons for maintenance or change of chromosome number
Significance of the number of cells produced.

Cells grow old and die. They need to be constantly replaced. This is done by a type of cell division called mitosis. Mitosis takes place in all cells except the sex organs(testes and ovaries) t is used for growth and replacement of cells. If you are going to grow you have to make more cells! Daughter cells have the same number of chromosomes as the parent cell. Meiosis - Also called reduction division - Is the cell division that produces gametes. Meiosis in animals takes place in the testes and ovaries. It results in the production of sex cells (sperm and egg) Sex cells have half the number of chromosomes as the parent cell. When the sperm and egg cells unite at fertilization, each contributes 23 chromosomes so the resulting embryo will have the usual 46. Meiosis also allows genetic variation through a process of DNA shuffling (Crossing over) while the cells are dividing. The two types of cell division are compared below:

Mitosis

Meiosis

In somatic cells (general body cells.

In cells producing the gametes (testes and ovaries)

One cell division, resulting in 2 daughter cells

Two cell divisions, resulting in 4 cells

Chromosome number remains the same (diploid)

Chromosome number halved (haploid)

No pairing of homologous chromosomes

Homologous cells pair together before first division

No "Crossing over"

"Crossing over" of homologous chromosomes when they pair

(exchanging genetic information)

Conservative process: Daughter cells genotypes

Identical to parental cell's genotype.

Process produces variation: The genetic makeup includes new

Combinations of genes not found in parental cell.

Occurs in many different cell types - important for

Animals to grow and repair.

Only occurs in specific cells in the body that will become egg

Cells or sperms cells (Gametes)

To understand how the differences between mitosis and meiosis occur, you must understand the sequence of events that takes place inside the cell for each type of cell division.

Remember that the point of mitosis is to produce identical copies of cells for rapid growth and repair. The point of meiosis is to produce sex cells that contain half the number of chromosomes. Also during meiosis, genetic information (bits of chromosomes called chromatids) is swapped between homologous chromosomes when they line up prior to separation. Mendel formulated what is now known as Mendel's law of independent assortment. The law states that allele pairs separate independently during the formulation of gametes. Therefore, Traits (characteristics) are transmitted to offspring independently of one another. It is a random process.

Mitosis

Before cell division, the chromosomes can't be seen. As division starts the chromosomes become fatter and more visible.
The DNA making each chromosome copied. Each chromosome has been copied. The copies are still attached at the centromere. Each copy is called a chromatid.
The chromosomes line up singularly n the middle of the cell (equator).
One chromatid of each chromosome goes to the opposite ends of the cell (poles), pulled by the swindle apparatus. The parent cell starts to split into two.
Two daughter cells are made. The number of chromosomes are the same in each daughter cell as the original parent cell.

Meiosis

Chromosomes become fatter and more visible.
The DNA making each chromosome is copied. Each chromosome has been copied. The copies are still attached at the centromere. Each copy is called a chromatid. The homologous pairs of chromosomes line up along side each other in the middle of the cell. Now some crossing over of bits of chromosomes can take place between homologous pairs. This swaps genes from one chromosome to another and leads to variation between offspring.
One of each homologous pair goes to opposite ends of the cell. The parent cell starts to split into two, making two daughter cells.
The chromosomes in each cell line up in the middle and this time the chromatids separate. We now have 4 daughter cells; each one has half the number of chromosomes are the original parent cell. Also, these chromosomes are not identical to the parent because crossing over took place and independent assortment occurred.

Mutations - When things go wrong with DNA.

It is important that DNA remains unchanged rom generation to generation. The complementary base pairing that occurs during DNA replication means this generally occurs. However, random, spontaneous mistakes can happen.

A mutation is a change in the type or the amount in the DNA in a cell.

Type of DNA: A mistake made in copying the DNA can produce a slightly different allele of a gene. Certain chemicals, ultra-violent rays, or radiation can increase the chances of changes occurring to DNA even when chromosomes are not being copied.

Changes to the base sequence are called point or gene mutations and can be (a) insertion (addition of a base) (b) deletion (loss of a base) c inversion (2 bases change position) (d) substitution (one base replaces another). If the mutation happens in the gametes, then this allele can be passed onto offspring - these diseases are called inherited diseases eg cystic fibrosis.

Mutations can be:

Harmful (causing death or disease): To function correctly, cells depend on many proteins. Gene mutations may stop one or more proteins from working properly. By changing a gene's instructions for a protein, the mutation can cause the protein to malfunction or to be missing entirely. If the protein plays a critical role in the body, it can disrupt development or cause a medical condition. Some mutations can cause illnesses such as cancer.
Beneficial( give some advantage or benefit to an organism): eg antibiotic resistance in bacteria is beneficial to the bacteria.
Silent/neutral - Appear to not affect the organism (does not affect the phenotype).they may not be evident if the mutation is a recessive allele.A change to a DNA triplet might not change which amino acid is introduced, or it might of change the amino acid to a chemically similar amino acid that works just as well. Silent/neutral mutations do however create the possibility of a future mutation having an effect.

Growth and repair
Produces two genetically identical cells
Cells are diploid (full chromosome set), 2n
Occur in somatic cells

New cells are needed throughout life - we do not have the same cells all throughout our life. These are for growth, to replace damaged cells and repair worn out tissues. Normal human body cells are diploid - they have two of each chromosome(they have a full set of chromosomes) . When new cells are made, these 46 chromosomes are copied exactly in a process called mitosis. Mitosis is the type of cell division used for growth and repair. Mitosis occurs whenever new cells are needed. It produces two cells that are identical to each other and the parent cell.

In mitosis, each chromosome is copied exactly. The new chromosomes are moved to opposite sides of the cell, before the cell divides leaving one complete set of 46 chromosomes in each of the two new cells.

Constant cell division ensures that cells never get two large. The larger the cell becomes, the smaller its surface area to volume ratio. Objects with a small surface area to volume ratio find it difficult to maintain exchange of materials with their enviroment. Large cells could run out of oxygen, and accumulate too much waste, such as carbon dioxide. For this reason it's more efficiant for large organisms to be multicellular, rather than single celled.

Fertilization

In humans, all reproduction is sexual. It involves joining together cells from each parent with half the normal number of chromosomes to make a new cell containing both parents genetic material.

The cells from each parent that combine to form the zygote are called gametes. In humans, the male gamete is called sperm, and the female gamete is called an egg. When the gametes join they form a cell called a zygote. Human sperm and egg contain 23 chromosomes. Human zygotes contain 46 chromosomes.

The type of cell division that produces gametes with half the normal chromosomes is called meosis.

Gametes contain different genetic information to each other and to the parent cell.

Meiosis is responsible for genetic variation.

These are helpful terms to learn:

Gamete - cell with half the normal number of chromosomes, and only used for sexual reproduction.
Zygote - cell formed when the two gametes combine.
Fertilisation - term to describe the joining of two gametes
Haploid - having half the normal number of chromosomes.
Diploid - having the normal number of chromosomes.

Gametes

All Gametes are haploid. They also have other adaptations to increase the chances of fertilisation and successful development of the embryo.

Sperm cells need to move the egg cell. They have a tail to propel them, and many mitochondria to provide energy. The front of the sperm contains enzymes to digest the egg membrane.

The egg contains a large food store to support the developing zygote until it can get food via the placenta.

Meiosis

Meiosis is the type of cell diviosion used in sexual reproduction. It occurs only in the testes and ovaries. It produces cells that are different to each other, and to the parent cell. The cells produced contain half the normal number of chromosomes.

At fertilisation, the nuclei of the sperm and an egg join to form the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg, thereby creating the correct number of 46 chromosomes for all body cells. It also means the zygote contains a complete set of chromosomes from each parent.

Amount of DNA: Sometimes when cells divide, the wrong number of chromosomes ends up in a gamete, and it may have a particular chromosome missing, or two copies of a chromosome.

How DNA “codes” for proteins

Gene is part of the DNA molecule/chromosome.
Is a sequence of bases/nucleotoids.
Codes for a specific sequence of amino acids

MITOSIS AND MEIOSIS

	Created by Ambionne Wilson over 10 years ago

1 Science (Genetics) Note on 1.9 Genetic variation , created by Ambionne Wilson on 09/08/2014.