3.1 - Genes

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What is a gene?

A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.

Genetics is the branch of biology concerned with the storage of information living organisms and how this information can be passed from parents to progeny.

The word genetics came from the word genes meaning origins.

In the 20th century, Scientists said that Genes came from DNA, but there are relatively very few DNA molecules (46 in a human body) in the cell whereas there are thousands of genes in the body.

Therefore we can deduce that each gene consists of a much shorter length of DNA than a chromosome and that a chromosome carries many genes.

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Where are genes located?

A gene occupies a specific position on one type of chromosome.

Genes are linked ina group that corresponds to one type of chromosome ina species.

Example: fruit flies (four groups of linked genes and you're types of chromosomes), humans (23 groups of linked genes and 23 chromosomes)

The locus of a gene is the specific position that a gene occupies on the type of chromosome.

Maps can show the sequence of genes along the chromosome, much detailed maps can be produced by the genome of a species is sequenced.

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What are alleles?

The various specific forms of genes are alleles.

Gorge Mendel is the father of genetics.

He crosse varieties of pea plants (tall with dwarf and white-flowered with purple-flowered)

He then deduced that the differences between the varieties that he crossed together were due to the different heritable factors.

we now that they are alternatives of the same gene.

Example: pea plant height; there are two sales one making them tall and the other making them short.

Ther can be more than two alleles off a gene.

Examples: rats the gene that influences coat color; they are three alleles making them grey, yellow, and black. Another one is humans and blood type; there are three A, O, and B.

There can be more than three as well and multiple; examples would be fruit flies and the gene that determines eye color.

They occupy the same position as the type of chromosome; they have the same locus.

Only one allele can occupy the locus of the gene on a chromosome.

and plants can have two copies of the same chromosome, so we can expect to have two copies of the gene to be present.

these could be two of the same alleles of the gene or different alleles.

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Differences between alleles

Alleles differ from each other by one or a few bases only.

A gene consists of a length of DNA, which has a base sequence that can be hundreds or thousands of bases long.

The different alleles of a gene have slight variation int he base sequence

There might be "Adanien" in one base sequence of an allele and then there might be "Cytosine" in the place of "adenine" of another base sequence of an allele.

Position in a gene where more than one base may be present are called single nucleotide polymorphisms also known as SNPs (snips)

Several snips can be present in a gene, but even then the alleles of the gene differ by only a few bases.

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Comparing gene

An outcome of the human genome project is that the techniques that were developed have enabled the sequences of other genomes.

This allows gene sequences to be compared.

The results of the comparison can be used to determine evolutionary relationships.

Also, the identification of the conserved sequences allows species to be chosen for exploring the function of that sequence.

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Mutation

New alleles are formed by mutation.

alleles are formed from other alleles by a gene mutation.

Mutation are:

Random changes

No mechanism for a particular mutation being carried out

The most significant type of mutation is base substitution.

One base int he sequence is replaced but another

Adenine to cytosine, guanine, or thymine.

Random changes to an allele that have developed by evolution over millions of years are unlikely to be beneficial.

Almost all mutation is neutral, harmful, or lethal.

Lethal means that they cause the death of the cell in which the mutation occurs.

Mutation in bodies is eliminated when the individual dies, but the mutation that developed into games are able to be passed to the offspring and cause genetic disease.

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Sickle cell anemia

Involves a base substitution mutation.

Change the base sequence of mRNA transcribed from it and ca change to the sequenced of a polypeptide in hemoglobin.

Most common genetic disease int the world.

Due to the mutation of the gene that codes for the alpha-globin polypeptide in hemoglobin.

Symbole of the gene is Hb; most humans have the allele HB^A

The sith codon mutates from GAG to GTG and forms a new allele called Hb^S

The mutation is only inherited by offsprings; occurs in a cell of the ovary or testis.

The Hb^S is transcribed by the mRNA to form GUG instead of GAG causing the hemoglobin molecules to stick together in tissues.

Hence causes low oxygen concentration.

The stickle cells cause damage to the tissues by becoming trapped in the blood capillaries, blocking them, and reducing blood flow.

And when the stickle cells return to the high oxygen concentration in the lungs the hemoglobin bundles start to break up and the cells return to their normal shape.

The hemoglobin and plasma membrane are then damaged.

The life of the red blood cell is then shortened by at least 4 days

The body won't be able to replace the red blood cells at that rate so anemia will develop.

Not known how often this mutation in some parts of the world, but it is widely common in east Africa.

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What are genomes?

The genome is the whole of the genetic information of an organism.

Genetic information is contained in DNA, a living organism's genome is the entire base sequence of each of it's DNA molecules.

In humans, the genome consists of the 46 molecules that form the chromosomes in the nucleus and the DNA molecule in the mitochondria

This is the pattern in other animals, but the number of chromosomes is usually different.

In plants, the genome is the DNA molecules of chromosomes in the nucleus and the DNA molecules in the mitochondrion and the chloroplast.

In prokaryotes, the genomes are much smaller and consist of the DAN in the circular chromosomes and any plasmids that are present.

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The human genome project

The entire base sequence of human genes was sequenced in the human genome project.

It began in 1990 to find the base sequence of the entire human genome.

A draft sequence was published in 2000 and then in 2003, the complete sequence was published.

Although the entire base sequence hasn't given us an immediate understanding of the human genome it has given us a "rich mine of data."

It is possible to predict which base sequence are protein-coding genes.

There are approximately 23,000 of these in the human genome, but the original estimate was much higher than this.

Another discovery is that most of the genomes are not transcribed, these are called "junk DNA".

Within these junk regions, there are elements that affect gene expression as well as highly repetitive sequences called satellite DNA.

The genome that was sequenced consists of one set of chromosomes.

It is a human genome rather than the human genome.

The vast majority of base sequences are shared by all humans giving us genetic unity, but there are also many single nucleotide polymorphisms that contribute to human diversity.

Comparing other species genomes reveal aspects of the evolutionary history of living organisms that were previously unknown.