3.1 Genes
Essential idea: Every living organism inherits a blueprint for life from its parents.
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Guiding Questions
Guiding Questions
What is the link between the DNA molecules in each cell and the inheritance of our genes?
How does the DNA control the cell and what happens if it goes wrong?
3.1.U1 A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.
3.1.U1 A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.
Be able to:
Describe how a gene is a heritable factor that consists of a length of DNA and influences a specific characteristic
Genes are make up of DNA, few DNA molecules in a cell (just 46) but there are 1,000’s of genes. From this we know that each gene consists of a much shorter length of DNA than a chromosome and that each chromosome carries many genes.
Genetics is the storage of information and how this information can be passed from parents to progeny.
Genes are make up of DNA, few DNA molecules in a cell (just 46) but there are 1,000’s of genes. From this we know that each gene consists of a much shorter length of DNA than a chromosome and that each chromosome carries many genes.
3.1.U2 A gene occupies a specific position on a chromosome.
3.1.U2 A gene occupies a specific position on a chromosome.
Be able to:
Discuss how a gene occupies a specific position on one type of chromosome
3.1.U3 The various specific forms of a gene are alleles.
3.1.U3 The various specific forms of a gene are alleles.
Be able to:
Define locus and allele
Explain why there can only be two alleles per pair of chromosome
Alleles are alternative forms of a gene that code for the different variations of a specific trait. For example, the gene for eye colour has alleles that encode different shades / pigments
Alleles are different heritable factors and these pairs of heritable factors are alternative forms of the same gene – etc. Height, one gene making the plant tall and the other making it small. This is called an allele. There can only be two alleles of a gene.
Alleles occupy the same position (locus) on the same type of chromosome – only one allele can occupy the locus of the gene on a chromosome
3.1.U4 Alleles differ from each other by one or only a few bases.
3.1.U4 Alleles differ from each other by one or only a few bases.
Alleles are alternative forms of a gene that code for the different variations of a specific trait
Genes consist of a certain sequence of DNA bases which can be 100’s to 1000’s bases in length
Usually different alleles of the gene vary by only one to a couple of different bases.
For example, the allele for Sickle Cell Anemia is created by a mutation of a single nucleotide.
Adenine is switched to Thymine (CTC to CAC) which results in glutamic acid being substituted by valine at position 6 in the Hemoglobin polypeptide.
This variation when one nucleotide is switched for another is called a single nucleotide polymorphism (SNPs for short)
3.1 U5 New alleles are formed by mutation.
3.1 U5 New alleles are formed by mutation.
Be able to:
Describe how a gene mutation involves a change in the nucleotide sequence of DNA
Explain the ultimate source of genetic diversity comes from mutation
New alleles are created by random changes in the base sequence called mutations.
Gene mutation – random changes
Significant types is base substitution – one base in the sequence is replaced by a different base
These changes can either be neutral or harmful, lethal – cause the death of the cell in which the mutation occurs.
Mutations –> develop into gametes–> passed on to offspring –> causing genetic disease
Gene mutations can be beneficial, detrimental or neutral
Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait
Detrimental mutations truncate the gene sequence (nonsense mutations) to abrogate the normal function of a trait
Neutral mutations have no effect on the functioning of the specific feature (silent mutations)
3.1.U6 The genome is the whole of the genetic information of an organism.
3.1.U6 The genome is the whole of the genetic information of an organism.
Be able to:
Define genome
The genome is the totality of genetic information of a cell, organism or organelle. This includes all genes as well as non-coding DNA sequences (e.g. introns, promoters, short tandem repeats, etc.
The whole of the genetic information of an organism's genetic information is contained in DNA, therefore a living organism’s genome is the entire base sequence of each of its DNA molecules.
In humans, the genome consists of 46 chromosomes plus the mitochondrial DNA
In plants, the genome also consists of chloroplast DNA on top of their chromosomes and mitochondrial DNA
Genome of the prokaryotes is much smaller and has the DNA in the circular chromosomes, plus any plasmids that are present.
Prokaryotes have a circular chromosome and plasmids in their genome
3.1.U7 The entire base sequence of human genes was sequenced in the Human Genome Project.
3.1.U7 The entire base sequence of human genes was sequenced in the Human Genome Project.
The entire base sequence of human genes was sequenced in the human Genome project. The aim was to find the base sequence of the entire human genome
The completion of the Human Genome Project in 2003 lead to many outcomes:
Mapping – The number, location, size and sequence of human genes is now established
Screening – This has allowed for the production of specific gene probes to detect sufferers and carriers of genetic diseases
Medicine – The discovery of new proteins have lead to improved treatments (pharmacogenetics and rational drug design)
Ancestry – Comparisons with other genomes have provided insight into the origins, evolution and migratory patterns of man
3.1.A1 The causes of sickle cell anemia, including a base substitution mutation, a change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.
3.1.A1 The causes of sickle cell anemia, including a base substitution mutation, a change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.
A mutation that causes the replacement of a single base nucleotide with another nucleotide in DNA.
When one of the bases is changed, this will cause a change in the mRNA sequence when the DNA is copied during transcription of the gene.
This change in the mRNA sequence may change the amino acid in the polypeptide coded for by the gene; in the process of translation.
Sickle-cell anemia is a disease that causes red blood cells to form a sickle shape (half-moon). These sickled blood cells cannot carry as much oxygen as normal red blood cells. They can cause clots in blood vessels because of their abnormal shape and inflexibility caused by crystallization of the abnormal hemoglobin.
Sickle cell is caused by a base-substitution when the adenine base in GAG is replaced by a thymine base, changing the triplet to GTG.
The normal triplet when transcribed and translated codes for the amino acid glutamic acid.
When the base substitution occurs, the amino acid that is translated is now valine.
Since valine has a different shape and charge, the resulting polypeptide’s shape and structure changes.
As a result, hemoglobin’s shape will change, as does the shape of the red blood cell, resulting in the problems associated with sickle cell anemia listed above.
3.1.A2 Comparison of the number of genes in humans with other species.
3.1.A2 Comparison of the number of genes in humans with other species.
3.1.S1 Use of a database to determine differences in the base sequence of a gene in two species
3.1.S1 Use of a database to determine differences in the base sequence of a gene in two species
Gene sequences from different species can be identified and then compared using two online resources:
GenBank – a genetic database that serves as an annotated collection of DNA sequences
Clustal Omega – an alignment program that compares multiple sequences of DNA
Epigenetics- a new frontier in medicine
Epigenetics- a new frontier in medicine
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