3.1 Genes Flashcards
What is a Gene?
A sequence of DNA that encodes for a specific trait (traits may also be influenced by multiple genes)
Section of DNA that codes for a protein
Heritable factor that consist of a length of DNA and influences a specific characteristic
Where is a gene?
On a specific position on a chromosome
What are alleles?
Alleles are alternative forms of a (one) gene that code for the different variations of a specific trait - they occupy the same position on one type of chromosome - they have the same locus
A version of a gene
For example, the gene for eye colour has alleles that encode different shades / pigments
Do alleles differ from each other by a lot?
Alleles differ from each other by one or only a few bases because they are alternative forms of one gene, so they possess very similar gene sequences
What are single nucleotide polymorphisms (SNPs)
positions in a gene where more than one base many be present
several snips can be present in a gene, but even then the alleles of the gene differ by only a few bases
How are new alleles formed?
By mutation (random changes)
What is a genome?
The genome is the whole of the genetic information of an organism
What is the Human Genome Project (HGP)
What are the outcomes?
The Human Genome Project (HGP) was an international cooperative venture established to sequence the human genome
- The HGP showed that humans share the majority of their sequence, with short nucleotide polymorphisms contributing diversity
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
What is DNA?
DNA is the genetic blueprint which codes for, and determines, the characteristics of an organism ( physical, behavioural and physiological features of the organism)
How are DNA packaged?
DNA is packaged and organised into discrete structures called chromosomes
How are specific traits determined?
By a gene or multiple genes
What is a locus (plural=loci)?
The position of a gene on a particular chromosome
What are the 2 main classes of DNA mutations?
- point mutation - only one nucleotide is affected
- insertion or deletion (indel) mutation - one or more nucleotides are inserted from a length of DNA, these may cause a frameshift
What is a frameshift?
- caused by an insertion or deletion mutation
- result in change to the aminio acid sequence whereas a substitution mutation may not
What is a gene mutation? Can they be good or bad?
A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait
Gene mutations can be beneficial, detrimental or neutral
What are the 3 types of point mutations?
- silent - codes for the same amino acid
- nonsense mutation - codes for stop codon
- missense mutation - codes for a different amino acid
What about the genetic code reduces the effect of point mutations?
the fact that it is degenerate - more than one triplet codes for each amino acid apart from Methionine (start codon)
What are beneficial mutations called?
Missense mutations - changes the gene sequence to create new variations of a trait
What are the detrimental mutations called?
Nonsense mutations - truncate the gene sequence to abrogate the normal function of a trait
codes for a stop codon
What is the relationship between genes, proteins and characteristics?
Different genes code for different proteins that code for different characteristics
What are neutral mutations called?
Silent mutation - no effect on the functioning of the specific feature
Codes for the same amino acid
When do mutations occur?
During DNA replication during S phase of the cell cycle
DNA is a relatively stable molecule when in a double helix but during replication, the bases are exposed
= is inherited
What is the difference between recessive and dominant chromosomes when representing the phenotype?
Recessive: two copies of the allele must be present in the genotype to be present in the phenotype
Dominant: Only one allele has to be present in the genotype to be present in the phenotype
Phenotype vs Genotype
A person’s genotype is their unique sequence of DNA
Phenotype is the detectable expression of this genotype
What are homologous chromosomes?
maternal and paternal pairs of the same chromosomes
What is the difference between heterozygous and homozygous chromosomes?
Heterozygous: Different alleles at a particular gene locus on a pair of homologous chromosomes
Homozygous: Identical alleles at a particular gene locus on a pair of homologous chromosomes
What is a human karyogram?
An arrangement of chromosomes of an organism in homologous pairs of decreasing length
What is the 23rd chromosome?
The sex chromosome
What are SNPS (single nucleotide polymorphisms)
Single base mutations that lead to different alleles that result in different phenotypes
What are hereditary mutations vs Somatic mutations?
Mutations present in the gametes before fertilization. Is present in every single cell and an be passed on through generations
Somatic: Mutations present in body cells. Only present in daughter cells of the mutated cell after mitosis. Not passed on through generations
What are some mutagens of somatic mutations?
Mutagen: an agent that increases the chance of a mutation
Physical agents: X rays, gamma rays, UV light
Chemical agents: Mustard gas, nitrous acid, free radicals, Benzopryene (found in tobacco smoke)
Biological agents: Some virus, food contaminants etc
How do mutagens cause mutations?
- Donate extra groups to the DNA bases
- Cause kinks in the DNA chain
- Change the hydrogen bonding between the two strands
What causes a frameshift?
Caused by an insertion or deletion mutation, is likely to result in change to the amino acid sequence
What is Indel mutation?
Insertion or deletion: one or more nucleotides are inserted or deleted from a length of DNA. May cause frameshift.
What are the 2 main classes of DNA mutation?
- Point mutation
- Insertion or deletion (indel) mutation
What does it mean if the genetic code is degenerate, universal and non-overlapping?
- Degenerate - more than one triplet codes for each amino acid apart from Methionine (start codon)
- Universal - the same triplet codes for the same amino acids in every species
- Non-overlapping - each triplet is distinct from the next
What is sickle cell anaemia?
An example of a disorder caused by a gene mutation
-The disease allele arose from a base substitution mutation - where a single base was changed in the gene sequence
What causes the sickle cell anaemia?
A base substitution mutation → a change to the base sequence of mRNA transcribed from it → a change to the sequence of a polypeptide in haemoglobin
- DNA: The DNA sequence changes from GAG to GTG on the non-transcribed strand (CTC to CAC on the template strand)
- mRNA: The mRNA sequence changes from GAG to GUG at the 6th codon position
- Polypeptide: The sixth amino acid for the beta chain of haemoglobin is changed from glutamic acid to valine (Glu to Val)
Consequence of sick cell anaemia?
The amino acid change (Glu → Val) alters the structure of haemoglobin, causing it to form insoluble fibrous strands
- The insoluble haemoglobin cannot carry oxygen as effectively, causing the individual to feel constantly tired
The formation of fibrous haemoglobin strands changes the shape of the red blood cell to a sickle shape
- Hydrophobic valine binds to adjacent haemoglobin molecules causing the haemoglobin to aggregate (clump together), deforming the red blood cell into a sick cell shape.)
- May form clots within capillaries, block blood supply to vital organs and cause myriad health issues
- sickle cells are also destroyed more rapidly than normal cells, leading to a low red blood cell count (anaemia)
What is the heterozygous advantage with sickle cell anaemia?
If hemoglobin gene is heterozygous, you make both normal haemoglobin and mutated haemoglobin (biconcave RBCs and sickle cell RBS)
- Produce enough RBS to not be affected by anaemia
- Produce enough sickle cells to be resistant to malaria as plasmodium falciparum cannot reproduce inside sickle cells.
What is a 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.)
What are the benefits of comparing genomes?
Can see if two species produce similar proteins, hormones…
Can purify and use (i.e. insulin)
We all share a common ancestor with all living things
Is there a relationship between number of genes and biological complexity?
No
How are the number of genes in a genome usually predicted?
by identifying sequences common to genes, which may include expressed sequence tags (ESTs) or sequences that are homologous to known genes
What presence makes making accurate counts of unique gene numbers very difficult?
A pseudogene is a non-functional sequence of DNA that is homologous to an active gene
- Pseudogenes may be processed (retrotransposed) or non-processed (duplicated)
A transposon is a segment of DNA that inserts itself into another section within the genome (i.e. ‘jumping’ genes)
- Transposition often results in the duplication of the transposable element
- If the duplicated sequence then undergoes random mutation, a pseudogene will result
What does it mean when individuals only possess the sickle cell allele or only the normal blood cell allele?
Individuals who only possess the sickle cell allele will have abnormally shaped red blood cells that are destroyed by the spleen
- This leads to a reduction in red blood cells and a variety of health complications associated with reduced blood cell circulation
Those who only possess the normal blood cell allele do not suffer from sickle cell anaemia but are more susceptible to malaria
- Malaria is caused by an endoparasite (Plasmodium falciparum) which reproduces inside red blood cells (but not sickle cells)
In areas where malaria is common, why is there a higher incidence of people who carry both alleles (are heterozygous)?
These individuals produce enough normal blood cells to avoid the more severe effects associated with sickle cell anaemia, but also produce enough sickle cells to confer an increased resistance to the malarial parasite
- This condition whereby the presence of both alleles is beneficial is known as heterozygous advantage
