Genetics - 3.1 Genes Flashcards
Understandings:
- A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic
- A gene occupies a specific position on a chromosome
- The various specific forms of a gene are alleles
- Alleles differ from each other by one or only a few bases
- New alleles are formed by mutation
- The genome is the whole of the genetic information of an organism
- The entire base sequence of human genes was sequenced in the Human Genome Project
DNA
codes for/determines characteristics of an organism (eg. physical, behavioural, physiological)
Genes =
lengths of DNA that code to make a proteins (ie sequence of DNA that encloses a specific trait)
Chromosoms =
DNA packaged and organisen into
Locus / Loci
= position on a chromosome where a gene for a particular trait is located
Homologous chromosome =
- same gene
- each homologous pair = same size has the centromere in the same place, has the same gene loci in the same sequence and has the same pattern of banding.
Alleles
alternative forms of a gene that codes for the different variations of a specific trait
= possess very similar gene sequences
= only differ by one or two bases
(“homologous pair carry alternative forms of a gene called alleles)
Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes
Mutations (definition, causes, types of mutations, locations)
Mutations = alternations in the DNA of the chromosomes that is not immediately and properly repaired (changed in nucleotide sequence of a sections of DNA for a specific trait)
Mutations may be:
- neutral
- harmful
- Beneficial
Causes:
- randomly/spontaneously
- induced
locations:
- somatic cells (are not inherited)
- Gametic (occur in the cells of the gonads (which produce sperm/eggs) and may be inherited)
neutral - mutations
neutral/silent (ie they have no observable effect on the organism) - no effect on the functioning of the specific feature
harmful - mutations
harmful (detrimental) evident because they may alter the survival capacity of the organism = + truncate the gene sequence to abrogate the normal function of a trait (eg. sickle cell anaemia)
beneficial - mutations
beneficial mutations form the bases for evolution - any mutation that improves survival of an organism will be passed on = change the gene sequence to create new variations of a trait
spontaneous mutations
arise from errors in DNA replication - different genes mutate at different rates
Induced mutations (eg by environmental factors)
induced mutations can be induced by mutagens (environmental factors that cause a change in DNA)
eg.
- radiation
- viruses
- microorganisms
- alcohol / diet
Gene mutation vs chromosome mutations (ie the two main types of mutations)
gene mutation = which is the change in the DNA base sequence
chromosome mutations = which can affect the number of chromosomes
Gene mutations
gene mutations = change the sequence of bases in DNA for a single gene (they may produce a new allele of a gene)
Gene mutations involving a single nucleotide = usually called POINT MUTATIONS
((
1. the new DNA sequence may result in a new sequence of the amino acids constituting a protein
2. Because of the DEGENERACY in the genetic code, not all changes in a DNA sequence will result in a new sequence of amino acids
3. even with a change in amino acid sequence, protein function may be unaffected (the mutation is neutral)
4. A change that forms the ‘stop’ codon would stop production of the polypeptide at the wrong place
5. A change in a ‘stop’ codon would chase the polypeptide to go on and on
6. Adding or taking away a nucleotide causes a READING FRAME SHIFT. They continue to read mRNA nucleotides 3 at a time (frame shift mutations are usually lethal)
))
Define gene mutation
gene mutation is the alteration of a sequence of bases in DNA for a single gene. They can be neutral, harmful or beneficial, spontaneous or induced by environmental factors
sickle cell anaemia
causes = results from a change to the 6th codon for the beta chain of haemoglobin
consequences = normal red blood cells containing normal haemoglobin maximizes oxygen-carrying. Sickle cells (destroyed more rapidly than normal cells = low red blood cell count (anaemia)) containing mutant haemoglobin are less efficient at carrying oxygen
Consequences = the haemoglobin clusters together to form fibre, which deforms the red blood cells into a sickle shape (alters the structure of haemoglobin)
mRNA 0 substitution of a single nucleotide base in the gene coding for the beta chain of haemoglobin which mutates GAG to GUG (at 6th codon position) which changes the amino acid from glutamic acid to valine - this changes the structure of the haemoglobin molecule
->
DNA = changes GAG to GTG on non-transcribed strand
->
polypeptide = 6th amino acid for beta chain of haemoglobin is changed from glutamic acid to valin (glu -> val)
Genome
Genome = the whole of the genetic information of an organism/cell/organelle (including coding and non-coding regions)
= established to sequence the human genome = showed that humans share the majority of their sequence with short nucleotide polymorphisms contributing diversity
->
outcomes: (MORE ON BIONINJA)
1. mapping
2. screening
3. medicine
4. ancestry
gene comparisons
- # genes present in organism = differ between species and is not a valid indicator of biological complaint
- # of genes in a genome is usually predicted by identifying sequences common to genes
- identifying regions may include expressed sequence tags/sequences that are homologous to known genes - presence of pseudogenes and transposons make accurate counts of unique gene #s very difficult
(scientists use diff approaches to prediction gene # = final estimations can vary significantly (eg. online resources used for bioninja = 1. GenBank, 2. Clustal Omega))
John Cairns (1963) = AUTORADIOGRAPHY
- first to determine the length of DNA molecules using a technique called = AUTORADIOGRAPHY
Autoradiography
- Cells are grown in a solution containing radioactive thymidine (tritiated thymidine – 3H-T)
- The tritiated thymidine is incorporated into the chromosomal DNA of the cell (3H-T is used as thymidine is not present in RNA)
- The chromosomes are isolated by gently lysing the cells and fixing the chromosomes to a photographic surface
- The surface is then immersed in a radioactively-sensitive emulsion containing silver bromide (AgBr)
- The radiation released from the tritiated thymidine converts the Ag+ ions in silver bromide into insoluble metal grains
- Following a period of exposure, excess silver bromide is washed away, leaving the silver grains to appear as small black dots
- When the photographic film is developed, the chromosomal DNA can be visualised with an electron microscope
