Genetic Variation in Eucaryotes Flashcards
What is the same over both procaryotes and eucaryotes
- Genetic Code
- Transcription
- Translation mechanism
- Basic strategies for DNA replication and repair.
What is the same across species
The Genome
Why do individual members of a species look different if their genome is the same
(Phenotypic Variation)
Difference in gene sequence.
Differences in regulation of gene expression.
Clonal population
Genomes of all members of the species could be the same
Usually procaryotes.
Some examples in eucaryotes e.g. dolly the sheep
In general what are the genomes within a species like
They are slightly different
Genetic changes are the drivers of phenotype variations
What are the types of genetic variation
- Mutations - point mutations, these are usually copying errors during DNA replication
- Homologous recombination ( gene duplication. exon shuffling)
- Mobile genetic elements ( transposons)
- Horizontal gene transfer ( homologous recombination prevalent in procaryotes)
- Sexual reproduction is a major source of genetic reassortment
What happens to genes in sexual reproduction
- ## Reshuffling genes appears to help species survive an ever changing and unpredictable environment
Somatic cell
These are the cells that make up most of our body
They are only diploid –> They have two sets of chromosomes
Germ Cell
Haploid cells
Have a single copy of each gene
These are reproductive cells
Generated by meiosis
Haploid gametes fuse to a diploid cells ( fertilised egg or a zygote
Meiosos
Somatic cells / Progenita germ cells where you have a diploid chromasome.
There is cell division but no DNA replication so daughter cells are produced with a single chromasomal copy.
Then two gametes come together to form a deployed organism
Allele
Variant of a gene
Homologous recombination
- Diploid germ cell precursor (germ cell progenita)
- One round of DNA replication occurs within this germ cell precursor –> Creating twice the usual number of chromosomes in the cell forming a tetraploid cell.
- Next there is pairing of the duplicated homologous chromosomes –> the chromosomes align in parallel with the corresponding genes next to each other (this is called pairing)
- Then homologous recombination occurs –> the chromosomes exchange genes
- This then splits from a tetraploid cell to a diploid cell and then cell division occurs again before DNA replication
- These means in one organism there are 4 different haploid cells within the germ cells –> only one of these will be fertilised
Occasionally mistakes occur during this process e.g. trisomy 21 ( three copies of chromosome 21 handed down from the parents –> causes downs syndrome)
Gene duplication ( a type of homologous recombination)
This arrises from a rare combination of events
- Occurs within organisms no just in the sex cells
The mammalian genome contains enormous amounts of non essential DNA –> Introns
e. g Globin protein
- -> Protein in carries oxygen to the different tissues within a mammalian organism
- -> In humans we have 4 differerent genes that code for globin
- Within the globin gene you have introns and exons –> zoom out for here
- outside the globin gene you have spacers which contain repetitive sequences of which the exact function we don’t understand
- During chromasomal pairing, these globin genes are then alligned before homologous recombination can be carried out. This is the pairing stage of miosis
- Sometimes this pairing is not 100% efficieint –> can have missalighnement during pair formation
- This causes unequal crossing over and means in one of the chromosomes you have two copies of globin genes in one of the chromosomes and the other has done
- This unequal cross over leads to gene duplication –> If the gamete that contains twice the gene is fertilised the individual goes on to have two copies of the globin gene which gives rise to phenotypic variation
This has created 4 different types of globin
Exon Shuffling
caused by homolougous duplication
Getting close into the gene
Between two exons is the interveining sequence – multiple repeats within them
- Missalignment can occur between introns –> caused by chromosomal crossover
- Two of the same exons can then end up in one of the gametes (meaning it have three exons overall) –> This causes changes in normal protein function if the gamete becomes a human
Why does misalignments happen
Because these repeated sequences exists and sometimes the two chromosomal copies can be shifted slightly
What caused the diversity of proteins in humans
It is thought that present day proteins arose from a handful of original exons
Present day proteins contain a patchwork of many different protein domains
What is the percentage makeup of DNA
70% is exon, intron and RNA coding
30% is repeated sequences of DNA
1/3rd of this 30% consists of satellite DNA
2/3rds of this 30% consists of more complex repeated sequences
10% of human DNA consists of 2 families of transposons
- Alu sequences
- L1 ( Line 1)
What is the function of satellite DNA
This has no known function and is found at centromeres and telomeres
Transposons
Mobile genetic elements
Sections of DNA that can randomly move within a chromosome
Encode for a particular protein called transposase –>This allows this section of DNA to cut itself out of the Genome where it is placed and randomly go and integrate itself into another position in the chromosome –> help by the enzyme integrase
It is thought that these were originally viral particles that lost their ability to leave the human genome –> retain their ability to move to different sections of the human chromosome
Transposons
Mobile genetic elements
Sections of DNA that can randomly move within a chromosome
Encode for a particular protein called transposase –>This allows this section of DNA to cut itself out of the Genome where it is placed and randomly go and integrate itself into another position in the chromosome –> help by the enzyme integrase
It is thought that these were originally viral particles that lost their ability to leave the human genome –> retain their ability to move to different sections of the human chromosome
Two examples of transposons
Alu sequence Line 1 (L1) genes
What happens during the random movement of transposons in the human genome
Sometimes separate the regulatory regions of the genes from the protein coding regions
–> This leads to inactivation of certain genes within the genome
Integrate at the junction between control elements and protein coding sequences
Can also integrate themselves into introns or exons –> If exon that particular protein domain no longer exists for that genome–> the protein looses function.
No complimentory sequence –> seemingly random integration
What is the makeup of a transposon
Can be DNA sequences
Can be RNA sequences
How do retrotransposons move
Via an RNA intermediate
What can mediate exon shuffling
Transposons
What does factor 8 mutation lead to
One of the proteins that helps blood clot so when there is a mutation it leads to haemophilia
Viruses
- Highly mobile pieces of DNA/RNA that can escape from cells
- They were first identified as causing disease
- Consist of genes that code for a protein coat
- Often lethal to host cells
- Can be made of DNA or RNA that can be single or double stranded
- The amount of protein that can fit into the protein coat is limited
- Can only reproduce inside a living cell by using the cells biochemical machinery
- Simple RNA viruses must contain genes that encode RNA replicases inside the cell
- The simplest viruses have 3 genes (minimum) –> Can have several hundred
retroviruses
Viruses that have RNA as their core genetic componenet
- Retro refers to the unnatural process where we have RNA as a component for producing DNA. The reverse transfer of information from RNA –> DNA
- Retroviruses can only infect eucaryotic cells
- They are a single stranded RNA virus
- Uses reverse transcriptase to synthesise DNA from an RNA template
- They encode and contain the enzyme
- Makes a DNA/RNA hybrid and then a double stranded DNA
- Integrase inserts DNA into the host genome
- Transcripts are made using the host RNA polymerase
- Proteins are translated using the host ribosomes which forms a new virus
- Often used in drug development–> For HIV drugs is uses reverse transcriptase as a biomarker–> Stops the conversion of RNA into DNA in turn stropping integration into the human genome
What else can this genetic change cause
Not only pheonotipic variation but also sometimes cause diseases such as cancer
Viruses than cause cancer are called tumour viruses
