mutations and meosis Flashcards
What is a genetic mutation and explain why mutations don’t always alter the genetic code
- Gene mutation is a change in the sequence of DNA base pairs that can alter the polypeptide
- Mutations occur continuously
- Most mutations do not alter the polypeptide or only alter it slightly so its structure or function is not changed
- This is because the genetic code is degenerate and the mutations can occur in introns
what are the different ways mutations can occur
- Deletion or insertion
- Substitution
What is deletion
- A nucleotide (therefore bases) is randomly deleted from the DNA sequence
- Changes the amino acid the triplet code would have coded for
- It has a knock-on effect by changing the rest of the triplet codes further on in the sequence
- Known as a frame shift mutation
What is insertion
same as deletion
- A nucleotide is inserted randomly in a DNA sequence
- Changes the amino acid the triplet code codes for
- It has a knock-on effect by changing the rest of the triplet code further on in the sequence
- Known as a frame shift mutation
What is substitution and what are the 3 forms
Substitution is when a base in the DNA sequence is randomly swapped for a different base
- It will only change the amino acid for the triplet code in which the mutation occurs (non-knock on)
3 forms:
- silent
- missense
- nonsense
Describe the 3 forms of substitution mutations
- Silent mutation - Doesn’t alter the amino acid sequence of the polypeptide chain - triplet code is degenerate
- Missense mutation - Mutation alters a single amino acid in the polypeptide chain (e.g. sickle cell)
- Nonsense mutation - Mutation creates a premature stop codon. Causing the polypeptide chain to be incomplete and effects the final protein structure and function.
What is cystic fibrosis caused by
Caused by one substitution mutation in the gene that codes for chloride channel proteins that effects the lungs and the digestive system
What are mutagenic agents
and examples
Environmental factors that increase the mutation rate of cells
e.g. - High radiation such as UV light
- Ionising radiation such as X-rays
- Toxic chemicals such as peroxides
How do mutations occur in chromosomes
By Non-disjunction - When chromosomes fall to separate cells it happens in meiosis
- Gametes may end up with one extra copy or no copies of a particular chromosome
- Mutated gametes will have different numbers of chromosomes compared to normal haploid numbers
- If abnormal gametes are fertilised the diploid cells will have the incorrect number of chromosomes
- Marfan and Turners syndrome occurs by Non-disjunction in the X and Y chromosomes
What are the chromosomes like at the start of meiosis
- Diploid cells have pairs of chromosomes. One maternal and one paternal.
- The chromosomes are said to be homologous
- These are not clones as one comes from each parent
- They pair up in meiosis 1the genes on the chromosomes are alleles of each other and code for the same protein but different versions
How does independent assortment cause variation
- In humans there are 23 pairs of chromosomes that are randomly assorted into each side of the cell during meiosis 1
- This means that alleles on separate chromosomes are independently assorted from each other and different combinations are present in each gamete
- Process is random and 2^23 variety’s of gametes. There can be 70 trillion different possible combinations of fertilised egg cells.
- This explains why no 2 people are the same (except identical twins)
What are the stages of meiosis
Meiosis I -
- Prophase 1
- Metaphase 1
- Anaphase 1
- Telophase 1
Meiosis II -
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
What happens in interphase of meiosis
Chromosomes duplicate to have 2 chromatids
Diploid (2n)
1 chromatid - 2 chromatids
(look up diagram if needed)
What happens in Prophase I in meiosis 1
- Chromosomes condense
- Nuclear envelope breaks down
- Spindle fibres begin to form as centrioles migrate to poles of the cell
Homologues chromosomes pair up and crossing over happens
Diploid cell (2n)
(Look up diagram if needed)
What happens in metaphase I of meiosis I
- Homologus pairs of Chromosomes line up on the cell equator ( in their pairs/ double file unlike single file in mitosis )
- Spindle fibre attaches to the centromere
Independent assortment occurs - Position of each homologues pair is independent of all of all the other chromosomes. The amount of paternal and maternal chromosomes each side of the equator is up to chance
Look up diagram if needed
What happens in Anaphase I of meiosis 1
- Homologous chromosomes are pulled by the spindle fibres to the poles splitting the pair
- This causes genetic variation
- Exchange between non-sister chromatids has occurred during crossover
look up diagram if needed
What happens during Telophase I of meiosis 1
- Nuclear envelope reforms
- Chromosomes uncoil
- Cells undergo cytokinesis
- Cells are now haploid
look up diagram if needed
What happens in prophase II of mitosis 2
- Chromosomes recondense
- Nuclear envelope breaks down again
- Spindle fibres reforms and centrioles move to opposite ends of the cell
look up diagram if needed
What happens in Metaphase II of meiosis 2
- Chromosomes are aligned on the equator by the spindle fibres
- Independent assortment of chromatids
- More genetic variation caused
look up diagram if needed
What happens in Anaphase II of meiosis 2
Same as anaphase in mitosis
- Microfibrils in the spindle fibre are pulled back towards the centrioles
- Causes the 2 sister chromatids to split apart and move to opposite poles of the cell
look up diagram if needed
What happens in Telophase II of meiosis 2
- Chromatids uncoil
- Nuclear envelopes reform
- The cell undergoes cytokinesis
- 4 haploid daughter cells are produced and are all genetically unique
How does meiosis produce cells which are genetically identical
Due to crossing over and independent assortment
What is having genetically different offspring important for
Natural selection
What mechanisms do meiosis have to increase genetic diversity of gametes
Crossing over
Independent assortment
