genetic diversity: mutations and meiosis Flashcards
what is a gene mutation
change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
how often do mutations occur
continuously
why do mutations effect what protein is made
the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to change in the polypeptide that the gene codes for
do most mutations alter polypeptide
no or only alter slightly so structure or function is unchanged
why do most mutations not alter the polypeptide chain
the genetic code if degenerate
what happens in the insertion of nucleotides mutation
a mutation occurs when a nucleotide (new base) is randomly inserted into the DNA sequence
why does insertion mutation effect the amino acid
-changes the amino acid that would have been coded for by the original triplet base and creates a new triplet of bases
-this has a knock on effect by changing the triplets further on in the DNA sequence
what type of mutation is an insertion of nucleotides mutation known as
frameshift mutation
insertion if nucleotides mutation effect on polypeptide
may dramatically change the amino acid sequence from this gene and therefore ability of the polypeptide to function
what is a deletion of nucleotides mutation
mutation that occurs when a nucleotide (and therefore base) is randomly deleted from the DNA sequence
deletion of nucleotides mutation effect on amino acids coded for
-a deletion mutation changes the amino acid that would have been coded for
-has a knock on effect by changing the groups of 3 bases further on in the DNA sequence
what type of mutation is a deletion mutation
frameshift
deletion mutation effect on polypeptide chain
dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function
what is a substitution of nucleotides mutation
mutation that occurs when a base in the DNA sequence is randomly swapped for a different base
what amino acids does substitution of nucleotides mutation effect
only changes the amino acid coded for by the triplet in which the mutation occurs, it will not have a knock on effect
what 3 forms can substitution mutations take
-silent mutations
-missense mutations
-nonsense mutations
silent mutation
the mutation does not alter the amino acid sequence of the polypeptide (because of degenerate code)
missense mutations
the mutation alters a single amino acid in the polypeptide chain
nonsense mutations
the mutation creates a premature stop codon causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function
effect of most mutations on polypeptide
-most mutations do not alter the polypeptide or only alter it slightly so appearance or function is not changed
effect of polypeptide when shape is changed
-a small number of mutations code for a significantly altered polypeptide with a different shape
-this may affect the ability of the active site on an enzyme changes the substrate may no longer be able to bind to the active site
-a structural protein may loose its strength if it changes shape
what are mutagenic agents
environmental factors that increase the mutation rate of cells
examples of mutagenic agents
-high-energy radiation
-ionising radiation such as X rays
-toxic chemicals such as peroxides
natural mechanisms in cells
there are natural mechanisms that take place in cells to ensure accuracy of DNA replication, these involve proofreading and repairing damaged DNA
what has happened when mutation rate of cells rises
when mutation rate of cells rises to above normal rate these mechanisms have become ineffective
Non-disjunction
occurs when chromosomes fail to separate during meiosis
when does Non-disjunction occur
spontaneously
how may gametes chromosome number be abnormal
gametes may end up with one extra copy of a particular chromosome or no copies of a particular chromosome
what will gametes with abnormal chromosomes have different
different haploid number compared to the normal
what happens if abnormal gametes take part in fertilisation
a chromosome mutation occurs as the diploid cell will have the incorrect number of chromosomes
what do chromosome mutations change
the number of chromosomes
example of a chromosome mutation
Down’s syndrome
chromosome mutation in Down’s syndrome
total of 47 chromosomes in genome as have 3 copies of chromosome 21
what does meiosis produce
daughter cells that are genetically different from each other and the parent cell
why are cells produced by meiosis genetically different to each other
-independent assortement
-crossing over
what is independent assortment
-alleles of two or more different genes get sorted into gametes independently from one another
-the allele a gamete received for one gene does not influence the allele received for another gene
-this is because homologous chromosomes line up in random orientations at the middle of cell at metaphase as they prepare to separate, meaning the same parent cell can produce different combinations of chromosomes in daughter cells
does alleles received influence independent assortment
the allele received for one gene does not influence the allele received for another gene
how does chromosomes lining up effect independent assortment
homologous chromosomes line up in random orientations at the middle of the cell at metapahse as they prepare to separate, meaning that the same parent cell can produce different combinations of chromosomes in daughter cells
what is crossing over
process whereby a chromatid breaks during meiosis and rejoins the chromatid of its homologous chromosome so that its allele are exchanged
what does meosis produce (haploid or diploid)
haploid cells from diploid cells
what type of cell does meiosis produce
gametes in plants and animals that are used for sexual reproduction
how many divisions does meiosis have
2
phases of meiosis
-prophase 1
-metapahse 1
-anaphase 1
-telophase 1
-cytokinesis
-prophase 2
-metaphase 2
-anaphase 2
-telophase 2
-Cytokinesis
what happens in prophase 1
-DNA condenses and becomes visible as chromosomes
-DNA replication has already occurred so each chromosome consists of 2 sister chromatids joined together by a centromemere
-the chromosomes are arranged side by side in homologous pairs
-a pair of homologous chromosomes are called a bivalent
-as the homologous chromosomes are very close together the crossing over non-sister chromatids may occurs. the point at which the crossing over occurs is called the chiasma
-in this stage centrioles migrate to opposite poles and spindle if formed
-the nuclear envelope breaks down and nucleotides disintegrates
chromosomes in prophase 1
condenses and becomes visible as chromosomes
once DNA replication has occurred what are the chromosomes consisted of
two sister chromatids joined together by a centromemer
how are chromosomes arranged in prophase 1
side by side in a homologous pair called a bivalent
when does crossing over occur
prophase 1
what is the point at which crossing over occurs
chiasma
spindle and centrioles in prophase 1
centrioles migrate to opposite poles and spindle is formed
nuclear envelope propahse 1
breaks down and disintegrates