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
what happens in metaphase 1
bivalents line up along the equator of the spindle, with spindle fibers attached to the centromeres
what happens in anaphase 1
-homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
-the centromeres do not divide
telophase 1
-the chromosomes arrive at opposite poles
-spindle fibers start to break down
-nuclear envelopes form around the two groups of chromosomes and nucleoli reform
-some plant cells go straight into meiosis 2 without reformation of the nucleus in telophase 1
telophase 1 chromosomes
arrive at opposite poles
telophase 1 spindle
spindle fibers start to break down
telophase 1 nucleus
nuclear envelopes form around the 2 groups of chromosomes and nucleoli reform
some plant cells in telophase 1
go straight into meiosis 2 without reformation of nucleus in telophase 1
cytokinesis meiosis 1
-division of cytoplasm occurs
-cell organelles also get distributed between 2 developing cells
-in animal cells the cell surface membrane pinches inwards creating a cleavage furrow in the middle of cell which contracts, dividing cytoplasm in half
-in plant cells, vesicles from golgi apparatus gather along the equator of the spindle. the vesicles merge with each other and form the new cell surface membrane and also secrets a layer of calcium pectate which becomes the middle lamella layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
-the end product of cytokinesis in meiosis 1 is 2 haploid cells as they contain half the number of centromeres
cytokinesis of animal cells meiosis 1
the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
cytokinesis in plant cells meiosis 1
vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
end product of cytokinesis meiosis 1
2 haploid cells
why are the end products of cytokinesis haploid
contain half the number of centromeres
meiosis 2
-no interphase between meiosis 1 and 2 so DNA is not replicated
-second division is almost identical to stages of mitosis
prophase 2
-nuclear envelope breaks down and chromosomes condense
-a spindle forms at a right angle to the old one
metaphase 2
-chromosomes line up in a single file along the equator of the spindle
anaphase 2
-centromeres divide and individual chromatids are pulled to opposite poles
-this creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell
telophase 2
-nuclear membranes form around each group of chromosomes
cytokinesis meiosis 2
-cytoplasm divides as a new cell surface membranes are formed creating 4 haploid cells
-the cells still contain the same number of centromeres as the did at the start of meiosis 1 but now have half number of chromosomes (previously chromatids)
causes of genetic variation due to meiosis
-crossing over
-independent assortment
-different combinations of chromosomes following meiosis
effect of genetic variation for offspring
genetically different offspring can be advantageous for natural selection
what do independent assortment and crossing over result in
different combinations of alleles in gametes
what is crossing over
process by with non-sister chromatids exchange alleles
outline process of crossing over
-during meiosis 1 homologous chromosomes pair up and are in very close proximity to each other
-the non-sister chromatids can cross over and get entangled
-these crossing points are called chiasmata
-the entanglement places stress on the DNA molecules
- as a result of this section of chromatid from one chromosme may break and rejoin the chromatid from the other chromosome
why is swapping of alleles significant
results in new combination of alleles on the two chromosomes
number of chiasmata present
usually at least one or more present in each bivalent during meiosis
where is crossing over more likely to happen
-further down the chromosome away from the centromere
what is independent assortment
production of different combinations of alleles in daughter cells due to random alignment of homologous pairs along the equator of the spindle during metaphase 1
what does different combinations of chromosomes in daughter cells increase
genetic variation between gametes
arrangement of chromosomes in phrophase 1
-chromosomes pair up and in metaphase 1 they are pulled to opposite ends of the spindle
-each pair can be e arranged with either chromosome on top, this is completely random
-the orientation of one homologous pair is independent / unaffected by the orientation of any other pair
chromosomes getting pulled apart leading to independence assortment
-homologous chromosomes are then separated and pulled apart to different poles
-the combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
calculating the different combinations of chromosomes following meiosis
The number of possible chromosomal combinations resulting from meiosis is equal to 2 to the power of n (n is the number of homologous chromosome pairs)
different number of combinations following meiosis humans
-diploid number is 46 then the haploid number is 23 so the calculation is 2 to the power of 23 = 8 388 608 possible chromosomal combinations
how does meiosis create genetic variation in gametes
-crossing over
-independent assortment
what does crossing over and independent assortment mean for alleles in gametes
each gamete carries substantially different alleles
how does random fertilisation cause genetic variation
-during fertilization any male gamete and fuse with any female gamete to form a zygote
-this random fusion of gametes at fertilisation creates genetic variation between zygotes as each will have a unique number of alleles
-there is almost no chance of individual organisms resulting from successive sexual reproduction being genetically identical
calculating different combinations of chromosomes following fertilisation
-in random fertilsation any two gametes combine
-Therefore the formula to calculate the number of combinations of chromosomes after the random fertilisation of two gametes is (2 to the power of n)squared
(n is the haploid number and squared is the number of gametes)
calculating different combinations of chromosomes following fertilisation in humans
Therefore in humans, when the haploid number is 23, the number of combinations following fertilisation is (2 to the power of 23) squared = 70368744177664
what does the calculation of combinations of chromosomes following random fertilisation explain
why relatives can differ so much from each other, even with the same parents, individuals can be genetically distinct due to variation at the meiosis and fertilisation stage (as well as other possible mutations and crossing over)
what do optical microscopes allow to be seen
tissues, cells and organelles
how can movement of chromosomes during mitosis be observed
using a microscope
how do optical microscopes work
-light is directed through the thin layer of biological material that is supported on a glass slide
-this light is focused through several lenses so that an image is invisible through the eyepiece
-the magnifying power of the microscope can be increased by rotating the higher power objective lens into place
key components of an optical microscope
-eyepiece lens
-objective lens
-stage
-light source
-coarse and fine focus
meiosis under a microscope - method for preparing a slide using a liquid specimen
-add a few drops of the sample to the slide using a pipette
-cover the liquid/smear with a coverslip and gently press down to remove air bubbles
-wear gloves to ensure there is no cross-contamination of foreign cells
meiosis under a microscope - method for a solid specimen
-use scissors to cut a small sample of the tissue
-peel away or cut a very thin layer of cells from the tissue sample to be placed in the slide (using a scalpel or forceps)
-some tissue samples need the be treated with chemicals to kill/make the tissue rigid
-gently place a coverslip on top and press down to remove any air bubbles
-a stain may be required to make the structures visible depending on the type of tissue being examined
-take care when using sharp objects and wear gloves to prevent the stain form dying skin
meiosis under a microscope - method for why do you, when using an optical microscope always start with the lower power objective lens
-it is easier to find what you are looking for in the field of view
-this helps prevent damage to the lens or coverslip incase the stage has been raised too high
meiosis under a microscope - method for unclear or blurry images
-switch to the lower power objective lens and try using the coarse focus to get a clearer image
-consider whether the specimen sample if thin enough for light to pass through to see the structures clearly
-there could be cross-contamination with foreign cells or bodies
meiosis under a microscope - method for preventing dehydration of tissue
-the thin layers of material placed in slides can dry up rapidly
-adding a drop of water to the specimen (beneath the coverslip) can prevent cells form being damaged by dehydration
meiosis under a microscope - method for using a graticule to take measurements of cells
- a graticule is a small disc that has an engraved ruler
-it can be placed into the eyepiece of a microscope to act as a ruler in the field of view
-as a graticule has no fixed units it must be calibrated for the objective lens that is in use. this is done by using a scale engraved on a microscope (stage micrometer)
-by using the two scales together the number of micrometers each graticule unit is worth can be worked out
-after this known the graticule can be used as a ruler in the field of view
limitations of meiosis under a microscope
-the size of cells or structures of tissues my appear inconsistent in different specimen slides
–> cell structures are 3D and the different tissue samples will have been cut at different planes resulting in inconsistencies when viewed on a 2D slide
-optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen
how can cells undergoing meiosis be observed and photographed
using specialised microscopes
how can each stage of meiosis be identified
each stage has distinctive characteristics so can be identified from photomicrograhps or diagrams
identifying meiosis 1 or 2
-homologous chromosomes line up side by side in meiosis 1 only
-this means if there are pairs of chromosomes in a diagram or photomicrograph meiosis 1 must be occurring
-the number of cells forming can help distinguish between meiosis 1 and 2
-if there a 2 new cells forming it is meiosis 1 but if there are 4 new cells forming it is meiosis 2
The distinguishing features at prophase 1 (Meiosis I)
homologous pairs of chromosomes are visible
The distinguishing features at metaphase1 (Meiosis I)
homologous chromosomes are lined up side by side along the equator of the spindle
The distinguishing features at anaphase 1 (Meiosis I)
whole chromosomes are being pulled to opposite poles with the centromeres intact
The distinguishing features at telophase 1 (Meiosis I)
there are 2 groups if condensed chromosomes around which the nuclei membranes are forming
The distinguishing features at cytokinesis (Meiosis I)
cytoplasm is dividing and cell membrane is pinching inwards to form 2 cells
The distinguishing features at prophase 2 (Meiosis II)
single whole chromosomes are visible
The distinguishing features at metaphase 2 (Meiosis II)
single whole chromosomes are lined up along the equator of the spindle in a single file (at 90 degree angle to the old spindle)
The distinguishing features at anaphase 2 (Meiosis II)
centromeres divide and chromatids are being pulled to opposite poles
The distinguishing features at telophase 2 (Meiosis II)
nuclei are forming around the 4 groups of condensed chromosomes
The distinguishing features at cytokinesis (Meiosis II)
cytoplasm is dividing and 4 haploid cells are forming
2 forms of cell division
-meiosis
-mitosis
what does mitosis contribute to
the growth of an organism or to replace dead/dying cells
what does meiosis produce
genetically different gametes for sexual reproduction
number of daughter cells meiosis
4
number of daughter cells mitosis
2
Ploidy of daughter cells mitosis
2n
Ploidy of daughter cells meiosis
n
Are the daughter cells genetically identical to the parent cells and each other? mitosis
yes
Are the daughter cells genetically identical to the parent cells and each other? meiosis
no
what changes throughout mitotic and meiotic divisions
chromosome content of a cell
mitosis produces two genetically identical daughter cells which is important because
-so that growth and cell replacement can occur within body continually
-every cell in an organism’s body (other than gametes) contain exactly the same genetic material - the full genome
why is important that meiosis produces 4 daughter cells that contain half the genetic material of the parent cell and are all different from each other and the parent
-this is important for genetic variation within families and the population
-genetic variation can reduce the risk of inheriting genetic diseases
what does random fertilation of gametes increase
genetic variation
meiosis creates genetic variation
meiosis creates genetic variation between gametes produced by an individual through crossing over and independent assortment
-this means each gamete carries substantially different alleles
random fertilisation
-during fertilisation, any male gamete can fuse with any female gamete to form a zygote
-this random fusion of gametes at fertilisation creates genetic variation between zygotes as each will have a unique combination of alleles
-zygotes eventually grow and develop into adults
-the presence of genetically diverse zygotes contributes to the genetic diversity of a species
