Meiosis and Genetic Variation Flashcards
What are the gametes foud in males?
Sperm cells
What are gametes found in females?
Egg cells
Why do gametes join together at fertilisation?
To form a zygote- this rapidly divides and develops into a new organism
What are diploid ?
Normal body cells having (2n) - cell contains two of each chromosome one from mother and father
What does it mean that gametes have a hapolid number?
(n) number of chromosomes
Only one copy of chromosome
What specifically happens at fertilisation?
Haploid sperm fuses with haploid egg
making a cell with normal diploid number of chromosomes
Half of chromosomes are from father (sperm) and half are from mother (egg)
How does gamete produce genetic diversity?
Fertilisation is random - any sperm can fertilise an egg
Random fertilisation produce a zygote with a different combination of chromosomes to both parents
Mixing of genetic material in sexual reproduction
Diagram of fertilisation
Where does meiosis take place?
Reproductive organs
What happens before Meiosis?
(Interphase I)
The DNA unravels and replicates
Two copies of chromosomes called chromatids - two centromeres
Nuclear membrane is present
Nucleolus is present
What happens in Meiosis 1- Early Prophase?
DNA condenses and becomes visible - double armed chromosomes each from two sister chromatids
Sister chromatids are joined together at the middle by a centromere
Centrioles begin to move
Nucleolus disappears
What happens in Meiosis 1 - Mid-prophase
- Homologous chromosomes pair up and lie along side each other (SYNAPSIS)
- Each pair claled a BIVALENT
- Centrioles move further towards poles
What happens in Meiosis 1 - Late Prophase 1
Chromatids tangle at a point called CHASMATA)
Chromosomes may break and rejoin
Crossing OVER occurs
Reformed chromatids called RECOMBINANTS
What happens in Meiosis 1 - Metaphase?
Nuclear membrane breaks down
Spindle forms
Bivalent moves to the equator and attach to the spindle by the centromere
What happens in Meosis 1 - Anaphase 1?
Contraction of spindle fibres pulls the whole chromosome towards opposite poles
Homologous chromosomes separate
What happens in meosis 1- telophase?
Nuclear membrane reforms around the set of chromosomes
Spindle fibres disappear
Centromere divides into two
What happens in cytokinesis in meiosis 1?
Division of cytoplasm
short phase
What happens in meiosis 2 -Prophase 2?
Centrioles move towards opposite poles
What happens to meiosis 2-Metaphase2?
Nuclear membranes disappear
Spindle forms
Chromosomes move to the equator and attach to the spindle fibres by the centromere
What happens to the meiosis 2- Anaphase 2?
Centromeres divide
Sister chromatids move towards opposite poles
What happens in meiosis 2-telophase2?
Each chromatid now called chromosomes
Nuclear membrane reforms around the group of chromosomes
Spindle fibres disappear and centrioles may divide
What happens to the cytokinesis in meiosis 2?
Division of cytoplasm
Four haploid daughter cells
Each cell is genetically differeny
What are homologous pairs?
Each pair is same size
Have different alleles
What are the two types of events that cause genetic variation in meiosis?
- Crossing over of chromatids
- Independent segregation of chromosomes
What does crossing of chromatids mean?
Crossing over of chromatids in meiosis 1 means that each of the four daughter cells formed from meiosis contains chromatids with different alleles
What happens in the independent segregation of chromosomes?
- Each homologous pair of chromosomes in cells is made up of one chromosome of mum and one from dad
- When homologous pairs are separated in meiosis 1 , completely random which chromosome from each pair ends up in daughter cell
- Four daughter cells produced by meiosis have completely different combination of maternal and paternal chromosomes
- Shuffling of chromosome lead to genetic variation in potential offspring
What is outcome of mitosis?
Produce cells with the same number of chromosomes as parent cells
Outcome of meiosis
Produces cell with half the number of chromosomes as parent cell
What is bivalent?
Pair of homologous chromosomes
What is chiasmata?
paired chromosomes remain in contact during the first metaphase of meiosis,
and at which crossing over and exchange of genetic material occur between the strands.
Give two differences between the nuclei removed from the embryo cells and the nuclei discarded from the unfertilised egg cells.
embryo cell diploid, egg cell haploid;
contain different alleles/forms of the colour gene;
Explain why meiosis is important in sexual reproduction , apart from producing gametes that are genetically different
produces haploid cells / chromosome number halved;
fertilisation;
maintains the diploid / chromosome number (in next generation
Most plants produce seeds after fertilisation in sexual reproduction. However, dandelions produce small, windblown seeds without fertilisation taking place.
Suggest two advantages to the dandelion of being able to reproduce from these seeds, as well as from pieces of root.
dispersal / prevent overcrowding / competition / colonise ;
increased number of (proven) offspring; (not quicker
If embryos with more than eight cells are split up, the separated cells fail to develop into new embryos. Suggest why.
cells lost ability to control development / no longer totipotent /
cells have differentiated/become specialised;
An egg cell from a cow was fertilised in a laboratory and allowed to develop into an eight-celled embryo. This was split into four parts, each of which developed into a new embryo. This is shown in the diagram. The new embryos were later transferred into different surrogate cows.
Give two advantages to a farmer of using embryos produced by this procedure.
offspring with favourable characteristics / high meat/milk yield;
pedigree embryos into non-pedigree mothers / not risking pedigree
mothers / rare breeds
conserved;
sex/gender selection;
What is the number that all daughter cells end up with meiosis?
23 chromosomes
What happens with mutations with meiosis?
Produce contain variations in the number of whole chromosomes or part of chromosomes - chromosome mutations
What do chromosome mutations lead to?
Errors caused by meiosis and inherited conditions - error in gamete (hereditary cells)
What chromosome mutation lead to Down Syndrone?
Non-disjunction - failure of chromosome to separate properly
chromosome 21 can lead to syndrome
What leads to Down’s syndrone?
- Person having an extra copy of chromosome 21 /extra part
- Non-disjunction mean that chromosome 21 fail to separate properly during meiosis , so gets cell an extra copy
- When gamete with extra copy fuses with another gamete at fertilisation , resulting zygote will have three copies of chromosome 21
