Meiosis Flashcards
Explain the meiosis cycle
- Zygote
- undergoes mitosis
- Forms a person
- undergoes meiosis
-Forms gametes - Leads to fertilisation
Why is sexual reproduction an advantage
- Related to genetic diversity
- Success shows the value
- Increases adaptations within population as genetic diversity is increased
Explain genetic diversity
- The higher the genetic diversity within a species the greater the chance of survival if the environment changes for any reason
What happens if all the species are genetically identical
- And the environment changes, none of the species may be adapted to those changes and all may die
- Genetically identical species are able to live under warm temperatures
- The environment changes and is now significantly colder – none of the individuals can survive – the species goes extinct.
What happens if individuals are genetically different
- Some individuals may be able to cope with changes in the environment while others may die.
- This means the species may experience some death but some individuals may be able to cope and reproduce under new environmental conditions
What leads to low genetic diversity
- Small populations
- Which means high inbreeding level
What type of reproduction leads to an identical population
- Asexual reproduction
- A single change in the environment can wipe out the population
What type of reproduction leads to a diverse population
- Sexual reproduction
- Boots chance that at least some members of the population will survive a changing environment
What is the outcome of meiosis
- Gametes
- each gamete contains one gene for each characteristic from the parent and contain half the number of chromosomes
Explain the expression of a specific trait
- Is determined by the two units of information received from the parents; one unit from each parent.
- We refer to each unit as an allele – alternate forms of a gene
- When two gametes combine (fertilization) the resulting zygote again contains two alleles for each characteristic
Explain homologous sets of chromosomes
- A sexually reproducing organism consists of diploid cells (2n) which contain two full sets of chromosomes, one set inherited from each parent
- Of the 23 pairs (46) of chromosome pairs in a human cell,
- 22 pairs are autosomes or chromosomes that are the same for both sexes
- A homologous pair of chromosomes is a matching pair of chromosomes that look alike and have the same sequence of genes
- Homologous chromosomes, however, are not identical and the two homologs differ in the combination of alleles or versions of the genes they carry
Explain meiosis being essential in sexual reproduction
- In the case of sexual reproduction, the chromosome number of each gamete must be half (haploid) that of the parent cell (diploid) in order for the zygote to be diploid again
- If this did not happen then each subsequent generation would have had double the chromosome number of the immediate parents
Explain meiosis in a nutshell
- In a nutshell we can say that meiosis involves three steps in terms of chromosome number, i.e.
- Doubling – DNA replication – sister chromatids
- 46 duplicated chromosomes (92 chromatids) - Halving – splitting of homologous chromosomes
- 23 duplicated chromosomes (46 chromatids) - Halving again – splitting of sister chromatids
23 chromosomes/chromatids
What happens in interphase
- Duplication of chromosomes takes place just before prophase I, and each duplicated chromosome (sister chromatids) remains attached at the centromere
What happens in prophase 1
- As in prophase of mitosis, similar behavior of nuclear membrane, nucleolus, centriole and spindle. Duplicated chromosomes are visible as condensed threads, and each duplicated form draws close to its homologue through process of synapsis.
- Crossing-over occurs between non-sister chromatids and results in exchange of genetic material - genetic variability is increased
What happens in metaphase 1
- Here we have the alignment of chromosomes at the spindle equator, also known as the metaphase plate. The spindle fibers are fully formed and will be attached to the kinetochores of the chromosomes.
- Random alignment of paternal and maternal chromosomes results in random distribution of paternal and maternal traits in offspring, i.e. the paternal and maternal chromosomes of the bivalents can be on either side of the metaphase plate resulting in a mixture of paternal and maternal chromosomes going to each daughter cell
What happens in anaphase 1
- The contraction of the spindle fibers leads to the separation of the homologous chromosome-pairs with attached sister-chromatids being pulled towards the poles.
- Thus, in humans, each pole will receive 23 duplicated chromosomes
What happens in telophase 1
Cytokinesis leads to the formation of two daughter cells, each with 23 duplicated chromosomes (46 chromatids)
What is interkinesis
A new spindle apparatus is formed
What happens in prophase 2
Duplicated chromosomes (sister chromatids) attach to the spindle fibers
What happens in metaphase 2
Duplicated chromosomes are arranged at the metaphase plate and this is assisted by the movement of the spindle fibers
What happens in anaphase 2
- Contraction of spindle fibers leads to separation of sister chromatids, now referred to as daughter chromosomes, and their movement to opposite poles, i.e. 23 daughter chromosomes move to each pole
What happens in telophase 2
Four daughter nuclei are formed and after cytokinesis with each cell containing half the number of chromosomes, i.e. 23
What does meiosis 1 and 2 do
- Meiosis 1 = separates maternal from paternal chromosomes randomly
- Meiosis 2 = separates sister chromatids
When does genetic variation takes place
Prophase 1 - crossing over
Metaphase 1 - line up in double file ( combo of maternal and paternal )
Explain crossing over fully
- Crossing over - DNA recombination — a reshuffling of bits of the parents’ chromosomes in the cells that give rise to eggs and sperm — creates new genetic combinations, half of which each parent passes to a child.
Synapsis – process of alignment
Chiasmata – points of exchange - Siblings will share about 50 percent of their DNA.
- The recombination means children don’t inherit the exact same mix from their parents (unless the kids are identical twins).
Explain twins fully
- Identical twins result from a single fertilization event
- The resulting zygote or embryo splits in half, creating separate, identical babies. They are therefore called monozygotic
- In contrast non-identical (fraternal) twins occur when two sperm cells fertilize two separate egg cells and they are called dizygotic
