Genetics - 10.1 Meiosis (HL) Flashcards
Understandings:
- Chromosomes replicate in interphase before meiosis
- Homologous chromosomes separate in meiosis I
- Sister chromatids separate in meiosis II
- Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I
- Chiasmata formation between non-sister chromatids can result in an exchange of alleles
- Crossing over is the exchange of DNA material between non-sister homologous chromatids
- Crossing over produces new combinations of alleles on the chromosomes of the haploid cells
Interphase:
Interphase is an active period that precedes meiosis and involves key events needed to prepare the cell for successful division
DNA is replicated during the S phase of interphase, resulting in chromosomes that contain two identical DNA strands
-These genetically identical strands are called sister chromatids and are held together by a central region called the centromere
- These chromatids separate during meiosis II, becoming independent chromosomes each made of a single DNA strand
If DNA replication did not occur prior to meiosis there would be no need for a 2nd meiotic division (meiosis I = diploid → haploid)
- The fact that DNA replication does occur suggests that meiosis evolved from mitosis (where initial DNA replication is necessary)
- One benefit of the duplication of chromatids is that it increases the potential for genetic recombination to occur (more variation)
Meiosis - process (bioninja) summary + in 3.3
Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase)
Meiosis is preceded by interphase, in which DNA is replicated to produce chromosomes consisting of two sister chromatids
A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage
Meiosis I
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
Meiosis II
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
The final outcome of meiosis is the production of four haploid daughter cells
These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)
Random Assortment
(basic information covered in SL content (pg 23) + 3.3 flashcards)
Independent assortment describes how pairs of alleles separate independently from one another during gamete formation
- According to independent assortment, the inheritance of one gene/trait is independent to the inheritance of any other gene/trait
- Independent assortment is due to the random orientation of pairs of homologous chromosomes in meiosis I
The orientation of each homologous pair is random and is not affected by the orientation of any other homologous pair
- This means an allele on one chromosome has an equal chance of being paired with, or separated from, any allele on another chromosome (their inheritance is independent of one another)
Independent assortment will not occur if two genes are located on the same chromosome (linked genes)
Chiasmata - synapsis
During prophase I of meiosis, homologous chromosomes become connected in a process known as synapsis
= The connected homologues are known as a bivalent (bi = two chromosomes) or a tetrad (tetra = four chromatids)
= The chromosomes are connected by a protein-RNA complex called the synaptonemal complex
- While autosomes always undergo synapsis during meiosis, sex chromosomes often remain unpaired
Chiasmata =
While in synapsis, non-sister chromatids may break and recombine with their homologous partner (crossing over)
These non-sister chromatids remain physically connected at these points of exchange – regions called chiasmata
Chiasmata (singular = chiasma) hold the homologous chromosomes together as a bivalent until anaphase I
Chiasmata formation between non-sister chromatids can result in the exchange of alleles
Chiasmata are X-shaped points of attachment between two non-sister chromatids of a homologous pair
Chiasmata form as a result of crossing over and hence non-sister chromatids should show an exchange of genetic material
The exchange of genetic material is most easily shown if homologous chromosomes are differentially colour-coded
“crossing over”
When chiasmata form between bivalents in prophase I = DNA can be exchanged between non-sister homologous chromatids
- This exchange of genetic material is called crossing over and produces new allele combinations on the chromosomes
= These chromosomes that consist of genetic material from both homologues are called recombinant chromosomes
= results = in new combinations of alleles in haploid cells and thus increases the genetic diversity of potential offspring
