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