Topic 10 Flashcards
When do chromosomes replicate?
During interphase
What phase of interphase does DNA replication occur?
S phase
What is produced from DNA replication?
Two sister chromatids that are held together by a centromere
Which meiosis do sister chromatids separate?
Meiosis 2
Which meiosis do sister chromatids separate?
Meiosis 2
Outline the stages of meiosis
P1 M1 A1 T1 P2 M2 A2 T2
What happens during prophase 1?
Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
What happens during metapahse 1?
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
What happens during Anaphase 1?
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
What happens during Teleophase 1?
Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
What happens during Prophase 2?
Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
What happens during Metaphase 2?
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
What happens during Anaphase 2?
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
What happens during teleophase 2?
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
Why is there an independent assortment of genes?
Due to random orientation of pairs of homologus chromosomes
What is the orientation of each pair of chromosome?
The orientation of each homologous pair is random and is not affected by the orientation of any other homologous pair
When will independent assortment not occur?
When genes are located on the same chromosome ( linked genes)
What chromatids do a chiasmata form between?
Non sister chromatids
Synapsis
- During prophase I of meiosis, homologous chromosomes become connected in a process known as synapsis
- The connected homologues are known as a bivalent
- 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
Draw a diagram to show chiasmata formation
What does crossing over produce?
-Crossing over produces new combinations of alleles on the chromosomes of the haploid cells
What is crossing over?
The exchange of DNA between non sister chromatids
What are recombinant chromosomes
The chromosomes that consist of genetic material of both homologues
What are unlinked genes
pairs of alleles are inherited independently of one another if their gene loci are on separate chromosomes
What does the independent segregation of unlinked genes lead to?
greater number of potential gamete combinations, as well as a greater variety of possible phenotypes
What method can you use to work out the gamete combination in a dihybrid cross?
FOIL method
What is a gene pool?
All the genes and alleles within an interbreeding population
What does evolution require?
There is a change in allele frequency with time in populations
What can cause the allele frequency within a gene pool to change?
Mutation: A random change in the genetic composition of an organism due to changes in the DNA base sequence
Gene flow: The movement of alleles into, or out of, a population as a result of immigration or emigration
Sexual reproduction: Sex can introduce new gene combinations and alter allele frequencies
Genetic drift: The change in the composition of a gene pool as a result of a random event
Natural selection: The change in the composition of a gene pool as a result of differentially selective environmental pressures
Allele frequency comparisons
- Allele frequencies represent the prevalence of a particular allele in a population
- Consequently, allele frequencies are either represented as a percentage or as a value from 0 to 1.0
- Changes in allele frequency can reflect either random processes (genetic drift) or differential processes (natural selection
What are the three types of selection?
- Stabilising
- Disruptive
- Directional
