asexual reproduction yields two identical organisms (with exception of mutations) sexual...
TRANSCRIPT
Asexual reproduction yields two identical organisms (with exception of mutations)
Sexual reproduction merges the genes of the two parents
Result: two versions of each chromosome◦ Called homologous chromosomes
Organism is diploid (2n=46) All somatic cells are diploid
Creation of Gametes
First 22 are autosomes Chromosome 23 is the sex chromosome Haploid (n=23) sex cells merge to create a
zygote in sexual reproduction Meiosis creates these sex cells (gametes) The overall process is the sexual life cycle
Homologous chromosomes
Chromosomes replicate
Meiosis I
Meiosis II
Homologous chromosomes separate
Sister chromatids separate
Interphase I of Meiosis
Similar to mitosis, chromatin replicates Centrioles replicate, proteins form around to
make the centrosomes◦ Also called the microtubule organizing centre◦ Collectively, with microtubules, is called mitotic
spindle
Interphase I
Longest part of meiosis Chromatin condenses, chromosomes
become visible◦ Homologous chromosomes come pair up, forming
a tetrad – called synapsis◦ The chromatids can overlap, called a chiasmata
All other features are similar to mitosis
Prophase I
Homologous chromosomes are at opposite poles of cell◦ Each end is now haploid
Cytokinesis takes place simultaneously Some cells proceed to interphase II, others
directly to prophase II No cells replicate chromatin again
Telophase I
Some of the processes can go awry If anaphase I or II does not proceed
correctly, the chromosomes may not be shared evenly
This is called nondisjunction More of a problem if it occurs during meiosis
I Can take place in mitosis, but not as much
of a concern
Errors
An abnormal chromosome number is called aneuploidy
If faulty gamete succeeds in fertilization, the offspring with have health consequences
If a gamate with an extra chromosome fertilizes, the zygote is trisomic (2n+1)
If the gamete has a chromosome missing, the zygote is monosomic (2n-1)
Ex. Extra chromosome 21 – Down syndrome
If the extra chromosome is an X in a male (XXY), called Klinefelter syndrome
The individual will have male sex organs, but the testes will be abnormally small, and he will be sterile
As well, femine characteristics such as breast enlargement is common
Final example is Turner syndrome Example of monosomy X Individuals are classified X0, and are female However, sex organs do not mature at
adolescence and secondary sex characteristics fail to develop
Also possible for anaphase I or II to completely fail and all chromosomes move to one cell
Referred to as polyploidy Could have three chromosome sets
(triploidy, 3n) or four (tetraploidy, 4n)
Polyploidy