Sexual Reproduction (lesson three) Flashcards
Sexual Reproduction
Sexual reproduction is a form of reproduction in which individuals are produced from the fusion of two sex cells
why are off-springs produced by sexual reproduction genetically unique?
because they obtain half of their genetic material from each of their two parents
disadvantages to reproducing sexually
- Specialized organs are required to produce the sex cells
- Specialized mating calls or bright colours to attract mates can also attract predators
- May require the loss of resources, like nectar in flowers
- The combining of genetic information may make the offspring weak and unable to survive
advantages to sexual reproduction
- Individuals that rely on sexual reproduction can adapt to a changing environment, due to their genetic variability which is not possible in organisms that reproduce asexually
what are the two key processes involved with Sexual Reproduction?
- The formation of haploid cells, or gametes, which contain genetic information from the parents
- Two sex cells join to form a zygote, the first cell of a genetically unique individual, through fertilization
Meiosis
- the process through which haploid gametes are formed
- Meiosis is a process that involves two divisions that produce 4 haploid gametes from 1 diploid parent cell
the difference between meiosis and mitosis
- In mitosis, the chromosome number in each generation stays the same
- and in meiosis, the chromosome number is cut in half
- For ex: sex cells produced in humans contain only 23 (haploid or n), not 46 (diploid or 2n), chromosomes
what happens in the combination of two gametes
The combination of two gametes (sperm and egg - both haploid) will result in a complete set of chromosomes (diploid, 2n = 46)
homologous chromosomes
You receive a complete set of DNA from each parent (one version of each gene) known as homologous chromosomes
what does homologous chromosomes ensure
ensures the offspring is genetically unique from each parent
gametogenesis
The formation of sex cells is called gametogenesis
what are the two types of gametogenesis in humans?
- Formation of sperm by spermatogenesis
- Formation of egg cells or ova by oogenesis
how are the two types of gametogenesis formed?
Both are produced through a process called meiosis
Interphase
- Cell grows to nearly twice in size
- Nucleus makes a copy of its DNA
- Organelles are also replicated
Meiosis I: Prophase I
- Chromatin condenses into chromosomes
- The nuclear membrane dissolves
- Centrioles move to opposite sides of the cell
- Spindle fibres begin to grow
synapsis
- Homologous chromosomes come together and intertwine to form a tetrad by a process called synapsis
- Each chromosome is made of 2 sister chromatids, and you have 1 chromosome from each parents
- These four sister chromatids together is called a tetrad
what happens to Homologous chromosomes in prophase I
Homologous chromosomes come together and intertwine to form a tetrad by a process called synapsis
explain crossing over which occurs during Meiosis I: Prophase I
- This is when intertwined chromatids from different chromosomes break off and reattach at the chiasmata
- This causes an exchange of genetic material in certain sections
- This works to mix or recombine genetic information further increasing genetic variation in the gene pool
Meiosis I: Metaphase I
- During this phase, the tetrads migrate towards the center of the cell, called the equatorial plate
- This is done through spindle fiber attachment at the centromeres like in mitosis
- The tetrads align their centromeres at the middle of the cell
Meiosis I: Anaphase I
- Homologous chromosomes move to opposite poles of the cell when the spindle fibers shorten
- This means that only one of the two chromosomes from each homologous pair will be in each new daughter cell and therefore are genetically unique
- EX. Each of your sperm or egg has only the chromosome you received from your mom or your dad, not both!
Meiosis I: Telophase I
- The nuclear membranes reform, spindle fibers dissolve and the cell begins to divide via the first cycle of cytokinesis
- Each new cell has only 1 (not two) duplicated copy of a chromosome at this point
- Only the maternal or paternal chromosome is in each new cell
- Therefore the cells are haploid
Meiosis II: Prophase II
- Each haploid cell has one chromosome made of two sister chromatids at this point
- The sister chromatids are not genetically identical though, due to the crossing over that occurred during prophase I
- Once again, the nuclear membrane dissolves, centrioles move to opposite poles and spindle fibers reform
Meiosis II: Metaphase II
- Again, the one chromosome made of two sister chromatids move to center of the cell, called the equatorial plate
- This is accomplished through spindle fiber attachment at the centromeres like in mitosis
- The chromosomes align their centromeres at the middle of the cell
Meiosis II: Anaphase II
- Sister chromatids now separate and move to opposite poles of the cell
- Each cell now has a single sister chromatid, not a duplicate copy
- These will not be identical due to the genetic exchange that occurred in crossing over
Meiosis II: Telophase II
- Just like in mitosis, the nuclear membrane reforms around the chromatids, the DNA relaxes into chromatin and the spindle fibers dissolve
- The second nuclear division is now completed
- Soon after, cytokinesis will occur for the second time
- This will result in 4 haploid daughter cells (that are NOT identical)
Genetic Variation
- There are many parts of meiosis that allows for genetic variation through random assortment of chromosomes
- In Prophase I, crossing over occurs randomly
- In Metaphase I and II, it is random as to which side each chromosome goes to
