Sexual reproduction and meiosis Flashcards
(34 cards)
What is sexual reproduction defined as?
The formation of offspring from the mixing (fertilisation) of genetic material from two cells. This usually involves two parents.
How does sexual reproduction occur in plants?
When pollen from a flower fertilises an ovum of the same flower.
How does sexual reproduction occur in mammals?
The male and female sex cells (gametes) are produced in different individuals and therefore require two parents.
How does fertilisation occur?
When a male and female gamete fuse, a new cell (zygote) with two sets of chromosomes (male and female) is formed, producing offspring with genetic variability.
How is genetic variation advantageous for survival?
- It offers an evolutionary advantage to adapt to changing environments
- To resist certain diseases that could otherwise wipe out entire populations.
- It masks the effects of harmful recessive genes by obtaining DNA from two sources
What is meiosis?
The creation of sex cells (gametes): during meiosis, chromosomes are divided so that each daughter cell receives half the original number - one member of each homologous pair.
How many stages of meiosis are there?
Meiosis I (the reduction stage) and Meiosis II
Describe the process of prophase I.
- Chromosomes condense and become visible.
- Homologous chromosomes pair up in a process called synapsis. Since each chromosome consists of two identical sister chromatids joined at the centromere, the structures formed by synapsis contain four chromatids and are called bivalents.
- In each homologous pair, non-sister chromatids may touch, break and rejoin, exchanging genetic material through a process called crossing over. The position this occurs at is called chiasmata, increasing genetic diversity.
- In animal cells only, the centrioles move towards the poles, a spindle apparatus begins to form and the nucleoli and nuclear envelope disintegrate. Plants cells lack centrioles, but the formation of the spindle apparatus and subsequent processes are the same as for animal cells.
Explain crossing over.
Occurring during prophase I of meiosis, it involves the exchange of genetic material between non-sister chromatids in homologous chromosome pairs. This results in genetic variation as different alleles from either the maternal or paternal chromosome have been swapped between chromatids, meaning that the resulting haploid cells will be genetically different from one another.
Describe the process of metaphase I.
- The homologous chromosome pairs (bivalents) migrate to the equator of the spindle where they become attached to the spindle fibres so that each member of the homologous pair faces a different pole.
- Each homologous pair comprises a maternal and a paternal chromosome; some of the maternal and paternal chromosomes will face one pole, and the rest the other
Explain independent/random assortment.
The side of the metaphase plate a chromosome lines up on is completely random, resulting in the random assortment of maternal and paternal genes in the resulting haploid cells. This greatly increases the number of possible combinations of genetic material in the gametes and consequent genetic diversity/variety.
Describe the process of anaphase I.
- Homologous pairs separate and whole chromosomes more towards opposite poles; the sister chromatids of each chromosome remain attached at the centromere.
- The diploid number has now be halved, which is labelled as reduction division.
Describe the process of telophase I.
- The two new haploid sets of chromosomes have gathered around opposite poles and the spindle has begun to disappear.
Describe the process of cytokinesis in meiosis I.
- Division of the cell, forming two daughter cells
- The nuclear membrane and nucleoli do not reform between telophase I and prophase II
Describe the process of prophase II.
- New spindle forms.
Describe the process of metaphase II.
- Chromosomes have migrated to the equator of the spindle and have become attached to the spindle fibres by their centromeres.
- Sister chromatids face opposite poles - there is only one member of each homologous pair present.
Describe the process of anaphase II.
- The centromeres divide, the spindle fibres contract, and the sister chromatids are pulled apart towards opposite poles.
Describe the process of telophase II.
- The nuclear membrane reforms, nucleoli reappear, and cytokinesis occurs.
- Four daughter cells have been produced as a result of one diploid cell undergoing meiosis; each one of these is haploid.
How is each haploid cell different?
Crossing over during prophase I, and independent assortment during metaphase I have ensured that each of these four daughter cells has a unique combination of genetic material.
What is an autosome?
A chromosomes that is not a sex chromosome.
What type of cells does fertilisation involve?
Sex cells (gametes) containing one representative chromosome from each homologous pair.
What happens when two gametes fuse?
When two gametes (haploid sex cells) fuse as part of fertilisation, the normal diploid condition is restored: the cell produced as a result of fertilisation is denoted as a zygote.
How is genetic variation of the offspring created?
Due to the random nature of fertilisation. However, there is a consistent chromosome number from one generation to the next.
Compare the characteristics of mitosis and meiosis (number of chromosomes).
Mitosis: diploid (2n)
Meiosis: haploid (n)
