Flashcards in 4.2 Meiosis Deck (19):
Haploid cells which fuse together during fertilisation to form a zygote (also called sex cells)
Describe a diploid cell
Contains 2 complete sets of chromosomes; chromosomes are in bivalents/tetrads. Body cells (somatic cells) are diploid.
Describe a haploid cell
Contains 1 complete set of chromosomes. Gametes (sex cells) are haploid.
A pair of homologous chromosomes joined by a centromere (also called a bivalent)
A reduction division of a diploid nucleus to form four haploid nuclei
Define homologous chromosomes
Homologous chromosomes are chromosomes that share:
• The same structural features
• The same genes at the same loci positions (while genes are the same, alleles may be different)
Outline Prophase I of Meiosis I
1. Nuclear membrane breaks down, DNA in form of chromatin supercoils, condensing into chromosomes.
2. Homologous chromosomes undergo synapsis whereby they form Tetrads
3. 'Crossing over' occurs
4. The centrosomes move to the poles of the cell and spindle fibres begin to form and attach to the centromeres of the chromosomes.
Outline Metaphase I of Meiosis I
The homologous pairs are lined up horizontally along the equator by contraction of the spindle fibres.
Outline Anaphase I of Meiosis I
Continued contraction causes the homologs to separate and pulls the chromosomes to the opposite poles of the cell.
Outline Telophase I of Meiosis I
Nuclear membrane reforms, spindle fibres break down, chromosomes decondense back into chromatin. Cell divides into two haploid daughter cells (cytokinesis)
State the result of meiosis
4 haploid cells produced from 1 diploid cell
When a set of homologs in a tetrad do not separate during Anaphase I
Explain the cause of down-syndrome due to non-disjunction
• Caused by extra chromosome 21.
• Chromosomes or chromatids do not separate and go to the same pole.
• non-separation of homologous chromosomes during anaphase I due to incorrect spindle attachment.
• non-separation of chromatids during anaphase II due to centromeres not dividing.
• Occurs during oogenesis or spermatogenesis, but more common in egg formation.
• Egg or sperm contain an extra copy of chromosome 21, therefore zygote has 3 copies of chromosome 21.
• Thus Down syndrome is also known as trisomy 21.
When chromosomes are arranged in pairs according to their structure
Outline the process of karyotyping
• Pictures can be taken of the human chromosomes during the metaphase
• They can then be arranged into pairs on the basis of size and structure
• Chromosome 1 is the shortest, chromosome 22 is the longest
• The chromosomes appear as pairs of sister chromatids
Outline one use for karyotyping
• Amniotic fluid or chorionic tissue can be sampled as they contain foetal cells in mitosis.
• Can be used for diagnosis of chromosomal abnormalities, such as Down syndrome, identifying extra chromosome 21.
The process by which tetrads are formed
Outline 'Crossing Over' of chromosomes in Prophase I
The homologous chromosomes in a tetrad cross over at a points called chiasmata. 'Crossing over' of genetic material between non-sister chromatids can occur at these points, resulting in new gene combinations (genetic variation).