Cellular Replication and Variation Flashcards
4.1.4 Within the process of meiosis I and meiosis II:
a) recognise the role of homologous chromosomes
b) describe the process of crossing over and recombination and demonstrate how they contribute to genetic variation
Define meiosis and describe the 8 different stages.
Meiosis is a type of cell division that produces sex cells (gametes) for the purposes of sexual reproduction. Interphase is when a cell is not dividing, but the DNA is being replicated. Phrophase I (spindle apparatus forms) is when homologous chromosomes pair up. Priot to cell division, the chromosomes condense into visible strucrures. Replicated chromosomes appear as two sister chromatids held together by a centromere. Crossing over may occur between two non-sister chromatids at this time. This makes the sister chromatids different to one another in the allele combinations. Metaphase I is when independent assortment occurs. Homolohoys pairs line up in the middle of the cell independently to each other. This results in paternal and maternal chromosomes assorting into the gametes. Anaphase I is when homologous pairs seperate, pulled apart by spindle fibres. Metaphase II, the second divisoion is merely miotic in nature, where the chromatids are pulled apart, but the number of chromosomes remains the same. By the end of telophase II seperate gametes are produced.
Describe the behaviour of the chromosome in the first and second division in meiosis.
In the first division homologous pairs of chromosomes pair to form bivalents (one pair of chromosomes in a tetrad which is four sister chromatids). Crossing over may occur. Homologous pairs seperate. In the second divsion, chromatids seperate (are pulled apart by spindle fibres), but the chromosome number stays the same. This is more or less a ‘miotic’ division.
How does independent assortment incease variation in gametes?
Independent assortment refers to the random distribution of maternal and paternal homologous to the gametes. In meiosis I, crossing over during prophase and independent assortment during anaphase creates sets of chromosomes with new combinations of alleles. Genetic variation is also introduced by random fertilization of the gametes produced by meiosis. When homologous chromosomes pair up during Meiosis, it is a matter of chance which side a chromosome ends up on. This random shuffling of the chromosomes is called independent assortment. It produces variation in the gametes by mixing up the maternal and paternal chromosomes.
How does crossing over increase the variation in the gametes and hence the offspring?
Crossing over is the mutual exchange of pieces of chromomes (alleles) between non-sister chromatids of homologous chromosome pairs, resulting in a mixture of parental characteristics in offspring. When two chromosomes — one from the mother and one from the father — line up, parts of the chromosome can be switched. The two chromosomes contain the same genes, but may have different forms of the genes. Crossing over increases variation by creating new combinations of alleles on the chromosome involved in the crossing over. The chromosomes cross over at points called chiasma. At each chiasma, the chromosomes break and rejoin, trading some of their genes. This recombination results in genetic variation
- 1.4 Within the process of meiosis I and meiosis II:
c) compare and contrast the process of spermatogenesis and oogensis (with reference to haploid and diploid cells)
How are gametes formed?
Spermatogenesis produces male gametes called spermatozoa (sperm) and occurs from puberty until old age. It starts with the spermatogonium cell, 2N, which undergoes mitosis to produce two primary spematocytes. This is when stage one of meiosis occurs and the primary spermatocyte will split into two secondary spermatocytes. In stage II of meoisis they will split further into four spermatids which then become motile to create four equal spermatozoa. As there are two primary spermatocytes involves in the process, one spermatogonium will produce eight spermatozoa. Oogensis produces female gametes called ovum (eggs). It starts before birth then pauses in prophase I until puberty. At puberty, oocte continues to develop but pauses again and doesn’t complete unless the egg is fertilised. Stage I of meiosis occurs when the girl reaches puberty and the primary oocyte develops into the secondary oocyte. The process is put on hold again and meiosis II doesn’t occur until fertilisation. In both stages of meiosis the cytoplasm divides unequally to form one egg and one polar body. These polar bodies eventually disintegrate whereas the egg continues to develop. Therefore one oogensis will prodiuce two egg cells by the end of meiosis II.
- 1.4 Within the process of meiosis I and meiosis II:
c) compare and contrast the process of spermatogenesis and oogensis (with reference to haploid and diploid cells)
Comapre spermatogensis and oogensis.
Similarities:
Both processes result in the formation of haploid gametes
Both processes involve mitosis, growth, and meiosis
In multiplication phase, the primordial germ cells of testes and ovaries proliferate mitotically, forming numerous gametogonia (spermatogonia/oogonia) in both processes.
Maturation phase in both processes comprises two successive divisions, first meiotic and second meiotic, resulting in the formation of secondary gametocytes and gametes respectively
Differences
Spermatogensis Oogensis
Location Testis Ovary
Number of gametes Life long production (many) Fixed amount (400)
Gametes per germ cell Four One
Beginning of process Begins at Puberty Fetal development
Timing of gamete form Continuous Once a month
End of process Lifelong but reduces Stops menopaus
Meiotic divisions Uninterrupted Arrested
Further differences include only oogensis forming polar bodies which reduces the number of gametes formed to just one per germ cell.
4.1.5. Demonstrate how the process of independent assortment and random fertilisation alter the variations in the genotype of offspring.
State the law of segregation.
Law of segregation - states that during gametogensis (formation of gametes), the two copies of each gene (one from mother, one from father) are seperated into different gametes (sex cells - eggs or sperm). Each individual has two alleles for each trait, and these alleles become segregated throughout the formation of gametes. Therefore the pair of alleles segregate from each other during meiosis cell division (gamete formation) so that only one allele will be present in each gamete.
4.1.5. Demonstrate how the process of independent assortment and random fertilisation alter the variations in the genotype of offspring.
State the law of independent assortment and how it leads to variations in the genitype of offspring.
Law of independent assortment states that when two or more genes are inherited, each gene is assorted independently into gametes, giving different traits equal chance to occur together. It leads to different combinations of maternal and paternal chromosomes in gametes. It depends on the orientation of homologous pairs during metaphase I. During meiosis, the pairs of homologous chromosome divide to form haploid cells. This seperation of homologous chromosomes is random, resulting in gametes with unqiue combinations of chromosomes. During meiosis, the process of recombination breaks and recombines pieces of DNA to produce new combinations of genes. Recombination scrambles pieces of maternal and paternal genes, whicn ensures that genes assort independently from each other. After meiosis, each haploid cell comtains a mixture of genes from the organism’s parent which leads to increases genetic variation in offspring.
