Meiosis (Meiosis vs Mitosis)

TOPIC: MEIOSIS Key Knowledge: • The production of haploid gametes from diploid cells by meiosis, including the significance of crossing over of chromatids and independent assortment for genetic diversity MEIOSIS VS MITOSIS Meiosis is the process by which haploid sex cells (gametes) are made in the reproductive organs. It follows a similar process to mitosis: it is preceded by interphase, separates the chromosomes in sequential stages (prophase, metaphase, anaphase and telophase) and divides cells via cytokinesis. However, there are also several key differences between the two processes. MEIOSIS MITOSIS Type of Cell Produced Sex cells / gametes Body cells Number of Divisions Two divisions (MI and MII) One division Number of Cells Formed Four haploid cells Two diploid cells Outcome of Process Cells are different (genetic variation) Cells are identical (cloning) Purpose of Process Sexual reproduction (more biodiversity) Growth and repair of tissues HOMOLOGOUS CHROMOSOMES Sexually reproducing organisms receive genetic information from both parents and so have two copies of every chromosome – a maternal copy and a paternal copy. The chromosome pairs are called homologous chromosomes and the cell is described as diploid (two chromosome sets). Meiosis functions to separate the homologous pairs to form sex cells that are haploid (one chromosome set). Via sexual reproduction, the male and female gametes can then be fused (fertilisation) to form a new, distinct diploid cell (zygote). Homologous chromosomes will have the same genes, but may have different versions of the gene (alleles). MEIOTIC DIVISIONS Meiosis consists of two cellular divisions which function to separate a germline cell (diploid) into four genetically distinct sex cells (haploid). Because meiosis is preceded by interphase, all of the chromosomes will consist of two sister chromatids (formed via replication of DNA). In the first meiotic division, homologous chromosomes will pair up via connection points called chiasmata to become single units (bivalents). These bivalents are then separated in a reduction division that halves the chromosome number in the daughter cells (i.e. diploid ® haploid). The second meiotic division separates the two sister chromatids that were formed via replication of DNA during the S phase (interphase). INTERPHASE MI M II GENETIC VARIATION Sexual reproduction promotes variation in populations by creating new gene combinations in offspring. Progeny inherit genetic characteristics from both parents and the ‘shuffling’ of these characteristics via sexual reproduction may result in new phenotypic variants. This recombination of traits occurs via meiosis and involves two distinct processes: crossing over (in prophase I) and random assortment (in metaphase I). Also, fusion of male and female gametes is random, meaning the same parents have distinctive offspring. 1. CROSSING OVER Crossing over Recombination Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I of meiosis. This exchange of genetic material occurs at points called chiasmata. As a consequence of this recombination, all four chromatids of the homologous chromosomes will be genetically distinct, leading to new phenotypic variants. 2. RANDOM ASSORTMENT During metaphase I, the homologous chromosomes line up at the equator in a random orientation. The orientation of each homologous pair occurs independently of all other chromosome pairs, meaning that different combinations of maternal and paternal chromosomes will be formed within gametes. As humans have 23 pairs of chromosomes, each person can produce 223 (over 8 million) distinct gamete combinations. Parent Cell Potential Gamete Combinations 3. RANDOM FERTILISATION Following meiosis, the haploid daughter cells undergo a process called gametogenesis to differentiate into functional sex cells. Males form spermatozoa (via spermatogenesis), whereas females will form egg cells or ova (via oogenesis). Fertilisation is a random process (specific egg and sperm fusion is different each time), which means that all siblings should be genetically distinct from each other. Identical twins will only result if the diploid zygote is split into two separate cell masses after fertilisation has occurred. Ovum (female egg) + Spermatozoa (male) = Fertilisation (micrograph) OVERVIEW OF MEIOSIS Meiosis consists of two divisions, each involving four phases (prophase, metaphase, anaphase, telophase). Interphase (S) MEIOSIS I SUMMARY • • • Is a reduction division (diploid ® haploid) Separates the homologous chromosomes Crossing over may occur during Prophase I to create genetically distinct sister chromatids MEIOSIS II SUMMARY • • • Akin to a mitotic division (but of haploid cells) Separates the sister chromatids Occurs because DNA is replicated in interphase to create chromosomes with sister chromatids