​Learning Objectives

• ​a) explain what is meant by homologous pairs of chromosomes

• b) explain the meanings of the terms haploid and diploid and the need for a reduction division (meiosis) prior to fertilisation in sexual reproduction​

  c) outline the role of meiosis in gametogenesis in humans and in the formation of pollen grains and embryo sacs in flowering plants

​Homologous Chromosomes

• Matching types of chromosomes

• ​In the zygote, one of each pair came form the mother and one from the father.

• In humans, chromosome numbers 1-22, are called autosomes.

​Sex Chromosomes

​The non-matching X and Y chromosomes are the sex-determining chromosomes. XX for females, and XY for males

​Chromosomes

• ​Chromosomes are thread-like structures located inside the nucleus of animal and plant cells.

• Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA).

• Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.​

​Chromosomes

• ​The number of chromosomes per species is fixed.

• ​The shape of a chromosome is characteristic; they are of fixed length and with a narrow region, called the centromere, somewhere along their length.

• ​The centromere links a pair of sister chromatids

​Chromosomes

• ​The centromere is always in the same position on any given chromosome, and this position and the length of the chromosome is how it can be identified in photomicrographs.

​Chromosomes

• Chromosomes hold the hereditary factors, genes. A particular gene always occurs on the same chromosome in the same position.

• ​The position of a gene is called a locus.

• ​Each gene has two or more forms, called alleles.

​Meiosis

• ​Meiosis 1 is reductive cell division as cells go from diploid to haploid (2n to n)

• Meiosis 2 is like mitosis

• Independent assortment and cross-over increase genetic diversity

• Independent assortment occurs during metaphase 1.

​Meiosis

• Cross-over occurs between prophase 1 and metaphase 1.

• ​Cross over forms the chiasmata

• ​Chiasmata is the point at which paired homologous chromosomes remain in contact as they begin to separate during the first prophase of meiosis, forming a cross shape.

• ​Cross-over leads to recombinant DNA

​Gametogenesis

• ​The process by which gametes, or germ cells, are produced in an organism.

• ​The formation of egg cells, or ova, is technically called oogenesis.

• The formation of sperm cells, or spermatozoa, is called spermatogenesis.​

Spermatogenesis​

​Spermatogenesis

• ​Spermatogonia (mitosis - diploid) - an undifferentiated male germ cell.

• ​Meiosis -

  • Primary spermatocyte (diploid)

  • Secondary spermatocyte (haploid)

  • Spermatid (haploid)​

• Spermatozoan (mature sperm - haploid) ​

​Oogenesis

​Oogenesis

• ​Oogonia (embryo – diploid - mitosis) - immature female reproductive cells that gives rise to primary oocytes by mitosis

  • Primary oocyte (embryo – diploid – meiosis)

  • Secondary oocyte (after puberty – haploid – meiosis)

• Ovum (after ovulation – haploid)​

Meiosis in Plants

• ​Inside the anthers, diploid pollen mother cells divide by meiosis to make 4 haploid cells

• These cells mature into pollen grains

• Inside each ovule, a large, diploid, spore mother cell develops.

• Only one matures to become an embryo sac.