Meiosis is a specialized cell division that produces haploid cells (gametes) from diploid progenitors. During meiosis parental chromosomes (homologs) need to pair, synapse and eventually segregate. Faithful chromosome segregation depends on chromosome recombination. In the beginning of prophase I programmed double strand breaks (DSBs) are introduced in meiotic cells by SPO11 enzyme. DSBs are positioned at hotspot sites that are specified by that action of DNA-binding histone methyltransferase PRDM9. Specific enzymes act at the site of breaks to create 5’ single stranded DNA ends. With the assistance of the strand exchange proteins DMC1 and RAD51 these ends invade homologous DNA sequence and DSB repair is initiated. DSB repair can be completed either as a crossover (reciprocal exchange of DNA) or as a non-crossover. Crossover events lead to the formation of chiasmata between homologs and ensure proper segregation during the first meiotic division. An interesting feature in male meiosis is the XY chromosomes. The shared region between sex chromosomes is short and is called pseudoautosomal region (PAR). Due to their large non synapsed region, XY chromosomes need to be transcriptionally silenced. Thus they are covered with the phosphorylated histone variant H2AX (γH2AX) forming the so called sex body. PAR region has higher density of DSBs than autosomes and it had been shown that sex chromosomes undergo delayed homologous pairing. Nevertheless little is known how meiotic recombination is regulated in PAR region of sex chromosomes. In close proximity with sex body it has been found a structure named dense body (DB). There are few reports suggesting that DB contains RNAs/proteins but no DNA. Its role in meiosis was unclear because no structural component had been described. In the present thesis the role of two meiotic expressed genes is described. In our group after performing RNA screens we identified several genes that are highly expressed during meiotic prophase I. Based on the expression profile we selected polycomb-related sex comb on midleg like 1 (Scml1) gene and the ankyrin repeat domain 31 (Ankrd31) to study their role in mammalian meiosis.:List of figures i List of abbreviations ii 1. Introduction 1 1.1 Gametogenesis 1 1.2 Meiotic prophase I 2 1.2.1 Meiotic recombination 4 1.2.2 Regulation of meiotic recombination 7 1.2.2.1 Meiotic recombination hotspots and PRDM9 activity 7 1.2.2.2 Meiotic surveillance mechanisms 8 1.3 Unique properties of XY recombination 9 1.4 Sex chromatin associated structure: The dense body 10 1.5 Aim of the thesis 11 2. Publications 12 3. Discussion 92 4. Summary 98 5. References 102 Acknowledgements 108 Declarations 109