Dense threadlike and rod-like structures present in the nucleus are chromatin and chromosomes.

Chromatin:
- Chromatin is a complex of DNA, RNA, and proteins that make up the genetic material of the nucleus.
- It appears as a mass of fine threads when seen under a microscope.
- Chromatin is composed of DNA molecules wrapped around proteins called histones.
- The main function of chromatin is to package and organize the DNA in a compact and manageable form.

Chromosomes:
- Chromosomes are condensed and highly coiled structures formed by the chromatin during cell division.
- They are visible under a light microscope and appear as rod-like structures.
- Each chromosome consists of a single, long DNA molecule and associated proteins.
- Chromosomes carry the genetic information in the form of genes, which are segments of DNA that code for specific traits or characteristics.
- During cell division, chromosomes ensure the proper distribution of genetic material to daughter cells.

Importance of Chromatin and Chromosomes:
- Chromatin and chromosomes play a crucial role in the regulation and transmission of genetic information.
- They help in the packaging of DNA, preventing it from becoming tangled and damaged.
- Chromatin structure also influences gene expression, as certain regions of DNA become more or less accessible for transcription.
- During cell division, chromosomes ensure accurate segregation of genetic material, ensuring each daughter cell receives the correct number and types of chromosomes.

Conclusion:
In summary, the dense threadlike and rod-like structures present in the nucleus are chromatin and chromosomes. Chromatin is the uncoiled and less condensed form of DNA, while chromosomes are the highly condensed and visible structures formed during cell division. They are essential for DNA packaging, gene expression, and accurate transmission of genetic information.