The gff file is the first thing came into my head. I believe that we should start this work with it.

I get this error when I try download this chromosome's genes in GFF format (http://www.ensembl.org/Homo_sapiens/Location/View?chr=21;end=46709983;r=21:1-46709983;start=1)

"The region selected is too large to export. Please select a region of less than 5Mb."

I could get this chromosome's genes and RNAs from databases like NCBI genome data viewer, ENSEMLBE and UCSC. but actually I couldn't get it's product and their size (the number of their amino acids)

Doesn't the Human genome project (I assume you're talking about human) have something like database of genes for each chromosome?

If there is list of all genes, you could select those located on Chr 21 - it's in their name, isn't it?

I mean the whole gff file, which contain all annotations for the human genome. The start and end position for the genes are recorded in this file. Then we can extract the gene from the fasta file according to the information gathered before.

This is my plan.

Thank you guys, your suggestions helped me so much.

I could finally do this and want to share with you too.

for finding the desire information of a particular chromosome, there is some databases like NCBI, UCSC, ensemble, biomart, uniprot and etc. you can search either chromosome's name in the search box or from the human reference genome assembly which you can choose a chromosome and it will give you all information about it. also for finding all genes based on their coding and non coding information, you can find it in ensemble or NCBI (after searching the chromosome's name in search field) and it gives you download option which you can choose the format you want, and then you can have excel file which include all the information you have selected to show you in that database (you can customize which information you want). and also there is some scripts for this purpose which I mention their links here:

https://www.biostars.org/p/113199/

https://www.biostars.org/p/91218/#91233

https://rdrr.io/bioc/goseq/man/getlength.html

and also a useful article:

GeneBase 1.1: a tool to summarize data from NCBI Gene datasets and its application to an update of human gene statistics