<p>I always assumed the designations of "forward" and "reverse" strand on DNA were chosen arbitrarily (i.e. the strands could just as easily have been named the other way around).</p>

<p>However, a quick browse of the chromosome maps for Human on the NCBI map viewer seems to imply that in all cases, base numbering starts on the side closest to the centromere. Assuming that this numbering applies to the "forward" strands of the reference genome, does this mean that, for humans at least, there is a policy whereby the forward strand is always the one which contains the centromere at the lowest numerical index?</p>

<p>Also, if this is the case, does anybody know if this convention would apply outside humans?</p>

<p>Thanks for your time.</p>

> ...does this mean that, for humans at least, there is a policy whereby the forward strand is always the one which contains the centromere at the lowest numerical index?

Yes, in humans the centromere is always closest to the start coordinate of each chromosome, since human chromosomes are oriented based on their p (for "petit") and q arms. The p and q designation comes from the size of the chromosomes arms observed classically during mitotic cytogenetic preparations.

> Also, if this is the case, does anybody know if this convention would apply outside humans?

No, this convention does not apply generally outside of humans. For example, the yeast <em>Saccharomyces cerevisae</em> has sixteen chromosomes, 9 of which have the centromere closer to the start of the chromosome than the end.

```
$ mysql --user=genome --host=genome-mysql.cse.ucsc.edu -A -D sacCer1 -e 'select sgdOther.chrom as "chrom.", size as "chrom. size", sgdOther.chromStart as "left arm", size-chromStart as "right arm", chromStart/(size-chromStart) as "left/right ratio"  from sgdOther, chromInfo  where name like "CEN%" and sgdOther.chrom=chromInfo.chrom order by size/(size-chromStart);'
+--------+-------------+----------+-----------+------------------+
| chrom. | chrom. size | left arm | right arm | left/right ratio |
+--------+-------------+----------+-----------+------------------+
| chr12  |     1078173 |   150826 |    927347 |           0.1626 | 
| chr8   |      562639 |   105578 |    457061 |           0.2310 | 
| chr5   |      576869 |   151959 |    424910 |           0.3576 | 
| chr13  |      924430 |   268030 |    656400 |           0.4083 | 
| chr2   |      813136 |   238167 |    574969 |           0.4142 | 
| chr4   |     1531914 |   449706 |   1082208 |           0.4155 | 
| chr15  |     1091285 |   326583 |    764702 |           0.4271 | 
| chr3   |      316613 |   114378 |    202235 |           0.5656 | 
| chr7   |     1090944 |   496920 |    594024 |           0.8365 | 
| chr6   |      270148 |   148503 |    121645 |           1.2208 | 
| chr10  |      745446 |   436001 |    309445 |           1.4090 | 
| chr16  |      948060 |   555951 |    392109 |           1.4178 | 
| chr1   |      230208 |   151456 |     78752 |           1.9232 | 
| chr11  |      666445 |   439773 |    226672 |           1.9401 | 
| chr14  |      784328 |   628756 |    155572 |           4.0416 | 
| chr9   |      439885 |   355625 |     84260 |           4.2206 | 
+--------+-------------+----------+-----------+------------------+
```