Re: What is meant by staggered ends in collagen molecules?

Covalent cross-links hold many collagen molecules side by side, forming fibres. The ends of the parallel molecules are staggered. If they were not, there would be a weak spot running right across the collagen fibre.
What is the meaning of staggered ends here? Why would there be a weak spot running right across the collagen fibre if the ends are not staggered?

In the case of collagen fibrils, the term 'staggered' means 'overlapping' as illustrated in the images below.

Here is a cartoon drawing of a single triple-stranded collagen molecule.

And here is a cartoon representation of a collagen fibril constructed from staggered (overlapping) collagen molecules. Each molecule is covalently cross-linked with its neighbors above and below, but I haven't drawn those cross-links here.

Finally, here is a cartoon representation of an ersatz collagen fibril constructed without staggering the individual collagen molecules. As above, the molecules are cross-linked with their neighbors above and below.

As you can see, the difference between the two fibrils is that the staggered molecules make covalent contacts with neighbors that extend beyond their own ends, whereas the non-staggered molecules only make contacts with adjacent neighbors. Therefore, the staggered structure is stronger than the non-staggered structure; when you pull on one collagen molecule, you are essentially pulling on all of them.

In contrast, you can easily break the non-staggered structure at any point where the collagen molecules end; these white gaps between cross-linked molecules are the weak-spots referred to in your text. When you pull on one collagen molecule in the lower figure, you are only pulling on the molecules in its immediate vicinity.

For further discussion of collagen fibril structure and formation, see:
Hulmes DJ. (2002) Building collagen molecules, fibrils, and suprafibrillar structures. J Struct Biol. 137(1-2):2-10. Review.