Question

Please complete tables for forensic biology lab using the gel image as reference.

If you could complete the whole table that would be much appreciated, however I do not expect anyone to do that much work for me. If you could just complete a few rows on each one and show me how you got there, then I'm sure I could complete the rest of it (I'm just confused on how to calculate the distance in cm)

DNA Marker (1)| Lane 2 | Lane 3 | Lane 4 | Lane 5 Lane 6 | Lane 7 | Lane 8 | Lane 9 Lane 10 Size Distance Dist. Size Dist. Size Dist. Size Dist. Size Dist. Size Dist. Size Dist. Size Dist. Size Dist. Size (bp) (cm) (cm) bp (cm) bp (cm) bp (cm) bp (cm) bp (cm) bp (cm) bp (cm) bp (cm) bp

My teacher gave me a walkthrough on completing some of the rows- but I'm still lost. Here is that walkthrough if you need it.

Thank you!

Answer 1

The distance traveled by each band in the Ladder is measured from the well. This can be done by placing a ruler adjacent to the gel and marking the distance of each band from its origin, the well. This can be done even if you don't have physicl access to the gel, by placing a ruler or a measuring tape on the computer screen and marking the distance traveled by each band. The size of the image doesnt matter as the relative distance between each band would remain the same. So, marking the distance of each band every band would be the first step.  The well would be marked zero and each band would be measured by marking the distance on the tape on each lane.

Now, the method used here to determine the size of a band is by choosing two of the ladder bands that 'flank' the unknown size band in the sample in the other lanes.

Let us look at the steps involved to determine the size of band X in the given example:

1. Find the difference in bp as well as distance between the two flanking bands. As given in the example, (Lane 3, S2), which are 200 bp and 100 bp ladder bands. The difference in bp is 200 - 100 = 100 bp; and the difference in distance migrated by these two bands is : 12.1 - 10.3 = 1.8 cm. This means the distance migrated by 100 bp is 1.8 cm.
2. Find the migration rate per cm. This is obtained by dividing the 100 bp by 1.8 cm: 100 / 1.8 = 56 bp / cm. Based on the rate of migration of the two ladder bands (200 and 100 bp), this means that a 56 bp band would cover 1 cm.
3. Next, we eliminate the distance between the well and the 200 bp band because now we take the point of origin for band X as the 200 bp band, not the well. From the 200 bp band, the distance migrated by band X is calculated as 11.3 - 10.3 = 1 cm.
4. The calculated rate per cm is 56 bp, so we multiply the distance of band X with 56 to get: 1 * 56 = 56 bp. AS we subtracted the distance values, to get 1 cm as the distance between X and 200 bp, the bp difference would be 1 cm = 56 bp. Therefore, we subtract the value from 200 bp to get the actual size of band X: 200 - 56 = 144 bp.

So, first we need to find the rate per cm of the ladder bands inbetween the unknown size band , change the point of origin from the well to the nearest ladder band , determine how many bp apart is the unknown band  from that of the ladder (origin) by multiplying the distance of the unknown band with the rate per cm (bp) and finally subtract the value from the origin (ladder) to get the actual size of the unknown band.

This appears a bit complex. Usually a standard curve is plotted using log molecular weight of the ladder bands against the distance, and the size of the unknown band is calculated using the equation of the straight line from the graph and converting the log value to the actual molecular size.

This probably is more accurate as it takes only the corresponding known size bands into consideration.

Let us look at the calculations for Lane 2:

Here the first two bands fall inbetween 300 and 200 bp whereas the 3rd band falls between 100 and 200 bp.

band#1:

Difference in bp = 300 - 200 = 100 bp

Difference in cm = 9.9 - 8.6 = 1.3 cm

Rate per cm = 100 / 1.3 = 76.9 bp

Distance cm for band #1 = 8.6 - 8.6 = 0 cm

rate per 0 cm distance = 76.9 * 0 = 0 bp

Size of band#1 = 300 - 0 = 300 bp.

band#2:

Difference in bp = 300 - 200 = 100 bp

Difference in cm = 9.9 - 8.6 = 1.3 cm

Rate per cm = 100 / 1.3 = 76.9 bp

Distance cm for band #2 = 9.6 - 8.6 = 1 cm

rate per 1 cm distance = 76.9 * 1 = 76.9 bp

Size of band#2 = 300 - 76.9 = 223.1 bp.

band#3:

Difference in bp = 200 - 100 = 100 bp

Difference in cm = 11.5 - 9.9 = 1.6 cm

Rate per cm = 100 / 1.6 = 62.5 bp

Distance cm for band #3 = 11.1 - 9.9 = 1.2 cm

rate per 1.2 cm distance = 62.5 * 1.2 = 75 bp

Size of band#3 = 200 - 75 = 125 bp.

Ladder	Blood on victim's body	Suspect 1	Suspect 2	Suspect 3	Suspect 4
Size (bp)	Distance (cm)	Size (bp)	Distance (cm)	Size (bp)	Distance (cm)	Size (bp)	Distance (cm)	Size (bp)	Distance (cm)	Size (bp)	Distance (cm)
1500	1.5	300	8.6		8.3		8.2		8.7		8.3
1200	2.3	223	9.6		9.3		9.5		9.6		8.5
1000	3	125	11.1		9.5		10.9		11		9.3
900	3.4				11.4						11.4
800	4.4
700	5.3
600	6.3
500	7
400	7.7
300	8.6
200	9.9
100	11.5