Genome sequences and gene number

Question 1

What is the average genome size per gene in prokaryotic cells?

Answer 1

1 kb per gene

Question 2

What is the minimum number of genes for a living cell?

Answer 2

500

Question 3

What is the minimum number of genes for a free living cell?

Answer 3

1500

Question 4

What accounts for the discrepancy between genome size and number of genes?

Answer 4

1) Gene organization

2) Amount of sequence repetition

Question 5

How are prokaryotic genes organized?

Answer 5

• Uninterrupted: they have no introns, no extra DNA

- The coding region on the DNA corresponds exactly to what is transcribed and translated

Question 6

How is complexity reflected in genome size and number of genes?

Answer 6

As complexity increases, the genome increases much more than the increase in number of genes

Question 7

How are eukaryotic genes organized?

Answer 7

• Most eukaryotic genes are interrupted

- So there is a lot more DNA per gene than what will be translated

Question 8

Why is there more DNA per gene than what is translated in eukaryotes?

Answer 8

• The genes are interrupted

Question 9

How do we go from the interrupted genes to the translated protein?

Answer 9

• Transcription yields the primary transcript or pre-mRNA

- Pre-mRNA is then processed and modified (process called splicing) to a mature mRNA which is then translated

Question 10

How does complexity affect gene interruption?

Answer 10

 Increase in percentage of interrupted genes
• Smaller percentage of uninterrupted genes
• Increase in number of introns
 Increase in the number of interruptions per genes (i.e. introns)
• Less complex organisms have less interruptions (less introns/gene)
• The majoriy of uninterrupted genes in mammals consist of 2-12 exons (i.e. they consist of 1-11 interruptions)

Question 11

What is sequence repetition?

Answer 11

The number of times a sequence is repeated within a genome

Question 12

What types of sequence repetition DNA do prokaryotes have?

Answer 12

They only have non-repetitive DNA

• they have 1 copy of every sequence per genome
• every sequence is protein-coding DNA

Question 13

What types of sequence repetition DNA do eukaryotes have?

Answer 13

Non-repetitive
Moderately repetitive
Highly repetitive

Question 14

What is non-repetitive DNA?

Answer 14

1 copy of a sequence per genome

Corresponds to the majority of protein coding sequences

Question 15

What is moderately repetitive DNA

Answer 15

Sequence repeated 10 to 1000 times in the genome

They can be gene clusters or mobile elements.

Question 16

What are examples of gene clusters?

Answer 16

• rRNA (about 100-200 copies of each rRNA gene

- Histone genes (over 300 copies of each)

Question 17

What are examples of mobile elements?

Answer 17

• Transposons
• Retroposons
• Retroviruses

Question 18

What is highly repetitive DNA?

Answer 18

Sequences repeated approximately 500,000 times

Question 19

What are examples of highly repetitive DNA?

Answer 19

Satellite DNA: very short sequences (less than 100 bp)

Question 20

What is satellite DNA?

Answer 20

Very short sequences (less than 100 bp) that are highly repeated
- associated with heterochromatin regions, especially telomere and centrosome