3.5 & 7.3 DNA Transcription

07/11/2012 § 1 Comment

Hi, I’m Kari, and I’m here to talk to you about DNA transcription today. I’m also going to touch upon translation but we’ll try and save more of that for later, okay? Oookay.

Proteins are what make an organism. And DNA is what codes to make a protein, which is essentially a long link of amino acids, a.k.a., a polypeptide. There are two processes that work to produce proteins – transcription and translation. Transcription, using DNA as a template, is the synthesis of RNA, typically mRNA (messenger RNA). There are other types of RNA such as tRNA (transfer RNA), which is involved in translation, and rRNA (ribosomal RNA), which is a part of the structure of the ribosome, where protein synthesis occurs. Remember that one of the main differences between RNA and DNA is that the thymine base in DNA is replaced by uracil in RNA. Translation is the decoding of the information on the synthesized RNA to then synthesize the polypeptide molecule. Usually one particular polypeptide is synthesized by one type of mRNA that was synthesized from one gene. This is the one gene–one polypeptide concept, which helps us to understand the amount of information within cells.

So, it’s like a story. What first happens in transcription is, the enzyme RNA polymerase binds to a site of the DNA called the promoter. This is where transcription starts. RNA polymerase unzips and unwinds the helix structure of the DNA and determines which of the two DNA strands is gig to be transcribed. The adult strand that isn’t going to be transcribed (therefore the strand that will match the final product that is complementary to the other adult strand) is called the sense strand because I think it makes sense with the daughter strand? The strand that will then provide a template for the RNA polymerase to transcribe is called the antisense strand because though it is complementary to the daughter strand, the contents of the strands are not the same.

The free RNA nucleoside triphosphates are added to the 3′ end of the RNA transcript so the process moves in a 5′ to 3′ direction. As the polypeptide continues to grow, the RNA polymerase moves along the DNA, unzipping the helix in the parts of the DNA that are being transcribed, winding it back up into the helix structure after transcription. When a termination sequence in the DNA is reached, the mRNA is released from the DNA.

When the mRNA is released, they’re still pre-mature and need to be modified through RNA splicing. What happens are there are sequences in the mRNA that aren’t needed for translation later to synthesize the polypeptide. These are called intervening sequences, or introns, and they must be removed by splicing. The rest mRNA’s code are called exons. Prokaryotes don’t have introns (eukaryotes do), neither do they have the mechanics to splice out those introns, so when manually synthesizing both prokaryotic and eukaryotic genes together, there would be a problem. The introns that aren’t edited out add extra parts to the polypeptide that hinder its ability to function. (Special copies from eukaryotes that don’t have any introns are then made for prokaryotes.)


Essay Questions

  • Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA. (5)
  • Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands. (5)
  • Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together. (8)
  • Outline the structure of the nucleosomes in eukaryotic chromosomes. (4)
  • State a role for each of four different named enzymes in DNA replication. (6)
  • Explain the process of DNA replication. (8)
  • Explain how the process of DNA replication depends on the structure of DNA. (9)
  • Describe the genetic code. (6)
  • Discuss the relationship between genes and polypeptides. (5)
  • Explain briefly the advantages and disadvantages of the universality of the genetic code to humans. (4)
  • Compare the processes of DNA replication and transcription. (9)
  • Distinguish between RNA and DNA. (3)
  • Describe the roles of mRNA, tRNA and ribosomes in translation. (6)
  • Outline the structure of tRNA. (5)
  • Outline the structure of a ribosome. (4)
  • Explain the process of translation. (9)
  • Compare DNA transcription with translation. (4)



Page 67, DNA and RNA structure

1. State two differences between the structure of DNA and RNA that are shown in Figure 3.

The sugar of DNA and RNA are different in that RNA has ribose (with a hydroxyl molecule at 2′) and DNA has dexoyribose (with a hydrogen molecule at 2′). Also, RNA has uracil while DNA has thymine.

2. State on difference between the structures of DNA and RNA that is not shown in Figure 3.

DNA comes in two strands while RNA comes in only one.

3. Distinguish between the structures of DNA and RNA.

While DNA has a structure that consists of two strands, RNA’s structure only consists of one. DNA contains the nitrogen base thymine while RNA contains the nitrogen base uracil. The sugars of DNA and RNA are different because DNA has a deoxyribose (H at the 2′) and RNA has a ribose (OH at the 2′).

4. State four similarities between DNA and RNA.

Both have the same phosphate group. Both have a pentose sugar that consists of five sides. Both have adenine, cytosine, and guanine. Both have the phosphate group (which holds the 5′ carbon) attached to the previous nucleotide’s carbon-3. 

Page 69, exons and species complexity

1. Determine the percentage of genes in yeast that have only 1 exon.


2. State the most common number of eons found in mammal genes.

5 exons.

3. With reference to the data, discuss the statement “The amount of noncoding DNA does seem to scale with complexity.”

We have a variety of different eukaryotes available to us in the data provided. We have yeast, a fruit fly (which is a little more complex than yeast), and mammals, which are generally very much more complex than yeast or a small fruit fly. If we consider the difference of these organisms, the amount of noncoding DNA (exons) also have to vary. This is shown in the data. The complexity range in this data goes from yeast to fruit flies to mammals. Similarly in the data, the complexity of the scale of exons increases as we increase in the complexity of the organism. Yeast only has three numbers of exons. Fruit flies have significantly more (11 more numbers) exons. Lastly, mammals have way more numbers of exons than either yeast and fruit flies. This data shows that the amount of noncoding DNA (exons) differs and ranges as complexity grows. The relationship is linear. As complexity increases, the scale of the amount of exons also increases.

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§ One Response to 3.5 & 7.3 DNA Transcription

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