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Concepts of Biology

Book by: OpenStax College

In both prokaryotes and eukaryotes, the second function of DNA (the first was replication) is to provide the information needed to construct the proteins necessary so that the cell can perform all of its functions. To do this, the DNA is “read” or transcribed into an mRNA molecule. The mRNA then provides the code to form a protein by a process called translation. Through the processes of transcription and translation, a protein is built with a specific sequence of amino acids that was originally encoded in the DNA. This module discusses the details of transcription.

Central Dogma: DNA Encodes RNA; RNA Encodes Protein

The flow of genetic information in cells from DNA to mRNA to protein is described by the central dogma (Figure), which states that genes specify the sequences of mRNAs, which in turn specify the sequences of proteins.

Figure 1. The central dogma states that DNA encodes RNA, which in turn encodes protein.

Question 1

1. What type of nucleic acid material is analyzed in forensic cases?

1. What type of nucleic acid material is analyzed in forensic cases?

Answer

What type of nucleic acid material is analyzed in forensic cases?

1

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Label

b

Nuclear chromosomal DNA

a

Cytoplasmic rRNA

c

Mitochondrial DNA

d

Nuclear mRNA

What type of nucleic acid material is analyzed in forensic cases?

1

Label

b

Nuclear chromosomal DNA

c

Mitochondrial DNA

d

Nuclear mRNA

Cytoplasmic RNA

What type of nucleic acid material is analyzed in forensic cases?

1

Next Question

DNA is used in forensic cases and not RNA.

Label

a

Nuclear chromosomal DNA

c

Mitochondrial DNA

d

Nuclear mRNA

Cytoplasmic rRNA

What type of nucleic acid material is analyzed in forensic cases?

1

Next Question

Forensics looks at the nuclear genetic material.

Label

a

Nuclear chromosomal DNA

b

Mitochondrial DNA

d

Nuclear mRNA

Cytoplasmic rRNA

Mitochondrial DNA would be shared by many people with the same maternal lineage.

What type of nucleic acid material is analyzed in forensic cases?

1

Next Question

Label

b

Nuclear chromosomal DNA

c

Mitochondrial DNA

Nuclear mRNA

a

Cytoplasmic rRNA

What type of nucleic acid material is analyzed in forensic cases?

1

Next Question

mRNA is not used in forensics.  DNA is used in forensics.

Question 2

2. The diagram shows different regions (1-5) of a pre-mRNA molecule....

2. The diagram shows different regions (1-5) of a pre-mRNA molecule....

Answer

The diagram shows different regions (1-5) of a pre-mRNA molecule, a mature-mRNA molecule, and the protein corresponding to the mRNA.A mutation in region ___ is most likely to be damaging to the cell.

2

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b

1

a

2

c

3

d

5

The diagram shows different regions (1-5) of a pre-mRNA molecule, a mature-mRNA molecule, and the protein corresponding to the mRNA.A mutation in region ___ is most likely to be damaging to the cell.

2

Label

b

1

c

3

d

5

2

Region 2 seems to be encoding a gene.  Any mutation in this region would likely produce a non-functional protein, damaging the cell. 

Next Question

The diagram shows different regions (1-5) of a pre-mRNA molecule, a mature-mRNA molecule, and the protein corresponding to the mRNA.A mutation in region ___ is most likely to be damaging to the cell.

2

Label

a

1

c

3

d

5

2

Next Question

The diagram shows different regions (1-5) of a pre-mRNA molecule, a mature-mRNA molecule, and the protein corresponding to the mRNA.A mutation in region ___ is most likely to be damaging to the cell.

2

A mutation in region 1 will not damage the cell.

Label

a

1

b

3

d

5

2

Next Question

The diagram shows different regions (1-5) of a pre-mRNA molecule, a mature-mRNA molecule, and the protein corresponding to the mRNA.A mutation in region ___ is most likely to be damaging to the cell.

2

Region 3 is present between exons 2 and 4. It is an intron, the non-coding sequence.

Label

b

1

c

3

5

a

2

Next Question

The diagram shows different regions (1-5) of a pre-mRNA molecule, a mature-mRNA molecule, and the protein corresponding to the mRNA.A mutation in region ___ is most likely to be damaging to the cell.

2

UTR is important for the process of translation and region 5 does not encode a gene.

Question 3

3. Label regions 1-5 in the diagram as introns or exons.

3. Label regions 1-5 in the diagram as introns or exons.

Answer

Label regions 1-5 in the diagram as introns or exons.

3

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Label

b

2 and 4 are introns; 1,3, and 5 are exons

a

1, 3, and 5 are introns; 2 and 4 are exons

c

1 and 5 are introns; 2, 3, and 4 are exons

d

2, 3, and 4 are introns; 1 and 5 are exons

Label regions 1-5 in the diagram as introns or exons.

3

Label

b

2 and 4 are introns; 1,3, and 5 are exons

c

1 and 5 are introns; 2, 3, and 4 are exons

d

2, 3, and 4 are introns; 1 and 5 are exons

Regions 1, 3 and 5 are the introns. These are the DNA regions that do not encode any part of the protein. Regions 2 and 4 encode the resulting protein, so these regions are exons.

Done

Label regions 1-5 in the diagram as introns or exons.

3

1, 3, and 5 are introns; 2 and 4 are exons

Label

2 and 4 are introns; 1,3, and 5 are exons

c

1 and 5 are introns; 2, 3, and 4 are exons

d

2, 3, and 4 are introns; 1 and 5 are exons

1, 3, and 5 are introns; 2 and 4 are exons

Introns do not code for a protein. Therefore, regions 2 and 4 cannot be introns.

Done

Label regions 1-5 in the diagram as introns or exons.

3

a

Label

2 and 4 are introns; 1,3, and 5 are exons

b

d

2, 3, and 4 are exons; 1 and 5 are introns.

1, 3, and 5 are introns; 2 and 4 are exons

Only exons code for a protein. Therefore, regions 3 is not an exon.

Done

Label regions 1-5 in the diagram as introns or exons.

3

a

1 and 5 are introns; 2, 3, and 4 are exons

Label

b

2 and 4 are introns; 1,3, and 5 are exons

c

1 and 5 are introns; 2, 3, and 4 are exons

a

2, 3, and 4 are introns; 1 and 5 are exons

1, 3, and 5 are introns; 2 and 4 are exons

Introns do not code for a protein. Therefore, regions 2 and 4 cannot be introns. Also, exons always code for proteins. Therefore, regions 1 and 5 cannot be exons.

Done

Label regions 1-5 in the diagram as introns or exons.

3

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Introduction to Biology

Themes and Concepts of Biology

The Process of Science

 

1

1.1

1.2

 

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The Chemistry of Life

The Building Blocks of Molecules

Water

Biological Molecules

 

2

2.1

2.2

2.3

 

Cell Structure and Function

How Cells are Studied

Comparing Prokaryotic and Eukaryotic Cells

Eukaryotic Cells

The Cell Membrane

Passive Transport

Active Transport

3

3.1

3.2

3.3

3.4

3.5

3.6

How Cells Obtain Energy

Energy and Metabolism

Glycolysis

Citric Acid Cycle and Oxidative Phosphorylation

Fermentation

Connections to Other Metabolic Pathways

4

4.1

4.2

4.3

4.4

4.5

Photosynthesis

Overview of Photosynthesis

The Light-Dependent Reactions of Photosynthesis

The Calvin Cycle

5

5.1

5.2

5.3

Reproduction at the Cellular Level

The Genome

The Cell Cycle

Cancer and the Cell Cycle

Prokaryotic Cell Division

6

6.1

6.2

6.3

6.4

The Cellular Basis of Inheritance

Sexual Reproduction

Meiosis

Errors in Meiosis

7

7.1

7.2

7.3

Patterns of Inheritance

Mendel's Experiments

Laws of Inheritance

Extensions of the Laws of Inheritance

8

8.1

8.2

8.3

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Molecular Biology

The Structure of DNA

DNA Replication

9

9.1

9.2

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Johnny

The copying of DNA to mRNA is relatively straightforward, with one nucleotide being added to the mRNA strand for every complementary nucleotide read in the DNA strand. The translation to protein is more complex because groups of three mRNA nucleotides correspond to one amino acid of the protein sequence. However, as we shall see in the next module, the translation to protein is still systematic, such that nucleotides 1 to 3 correspond to amino acid 1, nucleotides 4 to 6 correspond to amino acid 2, and so on.

Transcription: from DNA to mRNA

Both prokaryotes and eukaryotes perform fundamentally the same process of transcription, with the important difference of the membrane-bound nucleus in eukaryotes. With the genes bound in the nucleus, transcription occurs in the nucleus of the cell and the mRNA transcript must be transported to the cytoplasm. The prokaryotes, which include bacteria and archaea, lack membrane-bound nuclei and other organelles, and transcription occurs in the cytoplasm of the cell. In both prokaryotes and eukaryotes, transcription occurs in three main stages: initiation, elongation, and termination.

Initiation

 

Transcription requires the DNA double helix to partially unwind in the region of mRNA synthesis. The region of unwinding is called a transcription bubble. The DNA sequence onto which the proteins and enzymes involved in transcription bind to initiate the process is called a promoter. In most cases, promoters exist upstream of the genes they regulate. The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all of the time, some of the time, or hardly at all (Figure).

Figure 2. The initiation of transcription begins when DNA is unwound, forming a transcription bubble. Enzymes and other proteins involved in transcription bind at the promoter.

Elongation

 

Transcription always proceeds from one of the two DNA strands, which is called the template strand. The mRNA product is complementary to the template strand and is almost identical to the other DNA strand, called the nontemplate strand, with the exception that RNA contains a uracil (U) in place of the thymine (T) found in DNA. During elongation, an enzyme called RNA polymerase proceeds along the DNA template adding nucleotides by base pairing with the DNA template in a manner similar to DNA replication, with the difference that an RNA strand is being synthesized that does not remain bound to the DNA template. As elongation proceeds, the DNA is continuously unwound ahead of the core enzyme and rewound behind it (Figure).

Figure 3. During elongation, RNA polymerase tracks along the DNA template, synthesizes mRNA in the 5' to 3' direction, and unwinds then rewinds the DNA as it is read.

Termination

 

Once a gene is transcribed, the prokaryotic polymerase needs to be instructed to dissociate from the DNA template and liberate the newly made mRNA. Depending on the gene being transcribed, there are two kinds of termination signals, but both involve repeated nucleotide sequences in the DNA template that result in RNA polymerase stalling, leaving the DNA template, and freeing the mRNA transcript.

 

On termination, the process of transcription is complete. In a prokaryotic cell, by the time termination occurs, the transcript would already have been used to partially synthesize numerous copies of the encoded protein because these processes can occur concurrently using multiple ribosomes (polyribosomes) (Figure). In contrast, the presence of a nucleus in eukaryotic cells precludes simultaneous transcription and translation.

Figure 4. Multiple polymerases can transcribe a single bacterial gene while numerous ribosomes concurrently translate the mRNA transcripts into polypeptides. In this way, a specific protein can rapidly reach a high concentration in the bacterial cell.

Eukaryotic RNA Processing

The newly transcribed eukaryotic mRNAs must undergo several processing steps before they can be transferred from the nucleus to the cytoplasm and translated into a protein. The additional steps involved in eukaryotic mRNA maturation create a molecule that is much more stable than a prokaryotic mRNA. For example, eukaryotic mRNAs last for several hours, whereas the typical prokaryotic mRNA lasts no more than five seconds.

 

The mRNA transcript is first coated in RNA-stabilizing proteins to prevent it from degrading while it is processed and exported out of the nucleus. This occurs while the pre-mRNA still is being synthesized by adding a special nucleotide “cap” to the 5' end of the growing transcript. In addition to preventing degradation, factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.

 

Once elongation is complete, an enzyme then adds a string of approximately 200 adenine residues to the 3' end, called the poly-A tail. This modification further protects the pre-mRNA from degradation and signals to cellular factors that the transcript needs to be exported to the cytoplasm.

 

Eukaryotic genes are composed of protein-coding sequences called exons (ex-on signifies that they are expressed) and intervening sequences called introns (int-ron denotes their intervening role). Introns are removed from the pre-mRNA during processing. Intron sequences in mRNA do not encode functional proteins. It is essential that all of a pre-mRNA’s introns be completely and precisely removed before protein synthesis so that the exons join together to code for the correct amino acids. If the process errs by even a single nucleotide, the sequence of the rejoined exons would be shifted, and the resulting protein would be nonfunctional. The process of removing introns and reconnecting exons is called splicing (Figure). Introns are removed and degraded while the pre-mRNA is still in the nucleus.

Figure 5. Eukaryotic mRNA contains introns that must be spliced out. A 5' cap and 3' tail are also added.

Section Summary

In prokaryotes, mRNA synthesis is initiated at a promoter sequence on the DNA template. Elongation synthesizes new mRNA. Termination liberates the mRNA and occurs by mechanisms that stall the RNA polymerase and cause it to fall off the DNA template. Newly transcribed eukaryotic mRNAs are modified with a cap and a poly-A tail. These structures protect the mature mRNA from degradation and help export it from the nucleus. Eukaryotic mRNAs also undergo splicing, in which introns are removed and exons are reconnected with single-nucleotide accuracy. Only finished mRNAs are exported from the nucleus to the cytoplasm.

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