protein synthesis

types of rna

There are three different types of RNA. All three types are needed to make proteins. Messenger RNA (mRNA) copies genetic instructions from DNA in the nucleus. Then it carries the instructions to a ribosome in the cytoplasm. Ribosomal RNA (rRNA) helps form a ribosome. This is where the protein is made. Transfer RNA (tRNA) brings amino acids to the ribosome. The amino acids are then joined together to make the protein.

comparing rna with dna

RNA stands for ribonucleic acid. RNA is smaller than DNA. It can squeeze through pores in the membrane that encloses the nucleus. It copies instructions in DNA and carries them to a ribosome in the cytoplasm. Then it helps build the protein. RNA is not only smaller than DNA. It differs from DNA in other ways as well. It consists of one nucleotide chain rather than two chains as in DNA. It also contains the nitrogen base uracil (U) instead of thymine (T). In addition, it contains the sugar ribose instead of deoxyribose. You can see these differences in Figure 5.16.

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dna rna and proteins

DNA and RNA are nucleic acids. DNA stores genetic information. RNA helps build proteins. Proteins, in turn, determine the structure and function of all your cells. Proteins consist of chains of amino acids. A proteins structure and function depends on the sequence of its amino acids. Instructions for this sequence are encoded in DNA. In eukaryotic cells, chromosomes are contained within the nucleus. But proteins are made in the cytoplasm at structures called ribosomes. How do the instructions in DNA reach the ribosomes in the cytoplasm? RNA is needed for this task.

types of mutations

The effect of a mutation is likely to depend as well on the type of mutation that occurs. A mutation that changes all or a large part of a chromosome is called a chromosomal mutation. This type of mutation tends to be very serious. Sometimes chromosomes are missing or extra copies are present. An example is the mutation that causes Down syndrome. In this case, there is an extra copy of one of the chromosomes. Deleting or inserting a nitrogen base causes a frameshift mutation. All of the codons following the mutation are misread. This may be disastrous. To see why, consider this English-language analogy. Take the sentence The big dog ate the red cat. If the second letter of big is deleted, then the sentence becomes: The bgd oga tet her edc at. Deleting a single letter makes the rest of the sentence impossible to read. Some mutations change just one or a few bases in DNA. A change in just one base is called a point mutation. Table 5.1 compares different types of point mutations and their effects. Type Silent Missense Nonsense Description mutated codon codes for the same amino acid mutated codon codes for a different amino acid mutated codon is a prema- ture stop codon Example CAA (glutamine) ! CAG (glutamine) CAA (glutamine) ! CCA (proline) CAA (glutamine) ! UAA (stop) Effect none variable serious

causes of mutations

Mutations have many possible causes. Some mutations occur when a mistake is made during DNA replication or transcription. Other mutations occur because of environmental factors. Anything in the environment that causes a mutation is known as a mutagen. Examples of mutagens are shown in Figure 5.21. They include ultraviolet rays in sunlight, chemicals in cigarette smoke, and certain viruses and bacteria.

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effects of mutations

Many mutations have no effect on the proteins they encode. These mutations are considered neutral. Occasionally, a mutation may make a protein even better than it was before. Or the protein might help the organism adapt to a new environment. These mutations are considered beneficial. An example is a mutation that helps bacteria resist antibiotics. Bacteria with the mutation increase in numbers, so the mutation becomes more common. Other mutations are harmful. They may even be deadly. Harmful mutations often result in a protein that no longer can do its job. Some harmful mutations cause cancer or other genetic disorders. Mutations also vary in their effects depending on whether they occur in gametes or in other cells of the body. Mutations that occur in gametes can be passed on to offspring. An offspring that inherits a mutation in a gamete will have the mutation in all of its cells. Mutations that occur in body cells cannot be passed on to offspring. They are confined to just one cell and its daughter cells. These mutations may have little effect on an organism.

protein synthesis

The process in which proteins are made is called protein synthesis. It occurs in two main steps. The steps are transcription and translation. Watch this video for a good introduction to both steps of protein synthesis: http://w MEDIA Click image to the left or use the URL below. URL:

transcription dna rna

Transcription is the first step in protein synthesis. It takes place in the nucleus. During transcription, a strand of DNA is copied to make a strand of mRNA. How does this happen? It occurs by the following steps, as shown in Figure 5.19. 1. An enzyme binds to the DNA. It signals the DNA to unwind. 2. After the DNA unwinds, the enzyme can read the bases in one of the DNA strands. 3. Using this strand of DNA as a template, nucleotides are joined together to make a complementary strand of mRNA. The mRNA contains bases that are complementary to the bases in the DNA strand. Translation is the second step in protein synthesis. It is shown in Figure 5.20. Translation takes place at a ribosome in the cytoplasm. During translation, the genetic code in mRNA is read to make a protein. Heres how it works: 1. 2. 3. 4. 5. The molecule of mRNA leaves the nucleus and moves to a ribosome. The ribosome consists of rRNA and proteins. It reads the sequence of codons in mRNA. Molecules of tRNA bring amino acids to the ribosome in the correct sequence. At the ribosome, the amino acids are joined together to form a chain of amino acids. The chain of amino acids keeps growing until a stop codon is reached. Then the chain is released from the ribosome.

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the genetic code

How is the information for making proteins encoded in DNA? The answer is the genetic code. The genetic code is based on the sequence of nitrogen bases in DNA. The four bases make up the letters of the code. Groups of three bases each make up code words. These three-letter code words are called codons. Each codon stands for one amino acid or else for a start or stop signal. There are 20 amino acids that make up proteins. With three bases per codon, there are 64 possible codons. This is more than enough to code for the 20 amino acids plus start and stop signals. You can see how to translate the genetic code in Figure 5.17. Start at the center of the chart for the first base of each three-base codon. Then work your way out from the center for the second and third bases. Find the codon AUG in Figure 5.17. It codes for the amino acid methionine. It also codes for the start signal. After an AUG start codon, the next three letters are read as the second codon. The next three letters after that are read as the third codon, and so on. You can see how this works in Figure 5.18. The figure shows the bases in a molecule

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characteristics of the genetic code

The genetic code has three other important characteristics. The genetic code is the same in all living things. This shows that all organisms are related by descent from a common ancestor. Each codon codes for just one amino acid (or start or stop). This is necessary so the correct amino acid is always selected. Most amino acids are encoded by more than one codon. This is helpful. It reduces the risk of the wrong amino acid being selected if there is a mistake in the code.

instructional diagrams

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questions

Types of mutagens include

a. radiation

b. viruses

c. bacteria

-->  d. all of the above

When a mutated codon codes for a different amino acid, the mutation is called a

a. silent mutation

b. nonsense mutation

-->  c. missense mutation

d. chromosomal mutation

Each gene codes for one

a. codon

b. nucleotide

c. amino acid

-->  d. protein

Which type of RNA carries genetic information from the nucleus to a ribosome?

a. rRNA

b. tRNA

-->  c. mRNA

d. none of the above

The backbone of the DNA molecule consists of

a. sugars

b. phosphates

c. nitrogen bases

-->  d. two of the above

A section of DNA that codes for a protein is called a

a. base.

b. codon.

-->  c. gene.

d. chromosome.

Which statement about RNA is false?

a. RNA stands for ribonucleic acid.

b. RNA is smaller than DNA.

c. RNA can cross the nuclear membrane.

-->  d. none of the above

Each codon in the genetic code

-->  a. consists of three nitrogen bases.

b. codes for three amino acids.

c. represents start and stop.

d. none of the above

During the transcription step of protein synthesis

-->  a. DNA unwinds.

b. DNA is copied to form rRNA.

c. tRNA leaves the nucleus.

d. amino acids are assembled at a ribosome.

Some mutations occur when errors are made in copying DNA.

-->  a. true

b. false

The genetic code is the same in all living things.

-->  a. true

b. false

Mutations may be

a. beneficial.

b. neutral.

c. harmful.

-->  d. any of the above

The translation step of protein synthesis takes place in the nucleus.

a. true

-->  b. false

Which type of mutation has no effect on the organism?

-->  a. silent

b. missense

c. nonsense

d. frameshift

What happens during the translation step of protein synthesis?

-->  a. Amino acids are joined together.

b. The genetic code is carried to the nucleus.

c. A molecule of mRNA forms.

d. DNA moves to a ribosome.

RNA is a double-stranded nucleic acid.

a. true

-->  b. false

Uracil is a nitrogen base found only in RNA.

-->  a. true

b. false

___A nonsense mutation is caused by a premature stop codon.

-->  a. true

b. false

___The function of RNA is to help build proteins.

-->  a. true

b. false

___The genetic code is the sequence of nitrogen bases in DNA.

-->  a. true

b. false

___Down syndrome is caused by a point mutation.

a. true

-->  b. false

___The codon AUG is the start codon.

-->  a. true

b. false

___A mutated codon always codes for a different amino acid.

a. true

-->  b. false

___RNA contains the sugar deoxyribose.

a. true

-->  b. false

_nitrogen base found only in DNA

a. uracil

b. rRNA

c. RNA

d. mRNA

-->  e. thymine

f. DNA

g. tRNA

_type of RNA that copies DNA in the nucleus

a. uracil

b. rRNA

c. RNA

-->  d. mRNA

e. thymine

f. DNA

g. tRNA

_double-stranded nucleic acid

a. uracil

b. rRNA

c. RNA

d. mRNA

e. thymine

-->  f. DNA

g. tRNA

_nitrogen base found only in RNA

-->  a. uracil

b. rRNA

c. RNA

d. mRNA

e. thymine

f. DNA

g. tRNA

_type of RNA that helps form a ribosome

a. uracil

-->  b. rRNA

c. RNA

d. mRNA

e. thymine

f. DNA

g. tRNA

_any single-stranded nucleic acid

a. uracil

b. rRNA

-->  c. RNA

d. mRNA

e. thymine

f. DNA

g. tRNA

_type of RNA that brings amino acids to a ribosome

a. uracil

b. rRNA

c. RNA

d. mRNA

e. thymine

f. DNA

-->  g. tRNA

diagram questions

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