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Transcription and Translation

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Brandy Faye Lane

on 21 January 2014

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Transcript of Transcription and Translation

Transcription and Translation
Brandy Lane
Emelyn Walker
Dominique Lawrence

RNA carries DNA instructions
Transcription makes three types of RNA
The transcription process is similar to replication
Amino acids are coded by mRNA base sequences
Amino acids are linked to become a protein
Explain the base processes of transcription and translation, and how they result in the expression of genes.
Key COncepts:
Transcription converts a gene into a single-stranded RNA molecule (8.4).
Translation converts an mRNA message into a polypeptide, or protein (8.5).
is the process that converts, or translates, an mRNA message into a polypeptide. One or more polypeptides make up a protein. Nucleic acids uses four nucleotides (A, G, C, &T in DNA, or A, G, C ,&U in RNA). Proteins use twenty amino acids. The only way four nucleotides can code for twenty amino acids is using codons. A
is a three-nucleotide sequence that codes for an amino acid.
Many amino acids are coded for by more than one codon. There is a pattern to the codons. In most cases, codons that represent the same amino acid share the same first two nucleotides. The first two nucleotides are generally the most important in coding for an amino acid, and this makes DNA more tolerant of many mutations.
In addition to these codons that code for amino acids, there are three
stop codons
(which signals the end of the amino acid chain) and one
start codon
(which signals the start of translation).
For the mRNA code to be read correctly, codons must be read in the right order. Codons are read, without spaces, as a series of three non-overlapping nucleotides. This order is called the reading frame. Changing the reading frame completely changes the resulting protein. Therefore, pa clear start codon plays an important role in the genetic code.
Translation Diagram
A cell translates a codon into an amino acid by using ribosomes and tRNA molecules. Ribosomes are made of a combination of rRNA and proteins, and they catalyze the reaction that forms the bonds between amino acids. Ribosomes have a large and small subunit that fit together and pull the mRNA strand through. The small subunit holds onto the mRNA strand, and the large subunit holds on to the growing protein.
The tRNA acts as a type of the adaptor between the mRNA and amino acids. Cells need tRNA to carry free floating amino acids from the cytoplasm to the ribosomes. Then the tRNA molecules fold up in a characteristics of an L shape. On one end of the L is attached to a specific amino acid. On the other end, called the anticodon, was recognized as a specific codon. An
is a set of three nucleotides that is complementary to the mRNA codon.
Translation takes place in the cytoplasm in both prokaryotic and eukaryotic cells. A small ribosomal subunit must, first, bind to an mRNA strand in the cytoplasm. Then, a tRNA with methionine binds to the AUG start codon. This signals a large ribosomal subunit to join and pulls the mRNA strand through itself one codon at a time. As the strand moves, the start codon and its complementary tRNA shift to the second site inside a large subunit. This leaves the first site empty, which exposes the next mRNA codon.

1. The exposed codon attracts a complementary tRNA molecule bearing an amino acid. The tRNA anticodon pairs with the mRNA codon. This brings the new tRNA molecule very close to the tRNA molecule occupying the second site.
2. Next, the ribosome helps form a peptide bond between the two amino acids. The ribosome then breaks the bond between the tRNA molecule in the second site and its amino acid.
3. The ribosome pulls the mRNA strand the length of one codon. The tRNA molecule in the second site is shifted into the third site, which is the exit site. The tRNA leaves the ribosome and returns to the cytoplasm to be charged with another amino acid. The tRNA molecule that was in the first site shifts into the second site. The first site is again empty, exposing the next mRNA codon.
Francis Crick defined the
central dogma
of molecular biology after his discovery of DNA structure. It states that information flows in one direction (from DNA to RNA to proteins), and involves these three processes:
Replication copies DNA
Transcription converts a DNA message into an intermediate molecule, called RNA
Translation interprets an RNA message, called a polypeptide.
RNA acts as an intermediate link between DNA in the nucleus and protein synthesis in the cytoplasm. Like DNA,
(ribonucleic acid) is a chain of nucleotides. You can think of RNA as a temporary copy of DNA that is used and then destroyed. However, they differ in three significant ways:
The sugar in RNA is ribose, which has one additional oxygen atom not present in DNA's sugar.
RNA has the base uracil in place of thymine. Uracil, like thymine, is forms base pairs with adenine.
RNA is a single strand of nucleotides, in contrast to the double-stranded structure of DNA. This single-stranded structure allows some types of RNA to form complex three-dimensional shapes. As a result, some RNA molecules can catalyze reactions as much as enzymes do.
is the process of copying a sequence of DNA to produce a complementary strand of RNA. During the process of transcription, a gene (not an entire chromosome) is transferred into an RNA message. Just as replication is catalyzed by DNA polymerase, transcription is catalyzed by
RNA polymerases
(enzymes that bond nucleotides together in a chain to make a new RNA molecule.
In prokaryotic cells, replication, trancsription, and translation all occur in the cytoplasm at about the same time. In eukaryotic cells, these processes are seperated both in time and location. Replication and transcription occur in the nucleus, while translations occurs in the cytoplasm. Also, the RNA in eukaryotic cells goes through a processing step before it can be transported out of the nucleus.
With the help of other proteins and DNA sequences, RNA polymerase recognizes the transcription start site of a gene. A large transcription complex consisting of RNA polymerase and other proteins assembles on the DNA strand and begins to unwind the segment of the DNA molecule, until the two strands seperate from eachother.
STEP two
RNA polymerase, using only one strand of DNA as a template, strings together a complementary strand of RNA nucleotides. RNA base pairing follows the same rules as DNA base pairing, except that Uracil, not thymine, pairs with adenine. The growing RNA strand hangs freely as it transcribed, and the DNA helix zips back together.
step three
Once the entire gene has been transcribed, the RNA strand detaches completely from the DNA. Exactly how RNA polymerase recognizes the end of a transcription unit is complicated, and it varies with the type of RNA.
Transcription produces three major types of RNA molecules. Not all RNA molecules code for proteins, but most play an important role in the translation process. Each type of RNA molecule has a unique function.
Messenger RNA
(mRNA): an intermediate message that is translated to form a protein.
Ribosomal RNA
(rRNA): froms part of ribosomes (a cell's protein factories).
Transfer RNA
(tRNA): brings amino acids from the cytoplasm to a ribosome to help make the growing protein.
types of rna molecules
The processes of transcription and replication share many similarities.
Both processes occur in within the nucleus of eukaryotic cells.
Both are catalyzed by large, complex enzymes.
Both involve unwinding of the DNA double helix.
Both involve complementary base pairing to the DNA strand.
Both processes are highly regulated by the cell.
Just as a cell does not replicate its DNA without passing a critical checkpoint, so, too, a cell carefully regulates which genes are transcribed into RNA.
The end result of transcription and replication, however, are quite different. Replication ensures that each new cell will have one complete set of genetic instructions by making identical sets of double-stranded chromosomes. This double-stranded structure makes DNA especially well suited for long-term storage because it helps protect DNA from being broken down and from potentially harmful interactions with other molecules. Replication occurs only once during each round of the cell cycle because each cell needs to make only one copy of its DNA.
In contrast, a cell may need hundreds or thousands of copies of certain proteins, or the rRNA and tRNA molecules needed to make proteins. Transcription enables a cell to adjust to changing demands by making a single-stranded complement of only a segment of DNA and only when that particular strand is needed. In addition, many RNA molecules can be transcribed from a single gene at the same time to help produce more protein. Once RNA polymerase had transcribed one portion of a gene and has moved on, another RNA polymerase can attach itself to the beginning of the gene and start the transcription process again.
transcription diagram
TEM showing DNA being transcribed into numerous RNA strands.
How do DNA and RNA differ?

1. RNA uses ribose as its sugar (instead of the deoxyribose DNA uses)
2. RNA uses the base uracil (instead of the thymine DNA uses)
3. RNA is a single strand of nucleotides (instead of the double-stranded structure of DNA)
Explain why transcription occurs in the nucleus of eukaryotes.
The nucleus holds the DNA of the eukaryotic cell, and transcription creates a complementary copy of part of that DNA. Therefore, transcription needs to occur in the nucleus for access to the cells DNA.
How are the processes of transcription and replication similar?
Both processes occur within the nucleus of eukaryotic cells.
Both are catalyzed by large, complex enzymes.
Both involve the unwinding of the DNA double helix.
Both involve complementary base pairing to the DNA strand.
Both processes are highly regulated by the cell.
What is the central dogma?
central dogma:
a theory that states that, in cells, information only flows from DNA to RNA proteins.
why can the mRNA strand made during transcription be thought of as a mirror image of the dna strand from which it was made?
The DNA strand codes for the mRNA strand and acts as its template.
why might a cell make lots of rRNA but only one copy of DNA?
Basically, lots of rRNA means more ribosomes and more proteins being made. You only need one copy of DNA to be transcripted into mRNA and make more proteins.
if a DNA segment has the nucleotides agcctaa, what would be the nucleotide sequence of the complementary RNA strand?
What might geneticists learn about genes by studying RNA?
By studying RNA the geneticists can figure out BOTH DNA sequence and protein amino acid sequence, since RNA is the midway of biological information flow. The genetic code can be expressed as RNA, as a result.
translation process
suppose an mRNA molecule in the cytoplasm had 300 nucleotides. How many amino acids would be in the resulting protein?
Three nucleotides make a codon which codes for an amino acid. Looking at codon table, you can see how each of the possible sixty-four combinations map into one of the twenty-two amino acids, or one of three stops. Therefore, the most possible is 300/100 (or three) with no stops.
Explain the different roles of the large and small ribosomal subunits.
The large subunit binds to the tRNA, while small subunit binds to mRNA.
explain the connection between a codon and an amino acid.
A codon codes for an amino acid.
Briefly describe how the processes of translation is started.
A small ribosomal subunit binds to an mRNA strand in the cytoplasm. Next, a tRNA with methionine attached binds to the AUG start codon. This binding signals a large ribosomal subunit to join. The ribosome pulls the mRNA strand through itself one codon at a time. As the strand moves, the start codon and its complementary tRNA molecule shift into the second site inside the large subunit. This shift leaves the first site empty, which exposes the next mRNA codon.
suppose a tRNA molecule had the anticodon AGU. WHat amino a cidwould it carry?
serine (Ser)
the dna of eukaryotic cells has many copies of genes that code for rRNA molecules. Suggest a hypothesis to explain why a cell needs so many copies of these genes.
DNA is responsible for such protein like whether it is recessive or dominant (such as hair color, eye color.) rRNA makes the exact copy of that of the DNA. rRNA is single stranded when rRNA receive the copies of DNA it gives to tRNA.
How do the processes of transcription/translation result in the production of amino acids?
Essential Question
Nowicki, Stephen. "Chapter 8.4 and Chapter 8.5." Holt McDougal Biology. Austin, TX: Holt McDougal, 2010. N. pag. Print.
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