122 Flashcards
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Concept Card
➢ Genes provide information for building proteins. They don’t however directly create proteins. The production of proteins is completed through two processes: transcription and translation.
➢ Transcription and translation take the information in DNA and use it to produce proteins. Transcription uses a strand of DNA as a template to build a molecule called RNA.
➢ The RNA molecule is the link between DNA and the production of proteins. During translation, the RNA molecule created in the transcription process delivers information from the DNA to the protein-building machines. 1/16/2025 2
➢ Each strand of the DNA double helix contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand. Each strand can therefore act as a template, or mold, for the synthesis of a new complementary strand (Figure 6–2). In other words, if we designate the two DNA strands as S and S, strand S can serve as a template for making a new strand S, while strand S` can serve as a template for making a new strand S (Figure 6–3).
➢ Thus, the genetic information in DNA can be accurately copied by the beautifully simple process in which strand S separates from strand S`, and each separated strand then serves as a template for the production of a new complementary partner strand that is identical to its former partner.
➢ DNA replication produces two complete double helices from the original DNA molecule, each new DNA helix identical (except for rare copying errors) in nucleotide sequence to the parental DNA double helix. 1/16/2025 6 Figure 6–3 DNA acts as a template for its own duplication. Because Figure 6–2 A DNA strand can serve as a the nucleotide A will successfully pair only with T, and G with C, each template. Preferential binding occurs between strand of DNA in the double helix—labeled here as the S strand and its pairs of nucleotides (A with T, and G with C) that complementary S’ strand—can serve as a template to specify the can form base pairs. This enables each strand to sequence of nucleotides in its complementary strand. In this way, act as a template for forming its complementary double-helical DNA can be copied precisely. Keep in mind that although strand. they are colored differently here, the template strands (orange) and the new strands (red) are chemically identical. Page 173 1/16/2025 7
➢ The DNA double helix is normally very stable: the two DNA strands are locked together firmly by the hydrogen bonds formed between the bases on each strand. To begin DNA replication, the double helix must first be opened up and the two strands separated to expose unpaired bases. As we shall see, the process of DNA replication is begun by special initiator proteins that bind to double-stranded DNA and pry the two strands apart, breaking the hydrogen bonds between the bases.
➢ The positions at which the DNA helix is first opened are called replication origins. In simple cells like those of bacteria or yeast, origins are specified by DNA sequences several hundred nucleotide pairs in length. This DNA contains both short sequences that attract initiator proteins and stretches of DNA that are especially easy to open. There is an A-T base pair held together by fewer hydrogen bonds than is a G-C base pair. Therefore, DNA rich in A-T base pairs is relatively easy to pull apart, and regions of DNA enriched in A-T base pairs are typically found at replication origins. 1/16/2025 8 Stages of DNA replication DNA replication can be thought of in three stages: initiation, elongation and termination 1) Initiation
➢ DNA synthesis is initiated at particular points within the DNA strand known as ‘origins’, which have specific coding regions. These origins are targeted by initiator proteins, which go on to recruit more proteins that help aid the replication process, forming a replication complex around the DNA origin. Multiple origin sites exist within the DNA’s structure; when replication of DNA begins, these sites are referred to as replication forks.
➢ Within the replication complex is the DNA helicase. This enzyme unwinds the double helix and exposes each of the two strands so that they can be used as a template for replication. It does this by hydrolyzing the ATP used to form the bonds between the nucleobases, thereby breaking the bond holding the two strands together.
➢ DNA primase is another enzyme that is important in DNA replication. It synthesizes a small RNA primer, which acts as a ‘kick-starter’ for DNA polymerase. This enzyme is ultimately responsible for the creation and expansion of new strands of DNA. 1/16/2025 11 Stages of DNA replication 2) Elongation
➢ Once DNA Polymerase has attached to the two unzipped strands of DNA (i.e. the template strands), it is able to start synthesizing new strands of DNA to match the templates. DNA polymerase is only able to extend the primer by adding free nucleotides to the 3’ end.
➢ One of the template strands is read in a 3’ to 5’ direction, therefore the new strand will be formed in a 5’ to 3’ direction. This newly formed strand is referred to as the leading strand. Along the leading strand, DNA primase only needs to synthesize an RNA primer once, at the beginning, to initiate DNA polymerase. This is because DNA polymerase is able to extend the new DNA strand by reading the template 3′ to 5′, synthesizing in a 5′ to 3′ direction as noted above.
➢ However, the other template strand (the lagging strand) is antiparallel and is therefore read in a 5’ to 3’ direction. Continuous DNA synthesis, as in the leading strand, would need to be in the 3′ to 5′ direction, which is impossible as DNA polymerase cannot add bases to the 5′ end. Instead, as the helix unwinds, RNA primers are added to the newly exposed bases on the lagging strand and DNA synthesis occurs in fragments, but still in the 5′ to 3′ direction as before. 1/16/2025 These fragments are known as Okazaki fragments. 12 Stages of DNA replication 3) Termination
➢ The process of expanding the new DNA strands continues until there is either no more DNA template strand left to replicate (i.e. at the end of the chromosome) or two replication forks meet and subsequently terminate. The meeting of two replication forks is not regulated and happens randomly along the course of the chromosome.
➢ Once DNA synthesis has finished, the newly synthesized strands are bound and stabilized. For the lagging strand, two enzymes are needed to achieve this stabilization: RNAase H removes the RNA primer at the beginning of each Okazaki fragment, and DNA ligase joins these fragments together to create one complete strand. 1/16/2025 13 RNA polymerase is the enzyme responsible for making mRNA copies of genes Figure 7–6 Transcription produces an RNA complementary to one strand of DNA. The nontemplate strand of the DNA (the top strand in this example) is sometimes called the coding strand because its sequence is equivalent to the RNA product Page 352 1/16/2025 15 Translation is the process by which the genetic code contained within a messenger RNA (mRNA) molecule is decoded to produce a specific sequence of amino acids in a polypeptide chain.
➢ It occurs in the cytoplasm following DNA transcription and, like transcription, has three stages: initiation, elongation, and termination. In this article, we will discuss the components and stages of DNA translation. Components of Translation
➢ The key components required for translation are mRNA, ribosomes, and transfer RNA (tRNA).
➢ During translation, mRNA nucleotide bases are read as codons of three bases. Each codon codes for a particular amino acid.
➢ Every tRNA molecule possesses an anticodon that is complementary to the mRNA codon, and at the opposite end lies the attached amino acid. tRNA molecules are therefore responsible for bringing amino acids to the ribosome in the correct order, ready for polypeptide assembly. Page 247 1/16/2025 16
➢ A single amino acid may be coded for by more than one codon. There are also specific codons that signal the start and the end of translation.
➢ Aminoacyl-tRNA synthetases are enzymes that link amino acids to their corresponding tRNA molecules. The resulting complex is charged and is referred to as an aminoacyl-tRNA. 1) Initiation
➢ For translation to begin, the start codon (5’AUG) must be recognized. This codon is specific to the amino acid methionine, which is nearly always the first amino acid in a polypeptide chain.
