DNA Structure
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Imagine DNA as a set of instructions or a blueprint for building and maintaining living things, like plants, animals, and even us humans! The structure of DNA is like a twisted ladder, known as a "double helix." DNA is talked about as if we have known the structure for as long as we have known about atoms and molecules. But it wasn't until 1953 that the structure of DNA was discovered by two scientists, James Watson and Francis Crick.
Think of DNA as a long, thin chain made up of small individual chain links called nucleotides. These nucleotides come in four different types: adenine (A), thymine (T), guanine (G), and cytosine (C). Nucleotides pair up with each other in a specific way: adenine always pairs with thymine, and guanine always pairs with cytosine. It's like they have a special bond with their partner. Each nucleotide partnership is called a base pair. Now, let's talk about the twisted ladder part. Imagine that you have two chains twisted together to form a shape like a spiral staircase. That's what DNA looks like! The sides of the ladder are made of sugar and phosphate molecules, while the rungs (steps) of the ladder are the paired nucleotides we talked about earlier. This twisted ladder shape is very important because it keeps the genetic information safe and organized. It's like a strong protective armor that prevents damage to the precious genetic code inside. Each long strand of DNA can be coiled into a single chromosome. All the chromosomes in a cell make up your individual genome. All the information that is stored in your DNA is kept safe within the nucleus of the cell. Every living thing has its own unique DNA sequence, and this sequence of nucleotides is what makes each living thing special and different from others. The order of the nucleotides determines our traits, like eye color, hair type, and much more. In summary, DNA is like a double helix, a twisted ladder with nucleotides as building blocks, and it holds the instructions for building and maintaining all living things. It's a fascinating and essential molecule that makes life possible!
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A nucleotide consists of base, sugar, and phosphate group. OpenStax College / Wikimedia Commons / CC BY 3.0
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Protein Synthesis
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DNA is like a recipe book that contains all the instructions needed to make proteins, which are essential for the functioning and structure of our bodies. The process of making proteins from DNA is called protein synthesis, and it involves two main steps: transcription and translation.
1. Transcription: The first step takes place in the cell nucleus. Imagine the DNA in the nucleus as the recipe book, and the specific recipe we need to make a protein is the gene. Genes are segments of DNA that contain the instructions for a particular protein. During transcription, a special molecule called RNA (Ribonucleic Acid) is created based on the information in the gene. RNA is similar to DNA but has a slightly different structure. It acts as a messenger that carries the instructions from the DNA in the nucleus to the protein-making factory, which is located in the cytoplasm of the cell. 2. Translation: The second step, translation, occurs in the cell's cytoplasm on small structures called ribosomes. The messenger RNA (mRNA) moves from the nucleus to the ribosome, bringing the instructions for making the protein. Now, we have another type of RNA called transfer RNA (tRNA). tRNA acts as a helper that brings the necessary building blocks, called amino acids, to the ribosome. Each tRNA is specific to a particular amino acid, just like a key that fits into a lock. The ribosome reads the instructions on the mRNA and matches each triplet of nucleotides (called a codon) with the correct tRNA and its attached amino acid. This process continues, step by step, until the entire protein is assembled. The amino acids are linked together like beads on a string, forming a long chain. Once the ribosome reaches the end of the mRNA, the protein chain is complete. It then folds into a specific shape, and that shape determines its function in the cell. In summary, DNA is used to make proteins through a two-step process: transcription, where an RNA copy of the gene is made, and translation, where the RNA instructions are read by ribosomes to assemble a chain of amino acids into a functional protein. This process is crucial for the proper functioning of living organisms. |
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Transcription: a detailed lookTranscription takes place in the cell's nucleus, where the genetic information encoded in DNA is used to create a messenger RNA (mRNA) molecule. This mRNA carries the instructions from the DNA to the ribosomes, where they are used to make proteins during the process of translation.
Let's dive into the steps of transcription: |
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1. Initiation:
Transcription begins when a special enzyme called RNA polymerase binds to a specific region of the DNA called the promoter. The promoter acts like a signal, telling the RNA polymerase where to start transcribing the gene. Once the RNA polymerase is in place, it starts to "unzip" a small portion of the DNA double helix to expose the gene's sequence.
2. Elongation:
Now that the DNA is "unzipped," the RNA polymerase starts building the mRNA molecule. It reads the DNA sequence, using one strand of the DNA as a guide. The RNA polymerase builds the mRNA molecule by adding RNA nucleotides one by one that are complementary to the DNA strand. In DNA, A is complementary to T and C is complementary to G. But RNA nucleotides are slightly different from DNA nucleotides – RNA doesn't have thymine (T) but has uracil (U) instead. This means that if the DNA has an adenine then the complementary RNA nucleotide will be uracil.
Transcription begins when a special enzyme called RNA polymerase binds to a specific region of the DNA called the promoter. The promoter acts like a signal, telling the RNA polymerase where to start transcribing the gene. Once the RNA polymerase is in place, it starts to "unzip" a small portion of the DNA double helix to expose the gene's sequence.
2. Elongation:
Now that the DNA is "unzipped," the RNA polymerase starts building the mRNA molecule. It reads the DNA sequence, using one strand of the DNA as a guide. The RNA polymerase builds the mRNA molecule by adding RNA nucleotides one by one that are complementary to the DNA strand. In DNA, A is complementary to T and C is complementary to G. But RNA nucleotides are slightly different from DNA nucleotides – RNA doesn't have thymine (T) but has uracil (U) instead. This means that if the DNA has an adenine then the complementary RNA nucleotide will be uracil.
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For example, if the DNA strand has the sequence:
T A C A C A A AC The complementary mRNA strand would be: A U G U G U U G |
3. Termination:
Transcription continues until the RNA polymerase reaches a specific sequence on the DNA called the terminator. This terminator sequence signals the RNA polymerase to stop transcribing the gene. Once the RNA polymerase reaches the terminator, it releases the newly formed mRNA molecule, and the DNA strands come back together, closing the double helix. The RNA polymerase can bind to another promotor on the same or a different gene and begin the process all over again.
4. Post-transcriptional processing (in eukaryotic cells):
In eukaryotic cells (cells with a nucleus), the mRNA molecule undergoes some additional modifications before it can be used for translation. This includes the removal of non-coding regions called introns and the joining together of coding regions called exons through a process called splicing. After splicing, the mature mRNA molecule is ready to leave the nucleus and travel to the ribosomes in the cytoplasm.
In summary, transcription is the process by which an mRNA molecule is created from a gene's DNA sequence. It involves the RNA polymerase enzyme reading the DNA template strand and synthesizing a complementary mRNA strand. This mRNA molecule carries the genetic information from the nucleus to the ribosomes, where it is used as a template to build proteins during translation.
Transcription continues until the RNA polymerase reaches a specific sequence on the DNA called the terminator. This terminator sequence signals the RNA polymerase to stop transcribing the gene. Once the RNA polymerase reaches the terminator, it releases the newly formed mRNA molecule, and the DNA strands come back together, closing the double helix. The RNA polymerase can bind to another promotor on the same or a different gene and begin the process all over again.
4. Post-transcriptional processing (in eukaryotic cells):
In eukaryotic cells (cells with a nucleus), the mRNA molecule undergoes some additional modifications before it can be used for translation. This includes the removal of non-coding regions called introns and the joining together of coding regions called exons through a process called splicing. After splicing, the mature mRNA molecule is ready to leave the nucleus and travel to the ribosomes in the cytoplasm.
In summary, transcription is the process by which an mRNA molecule is created from a gene's DNA sequence. It involves the RNA polymerase enzyme reading the DNA template strand and synthesizing a complementary mRNA strand. This mRNA molecule carries the genetic information from the nucleus to the ribosomes, where it is used as a template to build proteins during translation.
Translation: a detailed lookTranslation is the second step in protein synthesis and takes place in the cytoplasm of the cell. It is the process by which the genetic information carried by mRNA is used to assemble a specific sequence of amino acids into a functional protein.
Let's explore the steps of translation: |
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1. Initiation:
Translation begins when the mRNA molecule, which has been transcribed in the cell nucleus and has undergone post-transcriptional modifications (in eukaryotic cells), arrives in the cytoplasm. The small ribosomal subunit binds to a specific sequence on the mRNA called the start codon. The start codon is usually AUG (adenine, uracil, guanine), which codes for the amino acid methionine (Met) and signals the beginning of protein synthesis. 2. Elongation: Once the ribosome has recognized the start codon, it recruits the tRNA molecules to the ribosome. Each tRNA molecule carries a specific amino acid, and its anticodon (a triplet of nucleotides on tRNA) pairs with the corresponding mRNA codon in a complementary manner. The ribosome moves along the mRNA, reading each codon in succession. With the help of tRNA molecules, it adds the appropriate amino acids one by one, forming a growing polypeptide chain. Each amino acid is joined to the previous one by a peptide bond, and the ribosome continues this process until it reaches a stop codon. 3. Termination: When the ribosome reaches one of the three stop codons (UAA, UAG, or UGA), the protein synthesis process stops. These stop codons do not code for any amino acid but instead act as signals to halt translation. At this point, the newly synthesized protein, called a polypeptide, is complete. |
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4. Post-translation processing (in some cases):
After translation, in some cases, the polypeptide may undergo further modifications to become a functional protein. This can include folding into a specific three-dimensional shape or combining with other polypeptides to form a complex protein structure.
The resulting protein is then ready to carry out its specific function within the cell or organism. Proteins play various crucial roles, such as enzymes that catalyze chemical reactions, structural components of cells and tissues, hormones that regulate body processes, and antibodies that help fight infections.
In summary, translation is the process of converting the genetic information in mRNA into a sequence of amino acids, forming a protein. It occurs in the cytoplasm and involves the ribosome, mRNA, and tRNA working together to read the mRNA codons and add the correct amino acids in the order specified by the genetic code. The resulting protein plays critical roles in the structure and function of living organisms.
After translation, in some cases, the polypeptide may undergo further modifications to become a functional protein. This can include folding into a specific three-dimensional shape or combining with other polypeptides to form a complex protein structure.
The resulting protein is then ready to carry out its specific function within the cell or organism. Proteins play various crucial roles, such as enzymes that catalyze chemical reactions, structural components of cells and tissues, hormones that regulate body processes, and antibodies that help fight infections.
In summary, translation is the process of converting the genetic information in mRNA into a sequence of amino acids, forming a protein. It occurs in the cytoplasm and involves the ribosome, mRNA, and tRNA working together to read the mRNA codons and add the correct amino acids in the order specified by the genetic code. The resulting protein plays critical roles in the structure and function of living organisms.