How do neural networks learn and improve their performance over time?
DNA replication in eukaryotes occurs in three stages: initiation, elongation, and termination. Initiation Eukaryotic DNA is bound to proteins known as histones to form structures called nucleosomes. During initiation, the DNA is made accessible to the proteins and enzymes involved in the replicationRead more
DNA replication in eukaryotes occurs in three stages: initiation, elongation, and termination.
Initiation
Eukaryotic DNA is bound to proteins known as histones to form structures called nucleosomes. During initiation, the DNA is made accessible to the proteins and enzymes involved in the replication process. There are specific chromosomal locations called origins of replication where replication begins. Certain proteins recognize and bind to the origin of replication and then allow the other proteins necessary for DNA replication to bind the same region.
Two copies of an enzyme called helicase are among the proteins recruited to the origin. Each helicase unwinds and separates the DNA helix into single-stranded DNA. As the DNA opens up, Y-shaped structures called replication forks are formed. Because two helicases bind, two replication forks are formed at the origin of replication; these are extended in both directions as replication proceeds creating a replication bubble. There are multiple origins of replication on the eukaryotic chromosome which allow replication to occur simultaneously in hundreds to thousands of locations along each chromosome.
Elongation
During elongation, an enzyme called DNA polymerase adds DNA nucleotides to the 3′ end of the newly synthesized polynucleotide strand. The template strand specifies which of the four DNA nucleotides (A, T, C, or G) is added at each position along the new chain. Only the nucleotide complementary to the template nucleotide at that position is added to the new strand.
DNA polymerase cannot initiate new strand synthesis; it only adds new nucleotides at the 3′ end of an existing strand. All newly synthesized polynucleotide strands must be initiated by a specialized RNA polymerase called primase.
Primase initiates polynucleotide synthesis and by creating a short RNA polynucleotide strand complementary to template DNA strand. This short stretch of RNA nucleotides is called the primer. Once RNA primer has been synthesized at the template DNA, primase exits, and DNA polymerase extends the new strand with nucleotides complementary to the template DNA.
The Leading and Lagging Strands
DNA polymerase can only synthesize new strands in the 5′ to 3′ direction. Therefore, the two newly-synthesized strands grow in opposite directions because the template strands at each replication fork are antiparallel. The “leading strand” is synthesized continuously toward the replication fork as helicase unwinds the template double-stranded DNA.
The “lagging strand” is synthesized in the direction away from the replication fork and away from the DNA helicase unwinds. This lagging strand is synthesized in pieces because the DNA polymerase can only synthesize in the 5′ to 3′ direction, and so it constantly encounters the previously-synthesized new strand. The pieces are called Okazaki fragments, and each fragment begins with its own RNA primer.
Termination
Once all the template nucleotides have been replicated, the replication process is not yet over. RNA primers need to be replaced with DNA, and nicks in the sugar-phosphate backbone need to be connected.
The group of cellular enzymes that remove RNA primers include the proteins FEN1 (flap endonulcease 1) and RNase H. The enzymes FEN1 and RNase H remove RNA primers at the start of each leading strand and at the start of each Okazaki fragment, leaving gaps of unreplicated template DNA. Once the primers are removed, a free-floating DNA polymerase lands at the 3′ end of the preceding DNA fragment and extends the DNA over the gap. However, this creates new nicks (unconnected sugar-phosphate backbone).
In the final stage of DNA replication, the enyzme ligase joins the sugar-phosphate backbones at each nick site. After ligase has connected all nicks, the new strand is one long continuous DNA strand, and the daughter DNA molecule is complete.
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Neural networks improve through iterative training, optimizing interconnected layers of artificial neurons: 1. Initialization: Begin the neural networks with random initialization of weights and biases. 2. Forward Propagation: Input data is fed through the network, where each neuron computes a weighRead more
Neural networks improve through iterative training, optimizing interconnected layers of artificial neurons:
1. Initialization: Begin the neural networks with random initialization of weights and biases.
2. Forward Propagation: Input data is fed through the network, where each neuron computes a weighted sum of inputs and applies an activation function to produce an output.
3. Error Calculation: Compare the network’s output to the actual targets using a predefined loss function to compute the error.
4. Backpropagation: Errors propagate back, computing gradients for each weight and bias.
5. Gradient Descent: Adjust weights and biases to minimize errors using gradients and a learning rate.
6. Iteration: Repeat steps 2-5 across batches to update weights and enhance performance.
7. Pattern Learning: Over epochs (iterations through the entire dataset), the network learns to discern relevant patterns and relationships within the data.
8. Generalization: Evaluate validation data to ensure robust performance.
9. Hyperparameter Tuning: Fine-tune parameters such as learning rate and batch size based on validation results to optimize performance.
10. Deployment: Apply networks to tasks like image recognition or natural language processing.
This systematic approach enables networks to learn from data, refine parameters, and excel at complex tasks efficiently.
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