General features of DNA replication - Week 1 Flashcards
what does DNA consist of and where is it found
DNA contains all the genetic structures for all the characteristics of living organisms.
DNA exists within cells, within the nucleus for eukaryotic cells, in the nucleus DNA is packed in specific structures and specialised cellular structures called chromosomes.
why is DNA important
The genetic material/DNA contains genetic information/instruction which is important to construct an entire organism, this genetic material is passed from cell to cell during cell division.
why is DNA important?
The genetic material/DNA contains genetic information/instruction which is important to construct an entire organism, this genetic material is passed from cell to cell during cell division.
what is the definition of DNA synthesis
molecular events that allow the assembly of new DNA strands
what is the definition of DNA replication
it’s a biological process by which DNA is copied and its very accurate.
what does DNA replication mainly include
- DNA chain synthesis
- Initiation, elongation and termination
- daughter strand separation
- distribution of chromosomes
why is DNA replication important
it is necessary for cell reproduction before cell division
what are the 5 stages a cell goes through and what happens at those stages
As a single cell divides, it goes through 5 stages:
1.G1
which is a growth phase
2. G0
the growth arrest phase
3. Synthesis phase
Where DNA synthesis/DNA replication occurs
4. G2
which is the 2nd growth phase
- M phases
Mitosis, where a cell duplicates and DNA will be distributed in both daughter cells
who were Watson and Crick
Watson and Crick were the first scientists that discovered the structure of DNA in 1954, they discovered the structure of DNA and also hypothesised how DNA is copied into 2 new molecules, they hypothesised that DNA is duplicated in a semi-conservative manner.
what does the semi-conservative model suggest
The semi-conservative DNA replication suggests that the 2 original strands of the DNA molecule split up, separates and each acts as a template for the formation of the new strand of DNA.
why wasn’t the semi-conservative approved at first
At that time the separation of the 2 strands of DNA seemed impossible, so the scientific community disagreed about this hypothesis of Watson and Crick and they proposed other models for DNA replication which were the conservative model and the dispersive model were still plausible until the theories could be disproved.
what does semi-conservative replication propose
According to the semiconservative model, after one round of replication, every new DNA double helix would be a hybrid that consisted of one strand of old DNA bound to one strand of newly synthesized DNA and after two rounds, two of the DNA molecules consist only of new material, while the other two contain one old and one new strand.
what does the conservative model propose
According to the conservative replication model, the entire original DNA double helix serves as a template for a new double helix, such that each round of cell division produces one daughter cell with a completely new DNA double helix and another daughter cell with a completely intact old (or original) DNA double helix. This is repeated in the second round.
what does the dispersive model propose
According to the dispersive model, every round of replication would result in hybrids, or DNA double helices that are part original DNA and part new DNA. thus indicating the two parental strands are distributed more or less randomly between two daughter molecules. Each subsequent round of replication would then produce double helices with greater amounts of new DNA.
which experiment was carried out to find out whether conservative, semi-conservative or dispersive is correct
In 1958, Meselson and Stahl devised an experiment to determine whether DNA replicated following a conservative, semi-conservative, or dispersive model.
the experiment is called the Meselson-Sahl experiment.
how was the Meselson-Stahl experiment carried out
how the experiment is carried out:
1. E.coli were grown for several generations in a medium with the heavier 15N isotope
2. E.coli cells with only 15N in their DNA were transferred to a 14N medium and were allowed to divide.
3. DNA was extracted periodically for example after 20 minutes, 40 minutes or 60 minutes, and was centrifuged on a salt density gradient.
4. The isolated DNA is isolated from these samples and then the DNA is detected, the outcome of the experiment is analysed
what do the results show from the Meselson-Stahl experiment
What the results show
After 1 cell doubling which is 20 minutes (which is the time that usually a population of bacteria cells duplicate) they obtain the DNA that these bacteria cells, which is lighter compared to the control, this means that the DNA after 1 cell doubling the DNA in these samples were half heavy because they display an intermediate density lighter than the control.
After the second cell doubling, like after 40 minutes, the DNA is made of 2 types, this is indicated by the fact that they obtained 2 different bands of DNA, 1 is light and the other is half heavy, this result after 40 minutes is consistent with the semi-conservative model only whereas the results of 1 cell doubling is consistent with both the semi-conservative model and the dispersive model.
thus the results prove that the semi-conservative model is the correct model.
what is the definition of the origin of replication
A unique DNA sequence of a replicon at which DNA replication is initiated and proceeds typically in a bidirectional fashion.
what is the replication fork
The point at which replication occurs is called the replication fork. The replication fork is the site of the progressive unwinding of the double-stranded DNA and the duplication of the template DNA, and it travels along the length of the DNA molecule, away from the ORI.
what is OriC and what does it contain
The origin of replication is prokaryotes is called OriC, this is a well-defined structure and consists of 245 base pairs in length and contains the following elements:
- AT-rich region (3 copies of 13 bp repeats)
- DNA boxes (the binding site of the protein, 5 copies of 9 bp repeats)
- GATC methylation sites ( methylation of the bacterial origin regulates initiation)
how many origins of replication do eukaryotes have
The eukaryote’s chromosomes are usually linear and contain multiple origins of replication, spaced throughout the chromosome that merges during the DNA replication.
Eukaryotes have many origins of replication that are called replicons.
how many origins of replication do bacteria have
Bacteria has a single orgin of replication (ORI) called OriC
where does the knowledge of DNA replication in eukaryotes come from
For eukaryotes, as these organisms are more complex than bacteria, there is no single model of the organism, so all our knowledge that we have regarding DNA replication is based on yeast and Saccharomyces cerevisiae.
DNA sequences at the replication origins in Higher Eukaryotes, including humans vary and have been much harder to isolate. Evidence suggests that the origin of replication in mammalian cells is similar to yeast-type origins.
why is the ARS1 sequence important in DNA replication
The autonomous replicating sequences (ARS1) are specific DNA sequences in the genome of eukaryotes that serve as the origin of replication. These sequences are recognized and bound by the Origin Recognition Complex (ORC), which is a protein complex that plays a crucial role in initiating DNA replication.
