DNA Structure and Replication Flashcards
Gene
Genes are region/s of DNA that are made of nucleotides; the molecular unit of heredity.
Major Components of Chromosomes
Eukaryotic and Prokaryotic chromosomes are quite different structurally but they contain genes composed of the nucleic acid called DNA.
In eukaryotic cells, chromosomes are thread-like structures that are made of DNA and protein found in the nucleus. They contain genetic information in the form of genes.
Eukaryotic chromosomes can be depicted as either single or duplicated (depending on the stage in the life cycle of a cell). Duplicated chromosomes result from DNA replication (and look like crosses). A duplicated chromosome is composed of two sister chromatids that are stuck together by a centromere.
Chromatids are duplicate copies of a chromosome (containing identical DNA sequences) that are produced via DNA replication and are held together for a time by the centromere
Nucleic Acids
Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA).
They are large organic molecules that are involved in inheritance and gene expression. DNA is the ‘coding molecule’ that chromosomes (and genes) are composed of.
Nucleic Acids are polymers made up of repeating subunits called Nucleotides.
Each DNA nucleotide has 3 parts: a nitrogenous base, a deoxyribose sugar and a phosphate group
How DNA is packaged in prokaryotic cells
There is often one main chromosome which is double-stranded and circular in shape. This main chromosome is located in the nucleoid region of the cytosol (cytoplasm).
Prokaryotic chromosomal DNA is not bound to histones. This main chromosome is not linear and has no telomeres. Many prokaryotic cells (and eukaryotic yeast cells) also contain small self-replicating loops of DNA called plasmids.
RNA occurs in different forms (messenger, ribosomal and transfer) but is made from DNA in the nucleus and has important roles in protein synthesis in the cytoplasm.
How DNA is packaged in eukaryotic cells
DNA is double-stranded and linear. In chromosomes, DNA is packaged in a highly organised / condensed fashion (eg. 6 feet lengths of DNA need to fit in a human cells nucleus). Eukaryotic chromosomes contain telomeres, which are protective structures at the ends with repetitive base codes. Telomeres erode with age (every time DNA replicates).
The chromosomal material in the nucleus which is composed of a DNA - protein complex is collectively called Chromatin.
Eukaryotic chromatin is composed of DNA bound to round proteins called Histones. Eight histones that are bundled together and wrapped approximately twice in DNA to form a DNA-histone complex called a Nucleosome. A small amount of DNA is also located inside the mitochondria and chloroplasts of eukaryotic cells (This organelle based DNA is circular and not bound to histones).
Major Components of DNA
Deoxyribonucleic acid (DNA) is a very long molecule that has the shape of a double helix (a twisted ladder).
Composed of three part repeating units called nucleotides, each containing a:
Sugar called deoxyribose (on the sides of molecule and often depicted as a pentagon. N bases attach to the sugars.)
Phosphate (on sides of molecule alternating with the sugars)
Nitrogen base
In DNA, there are 4 major types of N bases:
Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
N bases in pairs form the rungs of the molecule (A - T and G - C). Complimentary N bases are bonded together via weak Hydrogen bonds.
DNA Replication
DNA replication is a process where DNA copies itself. It occurs during the S (synthesis) phase of interphase (the resting stage) in a cells life cycle. This process is required before the nucleus of a cell undergoes either mitosis or meiosis. It results in chromosomes being duplicated (appearing like crosses).
Each of the daughter DNA molecules produced are composed of one new DNA side and one of the original (conserved) sides of the original parent DNA molecule. The new DNA strands have special names: the leading and lagging strands.
DNA Replication Terminology
Replication can occur in a 5’ to 3’ direction only (nucleotides are only ever added to the 3’ end of a DNA fragment / sequence being made).
The point where the double helix unzips (H bonds break) is called a replication fork.
The leading strand forms continuously as nucleotides are added in the 3’ direction towards the replication fork.
The lagging strand forms discontinuously as a series of DNA fragments (Okazaki fragments). The nucleotides are added in the 3’ direction away from the replication fork.
Okazaki fragments are 1000 - 2000 nucleotides long.
Enzymes Involved in DNA Replication
DNA helicase- unwinds and unzips the DNA double helix into two single parental strands exposing bases (it breaks the weak H bonds between complimentary N base pairs)
Primase- an enzyme that adds a small RNA sequence called a primer that will be the initiation / starting point for DNA replication.
DNA polymerase- reads the parental DNA in a 3’ to 5’ direction and adds DNA nucleotides to the new strand in a 5’ to 3’ direction using the parental DNA as a template. DNA polymerase also proofreads the newly synthesised DNA strands. There are at least three different types of polymerase that all have slightly different roles.
DNA ligase- joins Okazaki fragments together
3 Major Steps of DNA Replication
- The DNA double helix parent molecule unwinds and unzips to form two single strands (a replication fork forms) (H bonds holding complimentary N bases pairs together (A-T and C-G) in the middle of the molecule break). This involves the action of an enzyme called DNA helicase.
- Copying of the DNA proceeds by nucleotides bonding to parent template strands. On each strand of the ‘parent’ molecule free floating ‘complimentary’ nucleotides (found in the nucleoplasm) diffuse into position and bond to the exposed bases of the parent template strand and to each other via condensation reactions producing two identical copies of DNA. This bonding of nucleotides via condensation involves the action of the enzyme called DNA polymerase.
- Replication results in two identical molecules that then rewind. These identical copies of DNA molecules remain, for a time, connected together and while connected they are referred to as chromatids (forming both sides of a duplicated chromosome). 50% (one side) of each new daughter DNA molecule is from the original parent molecule and 50% is new.
How does DNA occur
In eukaryotes, DNA occurs bound to histone proteins in chromosomes in the nucleus of eukaryotes or as unbound circular DNA in the mitochondria. It can also occur as unbound circular DNA in the cytosol of prokaryotic cells
