DNA Structure Flashcards
Chapter 1.1
To be classified as organic, a compound must…
…contain carbon, be complex and be produced by or associated with living things
What are the four main organic compounds found in organisms?
Nucleic acids (DNA and RNA), proteins, carbohydrates and lipids.
What are the characteristics of RNA?
Single-stranded molecule, contains ribose sugars, replaces thymine with uracil, can leave the nucleus and is short.
What are the characteristics of DNA?
Double-stranded molecule joined by weak hydrogen bonds in a double helix shape, contains deoxyribose sugars, stays in the nucleus and is long.
What are nucleic acids?
Polymers made up of the subunits nucleotides.
What makes up a nucleotide?
A phosphate group, a sugar and a nitrogen base.
What is DNA?
Molecule which stores the information to direct complex cellular processes. It can self-replicate for cell division to pass on information to daughter cells.
What is DNA like in prokaryotes?
Prokaryotes contain a single circular chromosome made of DNA with no protein attached to it (unbound). Since there is no nucleus, the DNA is found in the cytosol.
Can DNA be found in organelles?
Yes, the mitochondria and chloroplasts contain their own DNA which has the same characteristics as prokaryotic DNA.
How are chromosomes formed in eukaryotes?
The long strands of DNA are wrapped around histones (a type of protein).
Compare linear and circular chromosomes.
Linear chromosomes are found in the nucleus of eukaryotic cells, and is DNA with two ends which are capped by telomeres. It undergoes tight coiling and dense packing inside the nucleus, and is too large to leave the nucleus.
Prokaryotic cells contain one circular chromosome–with no ends– which is attached to the cell membrane. It is located within either the cytosol, specifically the nucleoid region, or organelles (namely mitochondria and chloroplasts).
Describe six characteristics of prokaryotic chromosomes.
- Circular DNA
- No histone proteins
- One chromosome
- No introns
- Attaches to cell membrane during division
- Found in the cytosol
Describe six characteristics of eukaryotic chromosomes.
- Linear DNA
- Histone proteins present
- Two or more chromosomes
- Contains introns
- Attaches to spindle fibres during cell division
- Found in the nucleus
How many chromosomes are in each human somatic cell?
- There are 22 autosomal pairs, and one pair of sex chromosomes.
What is the role of telomeres?
It is a sequence of repeated DNA which prevents DNA from uncoiling, getting tangled or binding to other chromosomes.
When are chromosomes visible?
Only during cell division when the DNA is tightly coiled around the histones (condensed). The decondensed form of chromosomes are called chromatin, and are present during all phases of the cell cycle excluding cell division.
Why is it important that chromosomes are condensed and decondensed?
During cell division, it is important for chromatin to condense into a chromosome in order to protect the DNA and ensure that identical copies are transferred to both daughter cells. If chromatin (decondensed) was present in cell division, it would likely become tangled and result in DNA breakages.
If DNA is condensed all the time, this makes it inaccessible for protein synthesis, DNA synthesis and RNA synthesis, therefore meaning it is unlikely to occur.
What do chromosome pairs consist of?
One maternal (mum) and one paternal (dad) chromosome.
What is DNA replication?
Occurring prior to cell division during interphase, DNA replication is a process by which two identical copies of DNA are made - one for each daughter cell. Thus, information is inherited by subsequent generations.
Why is DNA replication known to be semi-conservative?
Each double-strand of DNA consists of one template strand (old) and one newly synthesised strand.
Summarise the steps of DNA replication:
- Helicase breaks the hydrogen bonds between the complementary bases connecting the two strands, causing it to unwind and expose their bases.
- The two original strands act as templates. DNA polymerase add free-floating DNA nucleotides to the exposed corresponding bases on each separate strand. The new DNA is synthesised in the 5’ to 3’ direction
- DNA ligase ensures all sections of the DNA molecule formed are sealed
- Two identical double-stranded molecules have now been formed, rewinding back into a double-helix and are joined at the centromere.
Is the number of times DNA can be replicated limited?
Yes, as each time DNA is replicated, the telomeres shorten. After a certain number of DNA replications, the telomeres are too short for further replication to occur.
What happens to the amount of DNA and number of chromosomes during replication?
DNA doubles, and chromosomes remain the same before separation.
What is the leading strand of DNA?
The strand that is synthesised continuously in the direction of the replication fork’s movement.
What is the lagging strand of DNA?
The strand that is synthesised in small, separate fragments (Okazaki fragments) that are eventually joined together to form a complete, newly formed strand. This is due to how it travels in the opposite direction from the replication fork’s movement.
