Nucleus Function Flashcards
(31 cards)
What organisms have what number of chromosomes
Many single-celled organisms, e.g yeast (despite being eukaryotic) carry a single set of chromosomes. They are haploid.
Most multi-cellular organisms carry two sets of chromosomes. They are diploid.
Some organisms carry more than two sets of chromosomes.
They are polyploidy. E.g xenopus (a genus of frog) is a tetraploidy (four copies of every chromosome)
Sets of chromosomes that different alleles
A gene can be the same on the different copies of the chromosome HOMOZYGOUS
A gene can be different versions on different copies of a chromosome HETEROZYGOUS
different versions of a gene (or DNA locus) are called ALLELES
The two copies of each chromosome is generally the same. However, at a given position, the chromosomes may vary. If they do not vary (are the same), then they are homozygous otherwise they are heterozygous
If these are changes are in a gene, we refer to them as alleles
A position on a chromosome is known as a locus.
What are codominant alleles and example
Codominance is when two alleles are both expressed in the phenotype rather than one being dominant over the other.
E.g blood AB group in humans
Transcription in the nucleus
The nucleus holds the genetic material (DNA)
Active DNA is transcribed (copied into RNA)
RNA
RNA is ‘processed’ in the nucleus prior to export to the cytoplasm where it is TRANSLATED into protein.
A small amount of our genome is transcribed. Transcription starts at a promoter and ends at a terminator.
RNA and DNA are both nucleic acids
DNA and RNA are related MACROMOLECULES
DNA is double stranded and holds the genetic information
RNA is a single stranded copy of parts of the DNA
The only chemical difference between DNA and RNA is that RNA has a 2’ OH ( no hydroxyl group on the 2 prime end) on the ribose ring which DNA lacks (hence deoxy)
However in biology there are quite a few differences between DNA and RNA
Instead of thymine RNA has the base uracil
Not all the DNA in the nucleus is transcribed
Only 3 or 4% of the DNA in a human is in the form genes i.e to be transcribed
Some of the DNA performs control functions e.g promotors and enhancers
Control functions tell when the genes should be transcribed and how much RNA should be made from that gene. These parts of the genes are called regulatory genes- these parts are not transcribed; these parts do not hold information about the proteins but they control how much of the gene is transcribed. Examples are these regulatory regions are promoters and enchancers.
Some of the DNA performs structural roles e.g telomeres and centromeres. Since chromosome is a large molecule, there are regions of genes which perform structural roles. Examples of these regions are telomeres and centromeres. Telomeres cap the ends of a chromosome. Centromeres are the centre part where the sister chromatids are held together after replication.
Telomeres are an usual structure. They are not in the usual b-form DNA structure that we normally see. They are normally in a g-quartet structure. This is a tough structure that stops the ends of the chromosome wearing away.
Some of the DNA performs no known function e.g introns and some repeating sequences
Some parts are the DNA have subtle roles that we are completely aware of. Some parts of the DNA are just junk which has been left with us by evolution because we can tolerate it.
Types of RNA- difference is in their function; NO CHEMICAL DIFFERENCE BETWEEN EACH OF THEM
tRNA, mRNA and rRNA
Each perform a different role in the functioning of a cell
RNA may have been the ancestral ‘molecule of life’.
mRNA definition
The DNA code holds the information to build proteins (the effectors of the cell). Messenger RNA (mRNA) is an intermediate in the flow of information from DNA to protein.
rRNA definition
Sometypes of RNA can play a structural/functional role. Ribosomal RNA plays such a role in the protein-rRNA complex known as the ribosome.
The factory at which translation occurs.
What is tRNA
Transfer RNA (tRNA) also has a structural/ functional role. It brings amino acids to the growing protein chain during translation of the mRNA into protein by the ribosome.
Quite a bit of tRNA is generally made but less than the amount of rRNA made.
What types of RNA is transcribed and which ones are translated
mRNA, tRNA and rRNA are all transcribed from DNA
Only mRNA is translated into protein
tRNA and rRNA helps the mRNA being translated.
What is mitosis
This is the replication of the chromosomes and the division of the nucleus to give two identical daughter nuclei
The division of the cytoplasm into two daughter cells each containing one nucleus is called cytokinesis
Mitosis + cytokinesis = cell replication
Chromosomes condense during mitosis and become discernible by microscopy
Prior to mitosis the chromosomes have replicated
(In S phase) Replication is the process of copying the DNA in the nucleus
In metaphase the copied chromosomes are held together at the centromere.
These are known as SISTER CHROMATIDS
Three components of a DNA
Replication origin (ARS)- the place where the DNA polymerase can initiate the replication of DNA
Centromere (where sister chromatids are held together at)
A telomere at either end(cap the ends)
These are needed to make an artificial DNA
The four stages of mitosis
Prophase: centrosomes migrate to opposite poles of the cell. The nuclear membrane breaks down. The replicated chromosomes condense
Metaphase: The chromosomes align on the equator of the cell
Anaphase: The sister chromatids separate and are pulled to different poles of the cell
Telophase: Nuclear membrane reforms and microtubule apparatus dis-mantles
The cell cycle
Mitosis is a part of the cell cycle
The stages of the cell cycle are regulated by rising and falling levels of proteins known as cyclins
Cyclins are phosphorylated by other proteins known as CYCLIN DEPENDENT KINASES (CDKs)
Mitosis can occur without cytokinesis
The cell cycle is like a clock and presumably needs to be timed in some way because you need to be ready for the next phase by the time it is supposed to start. The protein, cyclins, is timing this cell cycle. Cyclins activate another protein called cyclin dependent kinases.
These cyclin dependent kinases are inactive until a cyclin activates them and then they become active.
Kinases can phosphorylate other proteins. They can only become active when they are bound to cyclins though.
When you phosphorylate lamins, they disassociate (the nuclear envelope breaks down)- something you require for mitosis.
Phenotypes and genotypes
Phenotypes are sets of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
Genotype is the the genetic constitution of an individual organism.
Different phenotype can have various different genotypes.
RNA processing
RNA is made as a preRNA. It is first capped with a 5 prime cap. It is then tailed with a 3 prime polyA. Introns are removed (splicing). Then the RNA is exported to the cytoplasm where it is translated.
How to read the direction of a nucleic acid
We always start with the 5 prime end.
In nature, all polymerases can only synthesise new strands in the 5’ to 3’ direction. The type of polymerase does not matter.
So this means that in transcription, the new, growing RNA strand is made 5’ to 3’. This means that the polymerase is reading the strand 3’ to 5’.
Biological differences between RNA and DNA
The difference in role between RNA and DNA is that RNA will have uracil and DNA will have thymine.
RNA is copied as only parts of the template strand. Whereas when DNA is replicated, the whole DNA is copied.
RNA tends to be made as a single strand. This single strand will fold up on itself but was made as a single strand. Whereas, when DNA is made, it normally made in its double helical structure, at least in all cells.
What is the purpose of the DNA
The whole purpose of DNA is to be transcribed. It holds the information for the proteins.
Junk Genome
Humans carry quite a bit of junk genomes which we don’t necessarily need
These include leftover genes (through evolution)
Genes from viral infections called transposons (there is a quite a bit of it)
The human species can tolerate having quite a lot of unnecessary genome because we reproduce slowly.
As you go down the evolutionary tree (where the organisms can replicate faster and faster), the amount of junk genome or non-coding DNA goes down considerably.
What happens to the coding DNA (exons) in humans vs simple single-celled eukaryotic cell
They are quite widely separated within a gene (there is a lot of non-coding genes).
Whereas in yeast (single-celled eukaryotic organism), a section of gene has much faster replication time and much more of the genome is in the form of coding proteins and far less of it is in the form of non-coding.
Trend between junk genome and evolution
As you go up through the evolutionary tree, you tend to see genomes require more and more non-coding DNA as these organisms reproduce much slower than its ancestors.
On the other hand, with the organisms are further down the evolutionary tree, you tend to see organisms that have much more efficient and packed genomes as they replication much faster.
