Nucleus Function

Question 1

What organisms have what number of chromosomes

Answer 1

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)

Question 2

Sets of chromosomes that different alleles

Answer 2

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.

Question 3

What are codominant alleles and example

Answer 3

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

Question 4

Transcription in the nucleus

Answer 4

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.

Question 5

RNA and DNA are both nucleic acids

Answer 5

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

Question 6

Not all the DNA in the nucleus is transcribed

Answer 6

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.

Question 7

Types of RNA- difference is in their function; NO CHEMICAL DIFFERENCE BETWEEN EACH OF THEM

Answer 7

tRNA, mRNA and rRNA
Each perform a different role in the functioning of a cell
RNA may have been the ancestral ‘molecule of life’.

Question 8

mRNA definition

Answer 8

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.

Question 9

rRNA definition

Answer 9

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.

Question 10

What is tRNA

Answer 10

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.

Question 11

What types of RNA is transcribed and which ones are translated

Answer 11

mRNA, tRNA and rRNA are all transcribed from DNA
Only mRNA is translated into protein

tRNA and rRNA helps the mRNA being translated.

Question 12

What is mitosis

Answer 12

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

Question 13

Prior to mitosis the chromosomes have replicated

Answer 13

(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

Question 14

Three components of a DNA

Answer 14

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

Question 15

The four stages of mitosis

Answer 15

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

Question 16

The cell cycle

Answer 16

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.

Question 17

Phenotypes and genotypes

Answer 17

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.

Question 18

RNA processing

Answer 18

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.

Question 19

How to read the direction of a nucleic acid

Answer 19

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’.

Question 20

Biological differences between RNA and DNA

Answer 20

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.

Question 21

What is the purpose of the DNA

Answer 21

The whole purpose of DNA is to be transcribed. It holds the information for the proteins.

Question 22

Junk Genome

Answer 22

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.

Question 23

What happens to the coding DNA (exons) in humans vs simple single-celled eukaryotic cell

Answer 23

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.

Question 24

Trend between junk genome and evolution

Answer 24

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.

Question 25

How does tRNA and rRNA work together to translate mRNA

Answer 25

rRNA is the factory in which translation occurs.
tRNA shuttles in the raw materials to the factory (of the rRNA as rRNA goes to form the ribosome) for the translation to occur
The messenger inherent in the mRNA is translated
AGAIN, THE ARE THE SAME CHEMICALLY BUT DIFFER FUNCTIONALLY

mRNA is the middle of the central dogma

Question 26

Central dogma definition

Answer 26

This outlines the whole framework of self-replicating DNA being transcribed to form mRNA and the mRNA acts as a template for protein synthesis in translation.

Question 27

What is a multi-nucleated cell

Answer 27

A cell which has gone through the S, G2 phases and also mitosis. So, its DNA has been replicated and so has its nucleus. But, it hasn’t gone through cell division so has multiple nuclei. It is syncytium (multi-nucleated cell).

Skeletal muscle cells are syncytial. During the development of many insects, there is a syncytium stage where the cells become multi-nucleated. But usually in vast majority of the cases, mitosis is followed by cytokinesis (cell division)- which gives you two half cells; one of which has a daughter nucleus.
Cytokinesis occurs differently in animal and plant cells.

Question 28

Interphase

Answer 28

Consists of the G1, S and G2

Question 29

What are centrosomes

Answer 29

These are microtubules organising centres and at the beginning of mitosis, it replicates and go to the opposite sides on the nuclei.
From then, it extends spindle fibres. These fibres touch each of the sister chromatids (at the centromere) and then they shorten. As they shorten, they pull the sister chromatids apart.

Centrosomes is animal specific.

Question 30

Cell cycle

Answer 30

G1- where lots of RNA is made and this RNA is translated to lots of protein.
You do this for the preparation of the S phase where DNA is replicated. For DNA replication to occur, you need lots of specialised proteins.
G2 is again needed for forming specialised proteins (so again lots of RNA is made and this is translated to proteins) that will be needed for mitosis stage.

Question 31

G0 stage

Answer 31

A step in which once the cells are transferred, they stay there.
They leave the cell cycle- don’t go further to S phase.