The Eucaryotic Chromosomes and Genetic Regulation

Question 1

DNA

Answer 1

Double stranded polynucleotide formed from two separate chains of covalently linked deoxyribonucleotide units; serves as the carrier of genetic information

Question 2

Gene

Answer 2

Codes for proteins
contains information for the organism to decide when that protein is formed

Region of DNA that controls a discrete hereditary characteristic of an organism, usually corresponding to a single protein or RNA.

Question 3

Chromasomes

Answer 3

All organisms have chromosomes. Within procaryotic systems you have only one chromosome
In ucaryotes you can have multiple chromosomes.

Question 4

Chromosomes

Answer 4

All organisms have chromosomes.
Long thread like structures composed of DNA and associated proteins that carries part or all of the genetic information of an organism

Within procaryotic systems you have only one chromosome
In ucaryotes you can have multiple chromosomes.

Question 5

Genome

Answer 5

The entire chromosome makeup

All the chromosomes and DNA together

Question 6

The same genomes produce different cells within the body because

Answer 6

They express different proteins

Question 7

How many nucleotides are there in the human genomes

Answer 7

3 Billion

Each cell in the body has 2 billion nucleotides- proteins enable these to be folded into the nucleus of cells

Question 8

Chromatin

Answer 8

Combination of proteins and DNA (nucleotides) together that fit inside the nucleus

Question 9

How chromatin looks depends on the stage in the cell cycle

Answer 9

If you look at the m phase during cell division you have a condensed chromatin

In the interphase they can be looser and more unravelled

Question 10

In procaryotic systems what shape in the chormasomes

Answer 10

circular

Question 11

In Eukaryotic systems what shape are the chromasomes

Answer 11

Linear

Question 12

Structure of Eucaryotic chromosomes has three important elements

Answer 12

Replication Origin
- Double stranded DNA is unraveled in order to get DNA replication

Centromere

• Binds the two homologous chromosomes together
• There is order for the mitotic spidle to attach when the Chromosome is pulled apart in replication and myosis

Telomere

• Ends of the linear chromosomes
• These contain repeated nucleotide sequences that enable the ends of chromosomes to be replaced

Each is coded for by a specific DNA sequence –> Consensus sequence

Question 13

DNA replication

Answer 13

• The two strands of the DNA double helix seperate
• This creates a leading and lagging strand
• The lagging strand. Primase (a proteins) adds a short sections of nucelotides onto the lagging strand and then you have DNA replication from the 5’ end to the 3’ end.

Question 14

Having linear chromosomes is a problem when it comes to DNA replication because….

Answer 14

The last section of the lagging strand doesn’t have the template for it to attach the RNA primer to it. For discontinuous synthesis what you need it the RNA primer to be added and then synthesis happens and a protein removes that primer and then you have gaps filled.

Question 15

When it comes to the end of the DNA molecule

Answer 15

These is enough template for a primer to be added so what happens during repeated rounds of DNA replication is that the chromosome will become shorter , so you start loosing genes within the chromosomes.
This is a big problem for eucaryotic organisms.
This is where the telomere steps in.

Question 16

What happens during DNA replication?

what is the function of teleromase

Answer 16

-RNA primer is attached to the lagging strand
-Once it comes to the end, a protein called telomerase attaches itself to the end and starts adding the same sequence over and over again into the end of that lagging strand. \
The function of telemerase is to add the same sequence over and over again so that the lagging strand becomes stretched with repeats on the end which allows the primase to continue adding the RNA primers onto these repeats so extension of the last bit of the chromosome into these repeats. The repeats together form the telomeres.
The telemerase also stops the free end of the DNA from being digested.

Question 17

As you get older what happens to telemerase

Answer 17

The telemerase looses efficiency as you get older. So it doesnt produces as many telermeres onto the end of the DNA therfor replica DNA reducies

Question 18

How is DNA packing achieved

Answer 18

1st layer is nucleosome formation–> reduces DNA to 1/3 of its original length

Question 19

Nucleozome contains

Answer 19

The core histone proteins- These are a quaternary structure made up of many different polypeptides

Made up of different molecules each H2A, H2B, H3 and H4

Question 20

Nucleosome

Answer 20

• Beads on a string appearance
• Histones surrounded by ‘thread’ of DNA along with the linker DNA ( contains DNA and histone 1)
• Together condenses to a third the original length

1st level of condensation

These nucleosomes then stack on top of each other, further decreasing the DNAs length
When the DNA becomes so tightly packed the transcriptional messenary can’t contact the DNA. This stops RNA synthesis.

Question 21

What does chromatin packaging impact

Answer 21

The chromatin can be packed in different ways
Some areas are more tightly folded (heterochromatin), some are less folded (euchromatin)

Transcriptionally active interphase chromosome is the least condensed

Question 22

Heterochromatin

Answer 22

• Occupies 10% of interphase chomasomes
• Focussed around the centromeres and telomeres
• The compact regions are transcriptionally quiet
• it is found in X- chromosome inactivation

Question 23

Euchromatin

Answer 23

• Normal chromatin
• 90% of the interphase chromosome
• The more loosely folded DNA sections are the ones where protein synthesis is happening.
• These regions are being transcribed into RNA.

Question 24

Does packaging have an effect on transcription

Answer 24

Yes - this is known as the position effect.
Where the gene is placed in the chromosome also dictates how easily it can be transcribed and translated. e.g. if the gene is present closer to the telomer or centromere then it has a tendency to move into the heterocromatic region.

Question 25

The interface chromosomes are longer

Answer 25

But don’t get tangled in the nucleus
Because They have attachment sites on the nuclear envelope and laminar so the position of each chromosome within the nucleus is highly organised.

The nucleolus is a highly organised region within the nucleus which arises due to chromatin loop clusters

Question 26

Genetic regulation

Answer 26

Different cell types have different proteins complements and different RNA in them .
On average about 30% of genes within a cell are actively translating.

Question 27

There are 4 ways cells control protein expression

Answer 27

1. Controlling when and how often a gene is transcribed –> simplest and most popular
2. How the primary transcript is spliced
3. Selecting which mRNAs are translated
4. Activating or deactivating after translation

Question 28

Control of transcription

Answer 28

Promoters contain iniation sites which are required for binding of the RNA polymerase. It is a section of DNA present in the 5’end of genes to which the RNA polymerase attaches
Once you have RNA polymerase attaching then you get transcription

Binds onto the promotors

Question 29

Repressors

Activators

Answer 29

Proteins - Bind to DNA
reduce transcription rates

Increase transcription rates

Binds onto the operator

Question 30

Operators

Answer 30

Contain consensus sequences that bind specific repressors or activators

Question 31

Example of Genetic Regulation

Answer 31

E.coli
Prokaryotic bacteria

Production of tryptophan –> An amino acid. Synthesised within the ecoli cell

• 5Genes/proteins that work together to form tryptophan
• The production of these proteins is highly regulated in E.coli. Regulated in response to the level of tryptiphan present in the culture medium
• Low tryptophan levels then the RNA polymerase synthesises 5 enzymes
• If the levels are high then the Tryrophan molecules bind to a repressor protein that in turn binds to the operon and prevents RNA polymerase binding
• Activator proteins are needed to bind to a DNA site and to RNA polymerase in order to start transcription
• Eucaryotes have 3 types of RNA polymerase which all need general transcription factors for activation –> these have control over large distances.

Question 32

What is the TATA Box

Answer 32

a section (region) of every gene before the promotor 
Transcription factor 2D attaches to the TATA box which enables transcription factors  2B to attach to the TATA box aswell. This complex helps the RNA polymerase attach to the promotor enabling it to start transcription

Question 33

Enhancer sequences

Answer 33

Proteins that bind thousands of base pairs away from the transcription start site .
Enhances the transcription process

Question 34

What are the advantages of combinatorial control

Answer 34

Transcription can be regulated to a very high degree
As the cells go through different stages different transcription factors come into play.
Allows fine protein expression control