4. DNA, Chromosomes, Transcription and Translation

Question 1

Describe how the DNA is organised/arranged in eukaryotic cells

Answer 1

In eukaryotic cells (nucleus of eukaryotes)

DNA is linear (Linear DNA) with two ends

DNA molecules are much longer (compared to DNA molecules in prokaryotic cells)

The DNA molecules are tightly wrapped around proteins called histones (histone proteins) – forming complex structures called chromosomes

Question 2

Describe how the DNA is organised/arranged in prokaryotic cells

Answer 2

In Prokaryotic cells

DNA is circular with no free ends (Circular DNA)

DNA molecules are relatively short

DNA in prokaryotes is not bound to histones - so Prokaryotic DNA does not exist as a chromosome

Question 3

How does the DNA in mitochondria and chloroplasts compare to the DNA in prokaryotes

Answer 3

The DNA in mitochondria and chloroplasts are very similar to the DNA we find in prokaryotes

It is relatively short, circular and not attached to histones

Question 4

What is the dark material in the nucleus

What is it not possible to see at this point

Answer 4

The dark material in the nucleus is the DNA bound to histones

Not possible to see any distinct chromosomes
This is because at this stage, the chromosomes have a relatively open structure

At this point, we refer to the DNA and histones as chromatin

Question 5

what happen before a cell divides

What happens to these two copies

Answer 5

Before a cell divides all of the chromosomes are copied

These two copies remain attached at a point called the centromere

Question 6

Describe the structure of chromosomes

Answer 6

Before a cell divides all of the chromosomes are copied

These two copies (chromosomes) remain attached at a point called the centromere

Now the two DNA molecules are called chromatids (left hand and right hand chromatid)

At this stage, we now refer to the whole structure as a chromosome

At this point the chromosomes condense
The DNA and histones form densely packed loops and coils and the chromosomes become visible in the cell

Question 7

Describe what is meant by homologous chromosomes

Answer 7

a pair of chromosomes, one maternal and one paternal, that carry the same genes (but not necessarily the same alleles of the genes)

two chromosomes in a homologous pair have the same genes

Question 8

Define locus

Answer 8

The position of a gene on a chromosome is called the locus for that gene

A gene occupies a fixed position, called a locus, on a particular
DNA molecule

Question 9

how many chromosomes in human cells

Answer 9

In human cells, we find 46 chromosomes

Question 10

what would the homologous pair of chromosome 9 contain

Answer 10

chromosome 9
paternal and maternal chromosome containing genes

Question 11

What is a gene

Answer 11

A gene is a section of DNA which encodes (codes for) the amino acid sequence of a polypeptide

CERTAIN REGIONS OF CHROMOSOMES ARE CALLED GENES

A gene is a base sequence of DNA that codes for:
* the amino acid sequence of a polypeptide
* a functional RNA (including ribosomal RNA and tRNAs).

Question 12

How can genes occur in slightly different versions

Answer 12

Sometimes random mutations take place
This means that genes can occur in slightly different versions

Question 13

What is an allele

Answer 13

Versions of a gene

Question 14

how do humans inherit homologous chromosomes

Answer 14

They inherit one of each of the homologous chromosomes in a pair from their father and one from their mother

This means that on a homologous pair of chromosomes, the alleles do not have to be the same

e.g. inheriting the A allele from their father and the B allele from their mother

Question 15

They inherit one of each of the homologous chromosomes in a pair from their father and one from their mother

What does this mean about the alleles on a homologous pair of chromosomes

Answer 15

This means that on a homologous pair of chromosomes, the alleles do not have to be the same

e.g. inheriting the A allele from their father and the B allele from their mother

Question 16

Does the majority of the DNA found in chromosomes code for polypeptides?

Answer 16

A lot of the DNA that we find in chromosomes does not code for polypeptides

Question 17

Describe where non-coding sequences of DNA can be found

Answer 17

Between the genes there are large amount of repeating base sequences. These repeating sequences are non coding

Within functional genes, we find stretches of non-coding DNA. These are called introns

Question 18

What are introns

Answer 18

non-coding sequences of DNA found within function genes (exons)

Question 19

what is an exon

Answer 19

Even within a gene only some
sequences, called exons, code for amino acid sequences

sequences of DNA within a gene which code for amino acid sequences

Question 20

How the sequence of DNA is used to determine the amino acid sequence of protiens

Question 21

What do genes play a key role in

Answer 21

Genes play a key role in protein synthesis
This is because the nucleotide sequence of a gene, encodes the primary structure of a polypeptide (or protein) (the sequence of amino acids)

Question 22

What are the two main stages in protein synthesis

Answer 22

The first stage is called transcription and this takes place in the nucleus
During transcription, the base sequence of a gene is copied into the complementary base sequence of a molecule called messenger RNA (mRNA)

The mRNA molecule then moves to the cytoplasm
In the cytoplasm, the information encoded in the nucleotide sequence of the mRNA is used to join a specific order of amino acids, forming the polypeptide
This stage is called translation

Question 23

Describe the stages of transcription

Answer 23

Part of a gene encoding a specific polypeptide

https://cdn.kastatic.org/ka-perseus-images/20ce29384b2e7ff0cdea72acaa5b1dbd7287ab00.png

In the first stage of transcription, DNA helicase breaks the hydrogen bonds between the two strands

Now complementary RNA nucleotides move into place and form hydrogen bonds with the bases on the exposed nucleotides on one of the DNA strands

At this point, the enzyme RNA polymerase, joins the RNA nucleotides, by forming phosphodiester bonds

The enzymes continue making their way along the DNA until they reach the end of the gene

A strand of messenger RNA has been produced

The base sequence of the mRNA is the same as the base sequence of the top DNA strand (except thymine has been replaced by uracil)

The top DNA stand is called the sense strand

The mRNA is complementary to the other DNA strand
This is called the antisense or template strand

Once the mRNA has been synthesised, the RNA polymerase detaches from the DNA and the DNA goes back to its normal double helix structure

At this stage, the mRNA now moves out of the nucleus through a nuclear pore

Once in the cytoplasm, the mRNA can take part in translation

most mRNA molecules are hundreds of nucleotides long

Question 24

Describe how splicing leads to messenger RNA

Answer 24

a large amount of DNA is non-coding
non-coding DNA is found both between genes and also within genes

structure of a gene: exon intron exon intron exon

non-coding within a gene are called introns
coding regions within a gene are called exons

In humans, many genes contain a large number of introns

During transcription, both exons and introns are copied into RNA
That means that the RNA contains non-coding regions
This is called pre-mRNA

Once the pre-mRNA is formed, the introns are then removed and the ends of the exons connected
This process is called splicing

Splicing converts the pre-mRNA into functional mRNA

Gene
exon intron exon intron

pre-mRNA
exon intron exon intron

| splicing
\/

mRNA
exon exon

transcription
\/

Question 25

Explain why splicing does not take place in prokaryotes

Answer 25

Splicing does not take place in prokaryotes This is because introns are uncommon in prokaryotic cells

Question 26

What do some genes encode for rather than amino acid sequence of a polypeptide

Answer 26

Some genes do not encode for polypeptides Instead, these genes encode/codes for functional RNA molecules Examples of functional RNA molecules include transfer RNA and ribosomal RNA

Question 27

how many pairs of chromosomes do humans have

Answer 27

23 pairs of chromosomes

Question 28

Describe what is meant by the genome

Answer 28

All of the genes in a cell is called its genome In eukaryotes, this includes both the genes on the chromosomes and the genes in mitochondria and in chloroplasts

Question 29

Describe what is meant by the proteome

Answer 29

This is because the nucleotide sequence of a gene, encodes the primary structure of a polypeptide (or protein) (the sequence of amino acids) All of the proteins produced by the genome of an organism is called the proteome Only a fraction of those proteins will be produced in any particular cell type Cells can produce a different range of proteins depending on what the cell is doing (its function)

Question 30

How are mRNA nucleotides read

Answer 30

mRNA nucleotides are read as a series of triplets (three bases) These triplets are called the genetic code (UUU UUA -> Phe table etc.) These are the shorthand versions of each of the twenty amino acids that we find in proteins These are these are the mRNA triplets which encode those amino acids The mRNA triplets are the genetic code

Question 31

key features of the genetic code

Answer 31

Most amino acids have more than one triplet (multiple triplets code for the same amino acid) E.g. Leucine has 6 triplets Because of this, it is said that the genetic code is called a degenerate code The triplet code is non-overlapping This means that no base is read more than once e.g. AUG GCA CUG - triplet 1, triplet 2, triplet 3 if code was overlapping AUG would be triplet 1, UGG would be triplet 2 etc. The genetic code is universal The same triplets encode the same amino acids in the vast majority of organisms on planet Earth

Question 32

What is each triplet in the mRNA called

Answer 32

A codon Triplet - 3 bases

Question 33

Describe the structure of tRNA

Answer 33

tRNA has two important parts At the top of the tRNA molecule, we have a binding site for an amino acid At the bottom of the tRNA molecule, we have a triplet of bases called the anticodon tRNA for the amino acid methionine The tRNA anticodon is complementary to the mRNA codon for that amino acid e.g. on the mRNA, the first codon is AUG. This is the start codon but its also the codon for methionine The anticodon, on the tRNA carrying methionine (amino acid) is complementary to the mRNA codon for methionine There is a complementary tRNA for the codons encoding every amino acid Many amino acids are encoded by several triplets Each one of these will have a corresponding transfer RNA https://studymind.co.uk/wp-content/uploads/2022/03/Screen-Shot-2022-03-31-at-9.34.00-PM.png

Question 34

Describe the stages of translation

Answer 34

In the genetic code, we can see that one of the triplets determines where to start translating the mRNA molecule. This is called the start triplet The start triplet also encodes the amino acid methionine Three triplets determine where translation stops These are called stop triplets In translation the nucleotide sequence of the mRNA is used to determine the amino acid sequence of a polypeptide Each triplet in the mRNA molecule is called a codon In order for the mRNA to be read, another type of RNA is involved This is called transfer RNA (tRNA) Once the mRNA moves from the nucleus to the cytoplasm, the small subunit of a ribosome binds with the mRNA at the start codon Ribosomes contain a number of different proteins They also contain a type of RNA called ribosomal RNA Now a tRNA molecule with an anticodon complementary to the start codon attaches This is held in place by hydrogen bonds between the complementary base pairs on the mRNA and tRNA Now a second tRNA molecule moves into place The anticodon on this tRNA is complementary to the second codon on the mRNA This codon encodes for a specific amino acid. This amino acid is brought in by the tRNA Now a peptide bond is formed between the two amino acids This is catalysed by the enzyme peptidyl transferase - which is a part of the ribosomal RNA molecule The formation of the peptide bond requires energy provided by ATP Now the ribosome moves to the next codon and forms a peptide bond to the next amino acid The first tRNA molecule is released tRNA molecules that have been released are later attached to their amino acids by enzymes in the cytoplasm The ribosome continues moving down the mRNA, forming the polypeptide When the ribosome comes to a stop codon, it detaches and the polypeptide chain is released Once the first ribosome has started moving along, another ribosome can attach to the start codon and start translating. We might see a whole line of ribosomes making their way along the mRNA This means that a large number of polypeptide molecules can be produced rapidly Once the polypeptide has correctly folded, it can then carry out its function in the cell