DNA Flashcards Preview

A Level Biology 2 > DNA > Flashcards

Flashcards in DNA Deck (27):
1

What does DNA and RNA stand for?

-Deoxyribonucleic acid
-Ribonucleic acid

2

What are nucleotides?

They are made up of:
1. A nitrogen containing base
2. A pentose sugar
3. A phosphate group
-The base and sugar join with a glycoside bond
-The phosphate and sugar join with an ester bond
-Both require a condensation reaction to occur

3

If DNA is a polymer what is the monomer?

Nucleotides and therefore RNA and DNA are polynucleotides

4

What are the five different nitrogen containing bases in DNA and RNA?

-In a DNA molecule: adenine, thymine, guanine and cytosine
-In a RNA molecule: never contains the base thymine instead they contain uracil as well as adenine, guanine and cytosine

5

What is the difference between ribose and deoxyribose?

-The pentose (5 carbon) sugar can either be ribose or deoxyribose
-Deoxyribose has one fewer oxygen atoms in its molecule than ribose

6

How are polynucleotides formed?

1. Many nucleotides are linked together into a long chain and this takes place inside the nucleus during interphase of the cell cycle
2. In both DNA and RNA the polynucleotide strand is formed of alternating sugars and phosphates linked together with the bases projecting sideways
3. The covalent sugar-phosphate bonds (phosphodiester bonds) link the 5-carbon of one sugar molecule and the 3-carbon of the next
4. The polynucleotide strand is said the have 3’ and 5’ ends

7

What is the triplet code?

1. The sequence of nucleotide bases in a DNA molecule is a code for the sequence of amino acids in a polypeptide
2. The code is a three-letter, or triplet, code and each sequence of three bases stands for one amino acid
3. The sequence is always reads in the same direction and from only one of the two strands of the DNA molecule (the so-called sense strand)
4. The complimentary stand is referred to as the anti-sense strand

8

What is an example of a mutation?

-One mutation that has a significant effect is the one involved in the inherited blood disorder sickle cell anaemia
1. Haemoglobin is the red pigment in red blood cells which carries oxygen around the body a haemoglobin molecule is made up of four polypeptide chains, each with one iron containing haem group at the centre, two of these chains are called alpha chains and the other two are beta chains
2. The gene which codes for the amino acid sequence in the beta polypeptides is not the same in everyone, in most people the beta polypeptides begin with an amino acid acid sequence and this is coded from the HbsubscriptA (normal) allele of the gene
3. But in some people the base sequence CTT is replaced by CAT and raw amino acid sequence changes and this is coded from the HbsubscriptS (sickle cell) allele of the gene
4. This type of mutation is called substation and in this case, the small difference int he amino cid sequence results in genetic disease sickle cell anaemia in individuals with two copies of the HbsubscriptS allele

9

What type of mutation is in sickle cell anaemia?

Substitution

10

How does DNA control protein synthesis?

1. All chemical rections in cells, and therefore all the cells' actives are controlled by enzymes
2. Enzymes are proteins
3. DNA is a code for proteins, controlling which proteins are made and this DNA controls the cell's activities
4. Protein molecules are made up of strings of amino acids and the shape and behaviour of a protein molecule depends not eh exact sequence of these amino acids, that is, its primary structure
5. DNA controls protein structure by determining the exact order in which the amino acids join together when proteins are made in a cell

11

What is protein synthesis?

The code on the DNA molecule is used to determine the sequence of amino acids in the polypeptide

12

What is the first stage of protein synthesis?

-Transcription
1. In the nucleus a complimentary copy of the code from a gene is made by building a molecule of a different type of nucleic acid, called messenger RNA (mRNA), using one strand (the sense stand) as a template
2. Three RNA nucleotides are joined together by the enzyme RNA polymerase
3. This process copies the DNA code onto an mRNA molecule
4. Transcription of a gene begins when RNA polymerase binds to a control region of the DNA called a promoter and ends when the enzyme has reached a terminator sequence
5. At this point, the enzyme stops addicting nucleotides to the growing mRNA. The hydrogen bonds holding the DNA and RNA together are broken and double stranded DNA reforms. The last triplet code transcribed onto mRNA is one of the DNA triplets coding for 'stop'

13

What is the second stage of protein synthesis?

-Translation
1. The DNA code is translated into an amino acid sequence
2. The mRNA leaves the nucleus and acetates to a ribosome in the cytoplasm

14

What happens to the mRNA in the cytoplasm?

1. In the cytoplasm, there are molecules of transfer RNA (tRNA)
2. These have a triplet of bases at one end and a region where an amino cid can attach to the other
3. There are at least 20 different sorts of tRNA molecules, each with a particular triplet of bases at one end and able to attach to a specific amino acid at the other

15

What to the tRNA molecules do?

1. They pick up their specific amino acids from the cytoplasm and bring them to the mRNA on the ribosome
2. The triplet of bases (an anticodon) of each tRNA links up with a complimentary triplet (a codon) on the mRNA molecule
3. Two tRNA molecules fit onto the ribosome at any one time
4. This brings two amino acids side by side, and a peptide bond is formed between them
5. Usually, several ribose work on the same mRNA strand at the same time, they are visible using an electron microscope as polyribosomes

16

How is DNA, mRNA and tRNA related?

1. The base sequence ont he DNA molecule determine the base sequence in the mRNA, which determine which tRNA molecules can link up with them
2. Since each type of tRNA molecule is specific for just one amino acid, this determines the sequence in which the main acids are linked together as the polypeptide molecule is made

17

What is a gene and what is a mutation?

1. A part of a DNA molecule, where the nucleotide codes for just one polypeptide is called a gene
2. A change in the nucleotide sequence of a gene which may then result in an altered polypeptide is called a mutation
3. Most genes have several different variants called alleles which originally arose by the process of mutation

18

What is a DNA molecule made up of?

1. DNA molecules are made of two polynucleotide strands lying side by side, running in opposite directions
2. The strands are said to be in antiparallel
3. The two strands are held together by hydrogen bonds between the bases

19

What is a purine and what is a pyrimidine?

A purine has a double ringed structure (adenine and guanine) and pyrimidines have a single ringed structure (thymine uracil and cytosine)

20

What is the complimentary base paring?

A with T and C with G

21

What shape is DNA?

1. A double helix (the 3D shape that DNA molecules form)
2. The hydrogen bonds linking the bases, and therefore holding the two strands together, can be broken relatively easily, which happens during DNA replication and protein synthesis

22

What are RNA molecules like?

They remain as single strands of polynucleotide and can from very weak different 3D structure

23

What is semi-conservative replication?

1. The two strands of the DNA molecule split apart
2. New nucleotides could then line up along each strand, opposite their appropriate partners and join up to form complementary strands along each half of the original molecule
3. The new DNA molecules would be just like the old ones, because each base would pair with its complimentary one
4. Each pair of strands could then wind up again into a double helix, exactly like the original one
-Called this because half of the original molecules is kept in each of the new molecules
-DNA replication takes place when a cell is not dividing. This is in interphase in eukaryotic cells

24

How is a polypeptide formed?

A polypeptide is coded for by a gene

25

What is a gene?

A gene is a sequence of nucleotides that forms part of a DNA molecule

26

What is a gene mutation?

A gene mutation is a change in the sequence of nucleotides that may result in an altered polypeptide

27

Outline the process of protein synthesis

1. The RNA polymerase enzyme tells the DNA to unwind breaking the hydrogen bonds between the bases, to a template strand and a coding strand
2. The mRNA molecule which is a single strand is an exact copy of the coding strand but instead of the base thymine there is the base uracil
3. The formation of this mRNA molecule is called transcription
4. The mRNA then exits the nucleus via the nuclear pores in the nuclear envelope and attaches itself to a ribosome on the rough endoplasmic reticulum
5. The mRNA feeds through the small subunit (30s) ribosome part
6. Meanwhile tRNA molecules which are attached to a specific amino acid have the anticodon and the tRNA molecules enter the large subunit of the ribosome
7. The anticodon on the tRNA is complimentary to the codon on the mRNA and a peptide bond is then formed between two adjacent amino acids each in a different tRNA molecule in the same ribosome with energy from ATP and an enzyme, peptidyl transferase
8. These amino acids join the growing polypeptide chain which in turn pushes them out of the ribosome allowing the next tRNA molecule to enter. This process is translation
-Until STOP codon and MET amino acid is always at the start