Chapter 3 - Biological Molecules

OCR A Level Biology Module 2 - Chapter 3 - Biological Molecules

Why must the genetic code be copied onto mRNA?

DNA is too large to leave the nucleus, and proteins are synthesised by ribosome in the cytoplasm

What is the difference in the proteins made by ribosomes found on RER compared to proteins made by ribosomes that float in the cytoplasm?

ribosomes on the RER synthesise proteins which are later modified and packaged by the Golgi apparatus to be packaged into vesicle for excretion by exocytosis, whereas ribosome that float freely in the cytoplasm synthesis proteins that remain and are used within the cell, so aren’t packaged into vesicles

What is transcription?

the process by which the base sequence of a gene is converted into a complementary sequence of mRNA

What enzyme can ‘unzip’ a DNA molecule?

DNA helicase

How does the enzyme DNA helicase ‘unzip’ the DNA molecule?

it breaks the hydrogen bonds between the complementary base pairs

Which enzyme joins adjacent nucleotides by phosphodiester bonds during transcription?

DNA polymerase

Where does the mRNA pass through to leave the nucleus?

a nuclear pore

In transcription, what is the name of the strand that the free mRNA nucleotides binds to?

template strand

Fill in the blanks: At the start of transcription, the part of the DNA molecule, in the region of the gene, ____ and is _____ by the enzyme ____ breaking the ____ bonds between _____ ____.

unwinds, unzipped, DNA helicase, hydrogen, complementary bases

True or false: only 1 of the 2 strands of DNA contains the gene

true

Fill in the blanks: The gene being copied in transcription is found on the ____ strand. The other strand, the ____ strand is a ____ copy of the strand containing the gene. Free mRNA nucleotides from complementary base paires with the unpaired bases of the ____ strand.

coding, template, complementary, template

In transcription, describe the bonding between unpaired bases of DNA and RNA nucleotides?

A joins to U by 3 hydrogen bonds and C joins to G by 2 hydrogen bonds, always purine to pyrimidine

Fill in the blanks: In transcription, once the free RNA nucleotides have paired with the unpaired DNA bases, the enzyme ____ joins (a)____ nucleotides by ______ bonds, forming a (s) _____ (b)____ of the single strand of RNA, that is complementary to the ____ strand and a copy of the ____ strand.

RNA polymerase, adjacent, phosphodiester, sugar-phosphate backbone, template, coding

Fill in the blanks: Translation the assembly of ____ chains at the ___ based on the sequence of c____

polypeptide, ribosome, codons

Where in the cell are ribosomes formed?

nucleolus

True or false? In eukaryotic and prokaryotic cells, ribosomes are made of 2 subunits, one large and one small

false - only in eukaryotic cells

What are the subunits of ribosomes made of?

rRNA and proteins

In what stage of the cell cycle does DNA replication occur?

interphase

Why must DNA be copied when cells divide?

so that each daughter cell is genetically identical

Why must the copying of DNA be accurate? Why might it not be accurate?

To conserve the genetic information. A mutation can cause differences in copied DNA

What is a mutation?

a random and spontaneous change in the formation of DNA base sequences

What is the term given to the process by which DNA replicates?

semi-conservative replication

What does the term semi-conservative replication mean?

for every 2 molecules of DNA produced/formed, each molecule contains 1 parent strand and one new strand

What enzyme unzips the DNA molecule before replication?

DNA helicase

In what part of the cell are free, activated nucleotides found?

nucleoplasm

What enzyme joins adjacent nucleotides in the new strand of DNA?

DNA polymerase

What is the term describing the part of DNA where free nucleotides joins once it has been unzipped?

replication fork

Describe how DNA replicates by semi-conservative replication

The DNA helix unwinds and the enzyme DNA helicase unzips the molecule by breaking the hydrogen bonds between the CBP at the replication fork.

Free, activated nucleotides form CBP with each parent template strand - A→T by 2 H bonds, C→G by 3 H bonds, always purine to pyrimidine

DNA polymerase joins adjacent nucleotides on each new strand by phosphodiester bonds in condensation reactions (releasing water molecules) to create a sugar phosphate backbone

2 genetically identical DNA molecules are formed, each containing one parent strand that acted as a template strand for a new, complementary strand

Where does the energy to form the phosphodiester bonds come from?

the extra phosphate groups of the free activated nucleotides, which are hydrolysed off

How are free nucleotides activated in the nucleoplasm?

with extra phosphate groups

True or false? DNA in prokaryotes also replicates semi-conservatively

true

Describe how DNA replicates in prokaryotes, and inside mitochondria and chloroplasts

A bubble sprouts from the loops of DNA, unwinding and unzipping it so complementary nucleotides can join the exposed area until the entire loop is copied

This image shows semi-conservative replication. Which letter represents the replication fork?

B

What were the 3 theories of how DNA self-replicated called?

conservative, semi-conservative, dispersive

What was the theory of conservative DNA replication?

original molecules act as templates and new molecules are made

What was the theory of dispersive DNA replication?

original molecules break into nucleotides and each one joins to a complementary nucleotide, which join into strands

Using the diagram, briefly describe the Meselson and Stahl experiment that provided evidence of semi-conservative replication

• nucleotides contain nitrogen, so free nucleotides in heavy nitrogen isotopes contain N15, and the ones formed in light nitrogen contain N14

• bacteria was grown in a heavy (N15) nitrogen medium

• it was transferred to a medium of light nitrogen (N14) and after a single replication the DNA has a medium density, being made from both N15 (parental strand) and N14 (new strand)

• the next replication lead to some DNA only containing N14 isotopes and some contain N15-N14, forming 2 bands when spun in a centrifuge

• for future replications, the band at N15-N14 will get thinner, while the band at N14 gets thicker

Fill in the blanks: DNA contains the ___ ___ which codes for the synthesis of all ____

genetic code, polypeptides

What is a gene?

the sequence of DNA that codes for the synthesis of a specific polypeptide

Why is the genetic code contained in the nucleus?

so it can be conserved/protected

Fill in the blanks: polypeptides are chains of ___ ___ joined by ____ bonds

amino acids, peptide

How many different amino acids are there?

20

What is a genome?

the entire sequence of DNA in an organism

What is the position of a gene of a chromosome called?

locus (loci)

What is found in between genes of DNA?

non-coding DNA

Give 5 words/phrases to describe the genetic code

triplet code, degenerate, widespread (but not universal), contains punctuation, is non-overlapping

Why is the genetic code a triplet code?

the sequence of DNA nucleotide bases found within a gene is determined by a triplet code, with each sequence of three bases, a codon, coding for 1 amino acid

How many different codons are there if 4 DNA bases are grouped into triplets?

64 (4³)

Why is the genetic code degenerate?

there are 64 different codons that code for only 20 amino acids, so multiple codons can code for the same amino acid

Why is it important that the genetic code is degenerate?

it helps to limit the number of mutations that have an serious effect

Fill in the blanks: the genetic code is (w)____ but not (u)_____

widespread, universal

Why is the genetic code not completely universal?

a few organisms have different nitrogenous bases

How is the genetic code said to have punctuation?

it has a universal start codon (AUG) and stop codons

True of false? the start codon of the genetic code also codes for an amino acid, but the stop codons don’t

true

What is meant by the term gene expression?

the sequence of bases expressed as an amino acid through the processes of transcription and translation

What are the full names of the 2 types of nucleic acid?

deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)

Fill in the blanks: DNA and RNA are ____, made from monomers called _____

polynucleotides, nucleotides

What elements do nucleic acids contain?

carbon, hydrogen, oxygen, nitrogen and phosphorus

What are the 3 components of a nucleotide?

phosphate group, pentose sugar and organic nitrogenous base

What type of reactions joins adjacent nucleotides?

condensation reaction

What bonds hold adjacent nucleotides in place?

phosphodiester bonds

What are the pentose monosaccharides found in DNA and RNA called?

DNA - deoxyribose, RNA - ribose

True or false? the phosphate component in DNA and RNA is always the same but there are 5 possible different organic nitrogenous bases

true

What is the difference between a purine and a pyrimidine?

a purine has 2 rings and a pyrimidine has 1 ring

In terms of purines and pyrimidines, how to complementary base pairs form?

a purine always pairs with a pyrimidine

Which organic nitrogenous bases are purines?

adenine and guanine

Which organic nitrogenous bases are pyrimidines?

cytosine, thymine and uracil

True or false? the phosphate group in a nucleotide is inorganic, slightly alkaline and negatively charged

false - its acidic

Fill in the blanks: a molecule of DNA consists of __ polynucleotide strands (___ stranded) which are bonded together are twisted into a 3D structure called a ____ _____

2, double, double helix

Fill the blanks: each strand has a phosphate group at one end (__’ (prime)) and a hydroxyl group at the other end (__’ (prime))

5, 3

The two strands of DNA run in opposite directions. What term is used to describe this?

antiparallel

True or false? DNA always runs in the 5’ to the 3’ direction?

true

What part of a DNA molecule do the alternating sugars and phosphate groups form?

sugar-phosphate backbone

What holds the two strands of DNA together?

hydrogen bonds between the nitrogenous bases

Which nitrogenous base joins to adenine and by how many hydrogen bonds?

thymine, 2

Which nitrogenous base joins to cytosine and by how many hydrogen bonds?

guanine, 3

Why does DNA need to be a stable macromolecule?

to protect the coded information within the base sequences

Give 2 similarities between DNA and RNA

• both made of nucleotides and phosphate groups

• both have phosphodiester bonds formed in condensation reactions between nucleotides

• the order of nitrogenous bases forms a code

• both contain the bases A, G and C

• both have pentose sugars

Give 4 differences between DNA and RNA

• DNA contains 2 strands in a double helix but RNA is largely single stranded

• DNA contains sugar deoxyribose - ribose is found in RNA

• DNA contain hydrogen bonds between complementary base pairs. RNA has no hydrogen bonds or CBP

• DNA is a large stable, macromolecule but RNA is small and unstable

• DNA is only found in the nucleus but RNA can be found in the nucleus or the cytoplasm

What is the role of RNA?

It transfers the genetic information from DNA in the nucleus to the ribosomes

Give 2 reasons why DNA can’t leave the nucleus

it is too large and its genetic information must be conserved/protected

What are the 3 types of RNA and where are they found?

mRNA (messenger) - made in nucleus and transported to the cytoplasm

tRNA (transfer) - cytoplasm

rRNA (ribosomal) - ribosomes

What is a phosphorylated nucleotide?

a nucleotide containing more than 1 phosphate group

Give an example of a phosphorylated nucleotide?

ATP / ADP

What do all organisms need energy for? (3 points)

• synthesis/anabolic reactions

• transport

• movement/muscle contraction

Fill in the blanks: ATP is knows as the (u)___ ____ ____ as it supplies energy in the ___ of all known forms of life

universal energy currency, cells

What are the 3 components of an ATP molecule?

adenine (organic nitrogenous base), ribose (pentose sugar), 3 phosphate groups

Fill in the blank: adenosine is a ________ (a base -adenine - attached to a pentose sugar)

nucleoside

What does ATP stand for?

adenosine triphosphate

What is the full name of a molecule of ATP after a phosphate group has been hydrolysed off?

ADP - adenosine diphosphate

What are the monomer units of proteins called?

amino acids

What is the name given to a molecule made of 2 amino acids?

dipeptide

What type of reaction form dipeptides and polypeptides?

condensation