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A-Level Biology - OCR A > 2.6 - Nucleic Acids and Cell Division > Flashcards

2.6 - Nucleic Acids and Cell Division Flashcards

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1
Q

What is Chargraff’s rule?

A

A always bonds to T - 2 H bonds are formed

C always bonds to G - 3 H bonds are formed

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2
Q

What are pyrimidines?

A

Single ring bases - Thymine and Cytosine

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3
Q

What are purines?

A

Double ring bases - Adenine and Guanine

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4
Q

How many molecules of water are formed when a nucleotide is formed?

A

2

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5
Q

What shape do 2 chains of nucleotides make?

A

Double helix

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6
Q

What 2 bonds are formed in the formation of a nucleotide?

A

Phosphodiester bond - between phosphate and two sugars

Glycosidic bond - between the sugar and base.

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7
Q

What is a nucleotide comprised of?

A
  • Phosphate group
  • Ribose sugar
  • Nitrogenous base
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8
Q

How is DNA structured for its function?

A
  • Sugar-Phosphate backbone gives strength
  • Coiling gives compact shape
  • Double helix protects weak H bonds // makes molecule stable
  • Specific base sequence enables information replication
  • Lots of H bonds gives molecule stability
  • Weak H bonds allows molecule to be unzipped easily for replication.
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9
Q

Outline the process of DNA replication

A
  1. H bonds between bases are broken by DNA Helicase. Molecule unzips forming two single strands with exposed bases.
  2. Free nucleotides are attached to the exposed bases on the template strand by complementary base pairing.
  3. DNA polymerase joins the nucleotides together forming the phosphate-sugar backbone of the new strand.
  4. 2 new strands of DNA have been created as H bonds form between them.
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10
Q

What is DNA Helicase?

A

Enzyme which breaks H bonds - unzipping DNA.

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11
Q

What is DNA Polymerase?

A

Enzyme which attaches nucleotides to one another.

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12
Q

What is semi-conservative replication?

A

A (correct) model of DNA replication:

Two monomers of DNA are created which consists of one original DNA strand and one newly synthesised strand.

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13
Q

What is conservative replication?

A

An (incorrect) model of DNA replication:

Creates one original double strand of DNA and one entirely new double strand.

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14
Q

Describe Meselson and Stahl’s DNA replication experiment

A

Demonstrates that DNA replicates semi-conservatively.

E.coli bacteria grew in two different mediums: one containing ¹⁴N and one containing ¹⁵N.

The bacteria grown on ¹⁵N was transferred to the ¹⁴N so that any new nucleotides made and used in DNA replication would be made of the lighter isotope.

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15
Q

What is RNA?

A
  • Ribonucleic Acid
  • Ribose is its pentose sugar
  • Uracil is used instead of Thymine
  • Single polynucleotide strand
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16
Q

Name the three types of RNA

A
  • mRNA (messenger)
  • tRNA (transfer)
  • rRNA (ribosomal)
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17
Q

Outline the structure and function of mRNA

A
  • Used to transfer the DNA code from the nucleus to the cytoplasm
  • Once in the cytoplasm mRNA associates with ribosomes
  • Complementary to the template strand of DNA
  • The sequence of nucleotides in mRNA is referred to as the genetic code.
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18
Q

Outline the structure and function of tRNA

A
  • Small molecule folded into a clover leaf shape
  • Brings amino acids to the ribosome
  • One end of chain is slightly longer and attaches to an amino acid
  • 3 bases at the top of tRNA are called an anticodon and each amino acid has a different anticodon.
  • Anticodon pairs with the complementary codon on the mRNA.
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19
Q

Outline the structure and function of rRNA

A
  • Found alongside proteins in the ribosomes
  • Helps peptidyl transferase to catalyse the formation of peptide bonds between amino acids
  • Globular shape - long and large
  • Both single and double helix.
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20
Q

Outline the process of DNA transcription

A
  • mRNA is formed in the nucleus with RNA polymerase
  • mRNA forms on template strand of DNA
  • Leaves the nucleus via nuclear pores
  • Travels to the ribosomes for translation.
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21
Q

Outline the process of DNA translation

A
  • mRNA attaches to the ribosome at the ‘start’ codon
  • tRNA with complementary anticodon attaches to mRNA
  • This tRNA is attached to an amino acid
  • Ribosome moves along the mRNA bringing in 2 tRNA molecules at any one time
  • An enzyme and ATP are used to join the amino acids with a peptide bond
  • The first tRNA is released and can collect another amino acid
  • The process is repeated until a ‘stop’ codon is reached
  • Many ribosomes can travel along the mRNA at the same time.
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22
Q

What is a sequence of 3 bases on DNA referred to as?

A

Triplet

23
Q

What is a sequence of 3 bases on mRNA referred to as?

A

Codon

24
Q

What is meant by a degenerate code?

A

When amino acids have more than one codon which codes for them.

25
Q

What is meant by non-overlapping code?

A

Each nucleotide is only read once

26
Q

What is meant by universal code?

A

The same codons are used for the same amino acids in all organisms on Earth.

27
Q

List the phases of the cell cycle

A 
G₁ - Gap Phase 1
G₀ - G1 checkpoint
S - Synthesis
G₂ - Growth 2
M -  Mitosis and cytokinesis
28
Q

Detail what occurs during G₁

A
  • First position of interphase (cell prepares to divide)
  • Cell grows in size
  • Large amounts of proteins and cell organelles are produced in preparation for division
29
Q

Detail what occurs during G₀

A
  • Checkpoint during G₁ to make sure the cell is large enough and has enough proteins/organelles for DNA replication to occur.
30
Q

Detail what occurs during S Phase

A
  • DNA begins to replicate
  • S Phase begins when replication starts and ends once the final chromosome had been replicated.
  • Happens as quickly as possible due to high risk of exposed bases being affected by harmful chemicals such as mutagens.
31
Q

Detail what occurs during G₂

A
  • The gap between DNA synthesis and mitosis
  • Cell continues to grow
  • Makes sure that the DNA has been replicated without any damage. If it has, the cell will attempt to correct it or apoptosis will occur.
32
Q

Detail what occurs during the M Phase

A
  • Cell growth ceases
  • All energy is focused on division of the cell via mitosis
  • Cell completes cell division and the plasma membrane divides to form two new cells
33
Q

List the 5 phases of mitosis

A
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
  • Cytokinesis
34
Q

What happens during prophase?

A
  • Chromosomes condense
  • Centrosomes begin to move to opposite poles of cell. They form a network of spindle fibres around the cell.
  • Nuclear envelope breaks down to leave chromosomes free in the cytoplasm.
35
Q

What happens during metaphase?

A
  • Chromosomes (each with 2 chromatids) line up along the spindle equator of the cell
  • Chromosomes become attached to the spindle fibres through the centromere.
  • Metaphase checkpoint checks that chromosomes are all attached in preparation for division.
36
Q

What happens during anaphase?

A
  • Centrosomes divide, spreading sister chromatids

- Spindles contract to pull the chromatids to the opposite poles of the cell, centromere first.

37
Q

What happens during telophase?

A
  • Chromatids at the opposite poles of the cell begin to uncoil. They are now known as chromosomes again.
  • Nuclear envelopes reform around each group of chromosomes.
38
Q

What happens during cytokinesis?

A
  • Cytoplasm divides. In animals a cleavage furrow forms to divide the cell membrane.
  • Produces two genetically identical daughter cells.
  • Usually begins in anaphase and finishes in telophase.
  • Separate process to mitosis.
39
Q

What is a centriole?

A

Organelle in animal cells which produces the spindle.

40
Q

What is a heterosome?

A

The chromosome pair which determines sex.

41
Q

What is meant by homologous?

A

Chromosomes with the same genes in the same location.

42
Q

What are autosomes?

A

Chromosome pairs which are homologous.

43
Q

What are diploid cells?

A

Normal body cells - have 22 pairs of homologous chromosomes.

44
Q

What are haploid cells?

A

Sex cells - have half the normal genetic information - only one of each homologous chromosome.

45
Q

What is a bivalent?

A

A pair of associated homologous chromosomes held together by a complex after chromosome replication.

46
Q

Why is variation important for evolution?

A
  • Some have better adapted characteristics than others for a certain environment
  • Natural selection occurs // survival of the fittest
  • Eventually gene pool will change so much that a new species evolves.
47
Q

What is a stem cell?

A

An undifferentiated cell able to divide repeatedly and differentiate into specialised cells.

48
Q

What is self-renewal?

A

When a stem cell divides into two identical stem cells.

49
Q

What is differentiation?

A

When a stem cell divides into two specialised cells.

50
Q

What is meant by totipotent?

A
  • Can differentiate into all cell types (embryos/zygotes)
51
Q

What is meant by pluripotent?

A
  • Capable of producing all cells derived from a particular germ layer e.g. endo/ecto/mesoderm.
52
Q

What is meant by multipotent?

A
  • Can differentiate into a restricted range of related cell types. e.g. hemopoietic stem cells make RBCs, WBCs, platelets.
53
Q

How can stem cells be used to advance medicine?

A

Stem cells can help repair damaged tissues:

  • Can help repair damaged cardiac/neurological tissue and therefore help treat Alzheimer’s / Parkinson’s disease.
  • Can help repair skin in skin grafts.
54
Q

Define chiasma

A

Point of crossing.

A-Level Biology - OCR A (16 decks)

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