Chromosomes, Genes and Proteins

Section 1 of 9Chromosomes, genes and alleles

Inheritance

Inheritance the transmission of genetic information from generation to generation.

Chromosomes

• Chromosomes thread-like structures of DNA in the nucleus, carrying genetic information as genes.

Gene

Gene a short length of DNA on a chromosome that codes for a specific protein.

• The protein may be structural (collagen), an enzyme or a hormone.
• Genes control characteristics because proteins control what cells do.

Alleles

Alleles different versions of the same gene.

• Humans have two copies of each chromosome, so two alleles of each gene, one from each parent.
• For example one eye colour allele may code for brown eyes and the other for blue.

Zoom model

• Zoom model organism, cell, nucleus, chromosome, gene, DNA bases.

Section 2 of 9Haploid and diploid nuclei

Feature	Diploid (2n)	Haploid (n)
Sets of chromosomes	Two (pairs)	One (unpaired)
Human number	46 (23 pairs)	23
Found in	Body cells and the zygote	Gametes

Memory aid

• Memory aid haploid means half the normal number.

Other species have different chromosome numbers.

Section 3 of 9Inheritance of sex

Sex chromosomes

• Sex chromosomes sex is determined by a whole chromosome pair, XX in females and XY in males.

The father decides

• The father decides only a father can pass on a Y chromosome.

Every egg carries X, and about half the sperm carry X and half carry Y.

Outcome

• Outcome an X sperm gives a girl (XX) and a Y sperm gives a boy (XY).

Punnett square

• XX × XY gives XX, XX, XY, XY, a 1 : 1 ratio.

Section 4 of 9From DNA base sequence to amino acid sequence

1Base sequence

• Base sequence the sequence of bases in a gene determines the sequence of amino acids in a protein.

2Protein shape and function

• Protein shape and function different amino acid sequences give different shapes and functions.

3Transcription

• Transcription the base code of the gene is rewritten as mRNA.

4Translation

• Translation the ribosome uses the mRNA sequence to join amino acids in order.

5Why mRNA

• DNA is too large to leave the nucleus through a nuclear pore.

6Triplet code

• Triplet code each group of three bases codes for one amino acid.

Section 5 of 9Making a protein

1

The DNA double helix unzips at the gene as hydrogen bonds between bases break.

2

Free RNA nucleotides pair with the template strand to build mRNA.

3

The mRNA leaves the nucleus and attaches to a ribosome.

4

The ribosome reads the mRNA in triplets (codons).

5

tRNA molecules, each carrying a specific amino acid, bring them to the matching codons.

6

The amino acids are joined in the order set by the codons.

7

The finished chain is released and folds into the final protein.

• RNA compared with DNA
  • RNA is single-stranded, contains ribose and has uracil instead of thymine, so A pairs with U.

Section 6 of 9Which proteins are made

Gene expression

• Gene expression whether a gene is transcribed and translated in a particular cell.

Same genes

• Same genes all body cells contain the same genes but most are switched off in any one cell.

Needed proteins

• Needed proteins only genes whose proteins the cell needs are switched on.

Examples of proteins

• Examples of proteins enzymes, antibodies and receptors for neurotransmitters.

DNA controls cell function by controlling which proteins are made.

Section 7 of 9More from the specification

More from the specification

Gene Expression and Polypeptide Synthesis

• A gene is a sequence of DNA nucleotide bases that codes for a specific sequence of amino acids, determining the primary structure of functional proteins such as enzymes and haemoglobin.

Mitotic Division and Chromosome Replication

• Prior to nuclear division, DNA replicates to form sister chromatids joined at a centromere;
  • spindle fibres then align and separate chromatids to ensure both daughter cells inherit identical diploid chromosome complements.

Meiosis and Gamete Haploidy

• Meiosis is a reductive nuclear division in diploid germ cells producing four genetically non-identical haploid gametes, each carrying half the parental chromosome number.

Dominant and Recessive Allele Expression

• An individual possessing a homozygous genotype carries identical alleles at a gene locus,
• whereas a heterozygous genotype expresses the dominant phenotype while masking the recessive allele.

Probability

• The chance that an offspring has a given genotype or phenotype.

Daughter cell

• Cell produced by cell division.

Blood group

• A phenotype set by alleles I-A, I-B and I-O.

Monohybrid inheritance

• Inheritance of one gene.

Mitosis

• Cell division producing genetically identical cells.

Recessive

• Expressed only when no dominant allele is present.

Dominant

• Expressed when present in the genotype.

Meiosis

• Reduction division giving four genetically different haploid cells.

Genotype

• The alleles an organism has.

Homozygous

• Having two identical alleles.

Phenotype

• The observable characteristic.

Heterozygous

• Having two different alleles.

Section 8 of 9Key terms

Term	Meaning
Chromosome	A thread-like structure of DNA carrying genes.
Gene	A short length of DNA that codes for a protein.
Allele	A version of a gene.
Haploid	Having one set of chromosomes.
Diploid	Having two sets of chromosomes.
Transcription	Rewriting the DNA base code as mRNA.
Translation	Using the mRNA code to build a protein.
Gene expression	Whether a gene is switched on in a cell.

Section 9 of 9Exam technique

1

Saying genes and alleles are the same — an allele is one version of a gene.

2

Saying the mother decides the sex of a child — every egg carries X and the sperm carries X or Y.

3

Saying haploid human cells have 46 chromosomes — they have 23.

4

Saying DNA leaves the nucleus to make proteins — a copy, mRNA, leaves.

5

Confusing base sequences with amino acid sequences — bases are in DNA and RNA, amino acids make proteins.